Insects, Their Ways and Means of Living/Chapter VII
Chapter VII
The Periodical Cicada
It is to be observed, in most of our human affairs, that we give greatest acclaim to the spectacular, and, furthermore, that when once a hero has delivered the great thrill, all his acts of everyday life acquire headline values. Thus a biographer may run on at great length about the petty details in the life of some great person, knowing well that the public, under the spell of hero worship, will read with avidity of things that would make but the dullest platitudes if told of any undistinguished mortal. Therefore, in the following history of our most famous insect, universally known as the "seventeen-year locust," the writer does not hesitate to insert matter that would be dry and tedious if given in connection with a commonplace creature.
Most unfortunate it is, now, that we are compelled to divest our hero of his long-worn epithet of "seventeen-year locust," and to present him in the disguise of his true patronymic, which is cicada (pronounced sĭ-ka'-da). In a scientific book, however, we must have full respect for the proprieties of nomenclature; and since, as already explained in Chapter I, the name "locust" belongs to the grasshopper, we can not continue to designate a cicada by this term, for so doing would but propagate confusion. Moreover, even the praenomen of "seventeen-year" is misleading, for some of the members of the species have thirteen-year lives. Entomologists, therefore, have re-christened the "seventeen-year locust" the periodical cicada.
The cicada family, the Cicadidae, includes many species PERIODICAL CICADA
of cicadas in both the New World and the Old, and some of them are more familiar, at least by sound, than our periodical cicada, because not only are the males noto- riously musical, but they are to be heard every year (Fig. ??2). The cicadas of southern Europe were highly es- teemed by the ancient Greeks and Romans for their song, and they were often kept in cages to furnish entertainment
FIG. II?. One of the common annual cicadas whose loud song is heard every year through the later part of the summer
with their music. The Greeks called the cicada tettix, and Aesop, who always found the weak spot in every- body's character, wrote a fable about the tettix and the ant, in which the tettix, or cicada, after having sung all summer, asked a bite of food from the ant when the chill winds of coming winter found him unprovisioned. But the practical ant heartlessly replied, "Well, now you can dance." This is an unjust piece of satire because the moral is drawn to the disparagement of the cicada. Human musicians have learned their lesson, however, and sign their contracts with the box-office management in advance.
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In the United States there are numerous species of "annual" cicadas, so called because they appear every year, but their life histories are not actually known in most cases. These species are called "locusts," "harvest files," and "dog-day cicadas" (Fig. I I2). They are the insects that sit in the trees during the latter hall of sum- mer and make those long shrill sounds that seem to be the natural accompaniment of hot weather. Some give a rising and falling inflection to their song, which re- sembles zwing, zwing, zwing, zwing, (repeated in a long series); others make a vibratory rattling sound; and still others utter just a continuous whistling buzz. During the interval between the times of the appear- ancê of the adult cicadas, the insects live underground. The periodical cicada comprises two faces, one of which lives in its subterranean abodes for most of seventeen years, the other for most of thirteen years. Both faces inhabit the eastern part of the United States, but the longer-lived race is northern, and the other southern, though their territories overlap. Most of out familiar in- sects complete their life cycle in a single year, and many of them produce two or more generations every season. For this reason we marvel at the long life of the periodical cicada, l'et there are other common insects that normally require two or three years to reach maturity, and certain beetles have been known to live for twenty years or more in an immature stage, though under conditions adverse for transforming to the adult. Throughout the period of their underground lire the cicadas have a form quite different from that which they take on when they leave the earth to spend a brief period in the trees. The form of the young periodical cicada at the time it is ready to emerge from the ground is shown in Plate 5- It will be seen that it suggests one of those familiar shells so often found clinging to the trunk of a tree or the side of a post. These shells, in fact, are the empty skins of young cicadas that have discarded their
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PLATE 5
The mature nymph of the periodical cicada in the form in which it leaves the ground to transform to the winged adult after a subterranean life of nearly seventeen years
earthly form for that of a winged insect of the upper world and sunshine, though the skins_ ordinarily seen are those of the annual species. The cicada undergoes a striking transformation from the young to the adult, but it does so directly and not by means of an intervening stage, or pupa. "?'he young of an insect that transforms directly is termed a n.vmph by most American entomologists. The last nymphal stage is sometimes called a "pupa," but it is not properly so designated. The life of the periodical cicada stirs our imagination as that of no other insect does. For years we do not see the creatures, and then a springtime comes when countless thousands of them issue from the earth, undergo their transformation, and swarm into the trees. Now, for several weeks, the very air seems swayed with the mo- notonous rhythm of their song, while the business of ma- ring and egg-laying goes rapidly on; and soon the twigs of trees and shrubs are everywhere scarred with slits and punctures where the eggs have been inserted. In a few weeks the noisy multitude is gone, but for the rest of the season the trees bear witness to the busy throng that so briefly inhabited them by a spotting of their foliage with masses of brown and dying leaves where the punctured stems have broken in the wind. The young cicadas that hatch from the eggs later in the summer silently drop to the earth and hastily bury themselves beneath the sur- face. Here they lire in solitude, seldom observed by creatures of the upper world, through the long period of their adolescent years, only to enjoy at the end a few brief weeks of life in the open air in the fellowship of their kind. T?? NYMPHS Of the underground lire of the periodical cicada we still know very little. The fullest account of the history of this species is that given by Dr. C. L. Marlatt in his l?Ssl
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Bulletin, Tt?e Periodical Cicada, published by the United States Bureau of Entomology in I9O 7 . Doctor Marlatt describes six immature stages of the periodical cicada between the egg and the adult. The young cicada that first enters the ground is a minute, soft-bodied, pale-skinned creature about a twelfth of an inch in length (Fig. 126). The body is cylindrical and is supported on two pairs of legs, the front legs being the digging organs; the somewhat elongate head bears a pair of small dark eyes and two slender, jointed antennae. At no stage has the cicada _jaws like those of the grass- hopper; it is a sucking insect, related to the aphids, and is provided with a beak arising from the under surface of the head, but when not in use the beak is turned back- ward between the bases of the front legs. Throughout the period of its underground life, the cicada subsists on the sap of roots. During more than a vear the young cicada retains ap- proximately the form it'has at hatching, though the body changes somewhat in shape, principally by an increase in the size of the abdomen (Fig. 1?3). According to Doctor Marlatt, a nymph of the seventeen-year race first
Fro. 113. Nymph of the periodi- cal cicada in the first stage, about ? 8 months old, enlarged ? 5 rimes. (From Marlatt)
sheds its skin, or molts, some- rime during the first two or three months of the second vear of its lire. In its second stage it be- comes a little larger and is marked by a change in the structure of the front legs, the terminal foot part of each being reduced to a mere spur and the fourth section being developed into
a strong, sharp-pointed pick which forms a more efficient organ for digging. The second stage lasts nearly two years; then the creature molts again and enters its th}rd
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THE stage, which is about a year in length. In the fourth stage, which lasts perhaps three or four years, the nymph (Fig. 114) shows distinct wing pads on the two wing-bearing segments of the thorax. In the fifth stage the insect, sometimes now called a "pupa," takes on the form it has when it finally emerges from the earth; its front feet are restored and its wing pads are well developed, but it has entirely lost its small nymphal eyes. Once more, before its long underground sentence is up, the nymph molts, and enters the sixth and last stage of its subterranean life. When mature (Plate 5) it is about an inch and a quarter in length, thick-bodied, and brown in color; it appears to have a pair of bright-red eyes on the head, but these are the eyes of the adult inside showing through the nymphal skin.
Fig. 114. Nymph of the periodical cicada in the fourth stage, about 12 years old, enlarged 2¾ times. (From Marlatt)
According to the investigations of Doctor Marlatt, the nymphs of the periodical cicada do not ordinarily burrow into the earth below two feet, and most of them are to be found at depths varying from eight to eighteen inches. However, there are reports of their having been discovered ten feet beneath the surface, and they have been known to emerge from the floors of cellars at the time of transformation to the adult stage. There is no evidence that the insects, even when present in great numbers in the earth, do any appreciable damage to the vegetation on the roots of which they feed.
Some time before the mature nymphs emerge from the ground, probably in April of the last year of their lives, the insects come up from their subterranean burrows and construct a chamber of varying depth just below the 
Fig. 115. Outlines of plaster casts of underground resting chambers of the mature nymph of the periodical cicada (about one-half natural size).
surface. A good idea of the size and shape of these cham- bers may be obtained by filling the opened holes with a mixture of plaster of Paris in water, letting the plaster harden, and then digging up the casts. Figure I t5 shows castsofa number ofchambers madein this way. Some, it is seen, are mere cups about an inch in depth, but most of them are long and narrow, descending several inches into the ground, the longest being six inches or more in depth. The width is usually about five-eighths of an inch. All the chambers have a distinct enlargement at the bottom, and most of them are slightly widened at the top. The upper wall of each is separated from the surface by a laver of undisturbed soil about hall an inch in thick- ness, which is not broken until the insect is ready to enlerge. The shafts are seldom straight, their courses being more or less tortuous and inclined to the surface, as the miner had to avoid roots and stones obstructing the vertical path. The interior contains no débris of any kind, and the walls are smooth and compact. Below each chamber there is always evidence of a narrower burrow going irregularly downward into the earth, but this tunnel is filled to the chamber floor with black granu- lar earth. The burrows examined by the writer near Washington in ?9?9 were dug through compact red clay, and the lower tunnels here made a distinct black path through the red of the surrounding clay, where some could be followed for a considerable distance. The black color of the earth filling the tum?els was possibly due to an admixture of fecal matter. The chambers, as we have noted, are closed at the top until the cicada is ready to emerge. The largest chambers are many times the btilk of the nymph in volume, and it becomes, then, a question as to what the insect does with the material it removed in making a hole of such size. It seems improbable that it could have been carried down into the lower tunnel, for this would be filled with its own
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débris. The insects thernselves will give an answer to the question if several of thern are placed in glass tubes and covered with earth; but, to understand the cicada's technique, we must first study the rnechanisrn of its digging tools, the front legs. The front leg of a rnature cicada nyrnph (Fig. J x6 A) is
Fro. ??6. The digging organ, or front leg, of the mature cicada nymph A, right le[g, inner surface (4 times natural size). B, the tarsus (Tar) bent inward at right angles to the tibia (Tb), the posi- tion in which it is used as a rake Cx, basal joint or coxa; Tf, trochanter; F, retour; Tb, tibia; Tar, tarsus, with two terminal daws
cornposed of the sarne parts as any other of its legs. The (hird segment from the base, which is the.femur (F), is large and swollen, and has a pair of strong spines and a cornb of srnaller ones projecting from its lower edge. The next segment is the tibia (Tb). It is curved and terrninates in a strong recurved point (B). Finally, at- tached to the inner sur- face of the tibia, well up from its terminal point, is the slender tarsus (Tar). The tarsus can be extended beyond the
tibial point when the insect is walking or clirnbing, but can also be turned inward at a right angle to the latter, as shown at B, or bent back against the inner surface of the tibia. Let us now return to the insects in the earth-filled tubes, where they are industriously at work. It will be seen that they are using the curved, sharp-pointed tibiae as picks with which to loosen the earth, the tarsi being turned back and out of the way. The two legs, working alternately, soon accurnulate a srnall rnass of loosened rnaterial in front of the insect's body. Now there is a
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cessation of digging and the tarsi are turned forward at right angles to the tibiae to serve as rakes (Fig. J J6 B). The mass of earth pellets is scraped in toward the body, and -here comes the important part, the cicada's special technique--the little pile of rakings is grasped by one front leg between the tibia and the femur (.Fig. ??6 A, Tb and F), the former closing up against the spiny margin of the latter, the leg strikes forcibly outward, and the mass of loosened earth is pushed back into the surrounding earth. The process is repeated, first with one leg, then with the other. The miner looks like a pugilist training on a punching bag. Now and then the worker stops and rubs his legs over the protruding front of the head to clean them on the rows of bristles which cover each side of the face. Then he proceeds again, clawing, raking, gathering up the loosened particles, thrusting them back into the wall of the growing chamber. His back is firmly pressed against the opposite side of the cavitv, the middle legs are bent forward uhtil their knees are aimost against the bases of the front legs, their tibiae lying along the wing pads. The hind legs keep a normal position, though held close against the sides of the bodv. From what we know of the cicada's spring habits underground, we can infer that the nymphs construct their chambers on their arrival near the surface during April, and that, when the chambers are completed, the insects wait within for the signal to emerge and trans- form into the adult. Then they break through the thin caps at the surface and come out. |t would be difficult to explain how they know when they are so near the top of the ground, and" why some construct ample chambers several inches deep wh?le others make mere cells scarcely larger than their bodies. Do they burrow upward till the pressure tells them that the surface is only a quarter of an inch or so away, and then widen the débris-filled tunnel downward? Evidently not, because the chamber walls are made of clean, compacted clay in which there
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is no admixture of the blackened contents of the burrows. It is unlikely, too, that they base their judgments on a sense of temperature, because their acts are not regulated by the nature of the season, which, if early or late, would fool them in their calculations. Early in the spring, before the proper emergence season, cicada nymphs are often round beneath logs and stones. This is to be expected, for, to the ascending insect, some- thing impenetrable has blocked the way, and there is nothing to tell it that it has already reached the level of the surface. A more curious thing, often observed in some localities,
F1G. i17. Earthcn turrcts somctimes. erected by the nymphs of the period?cal cicada as continuations from their under- ground chambers. One cut open showing the tubular cavity within. (From photo- graph by Marlatt)
is that the insects some- rimes continue their chambers up above the surface of the ground within closed turrets of mud from two to several inches in height (Fig. 117). At certain places these cicada "huts" have been reported as occur- ring m great numbers; and it bas been supposed that they ma}" be built wherever there is some- thing about the nature of the soli that the insects do not like, the earth being perhaps too damp, for they are frequently round where the ground
is unusually wet. On the other hand, the turrets have been observed in dry situations as well, and towers and holes flush with the surface frequently occur intermingled. The writer has had no opportunity of studying the cicada turrets, but a most interesting description of them is given [Igal
PLATE 6
The cicada just after emergence from the nymphal
skin. (Enlarged two-thirds)
The Transformation
The period of emergence for most of the cicadas of the northern, or seventeen-year, race is the latter part of May. The time of their appearance over large areas is much more nearly uniform than with most other insects, which show a wide variation according to temperature as determined by the season, the elevation, and the latitude. Nevertheless, observations in different localities show that the cicada, too, is influenced by these conditions. In the South, members of the thirteen-year race may emerge even a month earlier, the first individuals of the southernmost broods appearing in the latter part of April.
By some feeling of impending change the mature nymph, waiting in its chamber, knows when the time of transformation is at hand. Somehow nature regulates the event so that it will happen in the evening, but, once the hour has come, no time is to be lost. The nymph must break out of its cell, find a suitable molting site and one in accord with the traditions of its race, and there fix itself by a firm grip of the tarsal claws. At the beginning of the principal emergence period large numbers of the insects come out of their chambers as early as
rive o'clock in the afternoon; but after the rush of the first few days not many appear before dusk. It is difficult to catch a nymph in the very act ofmaking its exit from the ground, and apparently no observations have been recorded on the manner of its leaving. Do the insects leisurely open their doors some time in advance of their actual need and wait below till the proper hour, or do they break through the thin caps ofearth and emerge at once? Digging up many open chambers revealed a living nymph in only one. Another issued from one of several dozen holes filled with liquid plaster for obtaining casts. Add to this the fact that great numbers of fresh holes are to be seen every morning during the emergence season, and the evidence would appear to indicate that the insects open their doors in the evening and come out at once. Only one chamber was round in the daytime partly opened. If the insects are elusive and wary of being spied upon as they make their début into the upper world, a witness of tl?eir subsequent behavior does not embarrass them at all. However, events are imminent; there is no time to waste. The crawling insects head for any upright object within their range of vision- a tree is the ideal goal if it can be attained, and since the creatures were born in trees there is likely tobe one near by. Yet it ffequently happens that trees in which many were hatched have been since cut down, in which case the returning pilgrims must make a longer journey perhaps than anticipated. But the transformation can not be delayed; if a tree is not accessible, a bush or a weed, a post, a telegraph pole, or a blade of grass will do. On the trees some get only so far as the trunk, others attain the branches, but the mob gets out. upon the leaves. Thotlgh thousands emerge almost simultaneously, they have not all been timed alike. Some have but a few minutes to spare, others can travel about for an hour or so before anything happens.
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The external phase of transformation, more strictly the shedding of the last nymphal skin, has been many times observed. It is nothing more than what all insects do. But the cicada is notorious because it does the thing in such a spectacular way, almost courting publicity where most insects are shy and retiring. As a conse- quence the cicada is famous; the others are known only to prying entomologists. Let us suppose now that our crawling nymph has reached a place that suits it, say on the trunk of a tree, or better still on a piece of branch provided for it and taken into a lighted room where its doings can be more clearly observed. Though the insects choose the evening for emergence, they are not bashful at all about changing their clothes in the glare of artificial light. The progress of this performance is illustrated by Figure I i8. The first drawing shows the nymph stiil creeping upward; but in the next (2) it has come to rest and is cleaning its t?ront feet and claws on the brushes of its face, just as did those confined to the glass tubes to give a demon- stration of their digging methods. The front feet done, the hind ones are next attended to. First one and then the other is slowly flexed and then straightened back- ward (3) while the foot scrapes over the side of the ab- domen. Several times these acts are repeated calmly and deliberately, for it is an important thing that the claws be well freed from any particles of dry earth that might impair their grip on the support. /?t last the toilet is completed, though the naiddle feet are always neglected, and the insect feels about on the twig, grasp- ing now here, now there, till its claws take a firm hold on the bark. At the same time it sways the body gently frona side to side as if trying to settle comfortably for the next act. Thirtv-five minutes may be consumed in the above preliminaries and there is "next a ten-minute interval of quietude before the real show begins. Then suddenly
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Ç l+. I18. Transformation of the periodical cicada from thc mature nymph to the adult
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the insect humps its back (4), the skin splits along the midline of the thorax (.5), the rupture extending forward over the top of the head and rearward into the first seg- ment of the abdomen. A creamy white back, stamped with two _large .let-black spots, now bulges out ((5, 7); next comes a head with two brilliant red eyes (8); this is followed by the front part of a body (9) which bends backward and pulls out legs and basés of wings. Soon one leg is free (`'o), then four legs (`'`'), while four long, glistening white threads pull out of the body of the issuing creature but remain attached to the empty shell. These are the linings of the thoracic air tubes being shed with the nymphal skin. Now the body hangs back down, when all the legs come free (`'2), and now it sags peril- ously (C?') as the wings begin to expand and visibly lengthen. Here another rest intervenes; perhaps twentv-five min- utes may elapse, while the sort new creature, like an in- verted gargoyle supported only by the rear end of its body, hangs motionless far out "fro?{? the split in the back of the shell. Now we understand why the nymph took such pains to get a firm anchorage, for, should the dead claws give way at this critical stage, the resulting fall most probably «ould prove fatal. The next act begins abruptly. The gargoyle moves again, bends its body upward ("4), grasps the head and shoulders of the slo'ugh (*'5), and pulls the rear parts of its body free from the gaping skin (,,6). The body straighten?" and hangs downward (`'7)- At last we be- hold the free imago, hOt ver mature but rapidly assum- ing the characters of an adult cicada. The new creature hangs for a while from the discarded shell-like skin, clinging by the front and middle legs, sometimes by the first alone; the hind ones spread out sidewavs or bend against the body, rarely grasping the skin. The wings continue to unfold and lengthen, finally hang fiat, fully formed, but soft and white (`'N). 14ere the creature
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usually becomes restless, leaves the empty skin (I9) , and takes up a new position several inches away At this stage the cicada is strangely beautiful. Its creamy-yellow paleness, intensified by the great black patches just behind the head and relieved by the pearly flesh tint of the mesothoracic shield, its shining red eyes, and the milky, semitransparent wings with deep chrome on their bases make a unique impression on the mind. There is a look of unreality about the thing, which out of doors (Plate 6) becomes a ghostlike vision against the night. But, even as we watch, the color changes; the unearthly paleness is suffused with bluish gray, which deepens to blackish gray; the wings flutter, fold against the back, and the spell is broken--an insect sits in the place of the vanished specter. The rest is commonplace. The colors deepen, the grays become blackish and then black, and after a few hours the creature bas all the characters of a fully matured cicada. Early the next morning it is fluttering about, restless to be off with its mates to the woods. The time consumed by the entire performance, from the splitting of the skin (Fig. I ? 8, 5) to the folding of the wings above the back (2I), varies with different indi- viduals, observed at the same time and under the same conditions, from forty-five minutes to one hour and twelve minutes. Most of the insects have issued from the nymphal skins before eleven o'clock at night, but oc- casionally a straggler may be seen in the last act as late as nine o'clock the following morning--probably a be- lated arrival who overslept the night before. Thus, to the eye, the burrowing and crawling creature of the earth becomes transfigured to a creature of the air; vet the visible change is mostly but the final escape of the mature ingect from the skin of its preceding stage. Aside from a few last adjustments and the expansion of the wings, the real change bas been in progress within the nymphal skin perhaps for years. We do not truly witness
[ ?9 8 ] PLATE 7
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The periodical cicada (Magicicada septendecim)
A female inserting eggs with her ovipositor into the under surface of an apple twig. (Enlarged two times) the transformation; we see only the throwing off of the shell that concealed it, as the circus performer strips off the costume of the clown and appears already dressed in that of the accomplished acrobat.
The Adults
The adult cicada bears the stamp of individuality. In form he does not closely resemble any of our everyday insects, and he has a personality all his own; he impresses us as a "distinguished foreigner in out midst." The body of the periodical cicada is thick-set (Fig. 119), the face is bulging, the forehead is wide, with the eyes set out prominently on each side; from the under side of the head the short, strong beak projects downward and backward between the bases of the front legs. The colors are distinctive but not striking. The back is plain black (Plate 7); the eyes are bright red; the wings are shiny transparent amber with strongly marked orange-red veins; the legs and beak are reddish, and there are bands of the same color on the rings of the abdomen. Each front wing is branded near the tip with a conspicuous dark-brown W.
With both the seventeen-year race and the thirteen-year race of the periodical cicada there is associated a small cicada, which, however, differs so little except in size from the others (Fig. 119) that entomologists generally regard it as a mere variety of the larger form, the latter always including by far the greater number of individuals in any brood.
The male cicada has a pair of large drumheads beneath the bases of the wings on the front end of the abdomen (Fig. 120, Tm). These are the instruments by which he produces his music, and we will give them more attention presently. The female cicada has no drums nor other sound-making organs; she is voiceless, and must keep silence no matter how much her noisy mate may disturb her peace. The chief distinction of the female is her ovipositor, a long, swordlike instrument used for inserting N S EC'I'S
the eggs into the twigs of trees and bushes. Ordinarily the ovipositor is kept in a sheath beneath the rear halfof the abdomen, but when in use it can be turned downward and forward by a hinge at its base (Plate 7). The oviposi- tor consists of two
lateral blades, and a ? guide-rail ai»ove. The blades excavate a cav- ? itv in the wood in- te; which the eggs j are passed through the space between the blades. I t was formcrly sup- posed that the period- ical cicada takes no fo??d during the brief Flç. 119. blales of the large and small form of the periodical cicada (natural size) rime of its adult lire, but ?e know from the observations of .?lr. W. T. l)avis, l)r. A. !...Quaintance, and others and from a study of the stomach contents made o by the writer that the insects do feed abundantly by st'?cking the sap from the trees on which they lire. The cicada, being a near relative of the aphids, has ?? / ?? also, as we have already noted, a pierci?{? and suck- in? belîk by which ? it ? the punctures plant tissues and r,? draws the sap up toitsmouth. ['n- like the other sucking insects v,ç. ,=o. ? ,?ale of ,?e p?r?,,??«a! «i?aaa ,?i,? ,?e wings spread, showing the ribbed sound-producing that infest plants, organs, or tympana (Tm),on thebaseofthe abdomen
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THE however, the cicadas cause no visible damage to the trees by their feeding. Perhaps this is because their attack lasts such a short time and comes at a season when the trees are at their fullest vigor.
The details of the head structure of the cicada and the exposed part of the beak are shown in Figure 121 which gives in side view the head of a fully matured adult, detached from the body by the torn neck membrane (NMb), with the beak (Bk) extending downward and backward below. The large eyes (E) project from the sides of the upper part of the head. The face is covered by a large protruding, striated plate (Clp). The cheek regions are formed by a long plate (Ge) on each side below the eyes; and between each cheek plate and the striated facial plate is partly concealed a narrower plate (Md). The cicada has no jaws. Its true mouth is shut in between the large flap (AClp), below the striated facial plate, and the base of the beak.
If the outer parts of the head about the mouth can be separated, there will be seen within them some other very important parts ordinarily hidden from view. In a specimen that has been killed in the act of emerging from the nymphal skin, when it is still soft, the outer parts are easily separated, exposing the structures shown at B of the same figure.
It is now to be seen (Fig. 121 B) that the beak consists of a long troughlike appendage (Lb) suspended from beneath the back part of the head, having a deep groove on its front surface in which are normally ensheathed two pairs of slender bristles (MdB, MxB), of which only the two of the left side are shown in the figure. In front of the bases of the bristles there is exposed a large tongue-like organ which is the hypopharynx (Hphy). Between this tongue and the flap hanging from the front of the face is the wide-open mouth (Mth), the roof of which (e) bulges downward and almost fills the mouth cavity. The way in which the cicada obtains its liquid food depends upon the finer structure and the mechanism of the parts before us.
Each one of the second pair of bristles has a furrow along the entire length of its inner surface, and the two
Fig. 121. The structure of the head and sucking beak of the adult cicada
A, the head in side view with the beak (Bk) in natural position
B, the head of an immature adult: the mouth (Mth) opened exposing the roof (e) of the sucking pump (see fig. 122), and the tonguelike hypopharynx (Hphy); the parts of the beak separated, showing that it is composed of the labium (Lb), inclosing normally two pairs of long slender bristles (MdB, MxB, one of each pair shown)
a, bridge between base of mandibular plate (Md) and hypopharynx (Hpy); Aclp, anteclypeus; Ant, antenna; Bk, beak; Clp, clypeus; e, roof of mouth cavity, or sucking pump; Ge, gena (cheek plate); Hpy, hypopharynx; Lb, labium; Lm, labrum; Md, base of mandible; MdB, mandibular bristle; Mth, mouth; Mx, maxilla; MxB, maxillary bristle; NMb, neck membrane; O, ocelli
bristles, small as they are, are fastened together by interlocking ridges and grooves, so that their apposed furrows are converted into a single tubular channel. In the natural position, these second bristles lie in the sheath of the beak (Fig. 121 A) between the somewhat larger first bristles. Their bases separate at the tip of PERIODICAI. CICADA
the tongue (Hphy) to pass to either side of the latter organ, but the channel between them here becomes con- tinuous with a groove on the middle of the forward sur- face of the tongue. When the mouth-opening is closed, as it always is in the fully matured insect, the tongue groove is converted into a tube which leads upward from the channel between the second bristles into the inner cavity of the mouth. It is through this minute passage that the cicada obtains its liquid food; but obviously there must be a pumping apparatus to furnish the sucking force. The sucking mechanism is the mouth cavity and its muscles. The mouth cavity, as seen in a section of the head (Fig. ? 22, Pmp), is a long, oval, thick-walled capsule having its roof, or anterior wall (e), ordinarily bent inward so far as almost to fill the cavity. Upon the midline of the roof is inserted a great mass of muscle fibers (Pmp:$lcl«) that have their other attachment on the striated plate of the face (Clp). The contraction of these muscles lifts the roof, and the vacuum thus created in the cavity of the mouth sucks up the liquid food. Then the muscles relax, and the elastic roof again collapses, but the lower end comes down first and forces the liquid upward through the rear exit of the mouth cavity into the pharynx, a small muscular-walled sac (P]zy) lying in the back of the head. From the pharynx, the food is driven into the. tubular gullet, or oesophagus (OE), and so on into the stomach. The bases of both pairs of bristles are retracted into pouches of the lower head wall behind the tongue, and tapon each bristle base are inserted sets of protractor and retractor muscle fibers. By means of these muscles, the bristles can be thrust out from the tip of the beak or withdrawn, and the bristles of the stronger first pair are probably the chier organs with which the insect punctures the tissues of the plant on which it feeds. As the bristles enter the wood, the sheath of the beak can
INSECTS
be retracted into the flexible membrane of the neck at i ts base. One other structure of interest in the cicada's head should be observed. This is a force pump connected with the duct (Fig. ?2"_, SalD) of the large salivary glands ?G/, GI) and used probably for injecting into the wound of the plant a secretion which perhaps softens the tissues of the latter as the bristles are inserted. Possibly the salira has ,'{lso a
Cl
GI
/VidB" /VixB"
FIG. 122. Median section of the head and beak of an aduIr cicada The sucking pump (Proie) is rhe mouth cavity, the collapsed roof of which (e) can be lifred like a piston by rhe large 'muscles (Pmp?Icls) arising on rhe clypeus (Clp). The liquid food ascends rhrough a channel between rhe maxillary brisrles (?IxB}, is drawn into rhe mourh opening (?Ith), and pumped back inro rhe pharynx (Phy), from which if goes inro rhe oesophagus (OE). A salivary pump (?alPmp) opens af rhe rip of rhe hypopharynx (Hp&v), dis- charging the secrerion of rhe large glands (GI, GI) inro rhe beak
digestive action on the food liquid. The salivary pump (SalPmp) lies behind the mouth, and its duct opens on the extreme tip of the tongue, where the salira can be driven into the charme} of the second bristles. .?lost sucking sects have two parallel channe]s between these bristles IFig. 9o), one for taking food, the other for ejectingsaliva, and the cicada probably bas two also, though in- vestigators diff-er as to whether there are two or on}y one.
[ 204]
THE PERIODICAL CICA1)A
The head of the cicada is thus seen to be a wonder(ul mechanism for enabling the insect to feed on plant sap. The piercing beak and the sucking apparatus, however, are characters distinguishing the members of a whole order of insects, the Hemiptera, or Rhvnchota. This order includes, besides the cicadas, such "familiar insects as the plant lice, the scale insects, the squash bugs, the giant water bugs, the water striders, and the bed bugs. "Fo the sucking insects properly belongs the name "bug," which is not a svnonwaa of "insect." It is believed, of course, that the parts of the sucking beak of a hemipteran insect correspond with the mouth parts of a biting insect, described in Chapter IV (Fig. 66), but it has been a difficult matter to determine the identities of the parts ira the two cases. Probably the anterior narrow plate on the side of the cicada's'head (Fig. ?2?, .lld) is a rudiment of the base of the true jaw, or mandible. The first bristles (aldB) are outgrowths of the mandibular plates, which have become detached from them and made independently movable by special sets of muscles. The second bri?'tles (M.vB) are out- growths of the maxillae, which are otherwise reduced to small lobes (M.v) depending from the cheek plates (Ge). The sheath of the beak (Lb) is the labium. We have here, therefore, a most instructive lesson on the manner ira which organs may be made over ira form, by the processes of evolution, "adapting them to new and often highly special uses. The abdomen of the cicada is thick, and strongly arched above, l ts external appearance of plumpness suggests that it would furnish a juicy meal for a bird, and birds do destroy large numbers of the insects. Yet when the interior of a cicada is examined (Fig. ? 23) , it is found that almost the entire abdomen is occupied by a great air chamber! The soft viscera are packed into narrow spaces about the air chamber, the stomach (&0m) being crowded forward into the rear part of the thorax.
[ 205 ]
INSECTS
[ = o6 ]
THE PERIODICAL CICADA
The air chamber is a large, thin-walled sac of the tracheal respiratory system, and receives its air supply directly through the spiracles of the first abdominal segment. From the sac are given off tracheal tubes to the muscles of the thorax and to the walls of the stomach. Many insects have tracheal air sacs of smaller size, and the purpose of the sacs in general appears to be that of holding reserve supplies of air for respiratory pur- poses. The great size of the air sac in the cicada's abdo- men, however, suggests that it has some special function, and it is natural to suppose that it acts as a resonating chamber in connection with the sound-producing drums. Yet the sac is as well developed in the female as in the male. Possibly, therefore, it serves too for giving buoy- ancy to the insects, for it can readily be seen that if the space occupied by the sac were filled with blood or other tissues, as it is in most other insects, the weight of the cicada would be greatly increased; or, on the other hand, if the body were contracted to such a size as to accommodate only its scanty viscera, it would lose buoyancy through lack of suflîcient extent of surface-- a paper bag crumpled up drops immediately when re- leased, but the same bag inflated almost floats in the air. THE SOUND-PRODUCING ORGANS AND THE SONG The cicadas produce their music by instruments quite different from those of any of the singing Orthoptera --the grasshoppers, katydids, and crickets, described in Chapter ll. On the body of the male cicada, just back of the base of each hind wing, as we have already observed, in the position of the "ear" of the grasshopper (Fig. 63, Tre), there is an oval membrane like the head of a drum set into a solid frame of the body wall (Fig. lzo, Tre). Each drumhead, or tympanum, is a mem- brane closely ribbed with stiff vertical thickenings, the number of ribs varying in different species of cicadas and perhaps accounting in part for the different qualities [ "-o7 1
INSECTS
of sound produced. In the periodical cicada, the drum- heads are exposed and are easily seen when the wings are lifted; ira our other common cicadas each drum lS concealed by a flap of the body wall. The sound made by ara ordinary drum is produced by the vibration of the drumhead that is struck by the player, but the tone and volume of the sound are given by the air space within the drum and by the sympathetic vibration of the opposite head. The air within the drum, then, must be in communication with the air outside the drum, else it would impede the vibration of the drumheads. All these conditions imposed tapon a drum are met by the cicada. The abdomen of the insect, as we have seen, is largely occupied by a great air chamber (Fig. ?z3) , and the air within the chamber communicates with the outside air through the spiracles of the first abdominal segment (ISp). In addition to the two drumheads whose activity produces the sound, there are two other thin, taut l?mmbranous areas set into oral frames in the lower side walls of the front part of the abdomen (hOt seen in the figures). These ventral drumheads have such smooth and glistening surfaces that they are often desig- nated the "mlrrors." The wall of the air sac is applied closely to their inner surfaces, but both membranes are so thin that it is possible to see through them right into the hollow of the cicada's bodv. The ventral drum- heads are hOt exposed externally, however, for they are covered by two large, fiat lobes projecting back beneath them from the under part of the thorax. The cicada does not beat its drums or play upon thern with any external part of its body. When a male is "singing," the exposed drumheads are seen to be in very rapid vibration, as if endowed with the power of auto- matic movement. An inspection of the interior of the body of a dead speclmen, however, shows that con- nected with the inner face of each drumhead is a thick
I =og ]
THE PER1OD1CAI. CICADA
muscle which arises below from a special support on the ventral wall of the second abdominal segment (Figs. ?23, I24, Tm.rtlcl). It is by the contraction of these muscles that the drum membranes are set in motion.
TITI Trnl?cl
A
IT IIT IIIT Tre- . .Trrd?cl B IS IIS IIiS
FIG. 1:24. The abdomen and sound-making organs of the male periodical cicada A, the abdomen cut open from above, exposing the air chamber (HirSc), and showing the great tympanal muscles (TmMcl) inserted on the tympana (Tre). The arrows indicate the position of the first spiracles opening into the air chamber (see fig. 123, ISp) B, inner view of right hall of first and second abdominal segments, showing the ribbed tympanum (Tre), and the muscles that vibrate it (TmMcl) AirSc, air chamber; DMcl» dorsal muscles; IS, IlS, IIIS, sternal plates of first three abdominal segments; ISp, first abdominal spirade; IT, liT, IIIT, tergal plates of first three abdominal segments; ,??, tergal plate of third thoracic seg- ment; Trn, tympanum; TmMcl, tympanal muscle; I/?a, base of hind wing; I?Mcl, ventral muscles
But a muscle pulls in only one direction; the drum muscles produce directly the inward stroke of the drumhead membranes; the return stroke results flore the outward convexity and the elasticity of the heads themselves and the stiff ribs in their wal]s. When a cicada starts its music, it lifts the abdomen a little, thus opening the space between its ventral drum-
[ 2o9 ]
INSECTS
heads and the protecting flaps beneath, and the sound comes out in perceptibly increased volume. There can be little doubt that the air chamber of the body andthe ventral membranes are important accessories in the sound-producing apparatus. Living cicadas are often found with hall or more of the abdomen broken off, leav- ing the air sac open to the exterior. Such individuals may vibrate the drumheads, but the sound produced is weak and entirely lacks the quality of that made by the perfect insect. Wherever the periodical cicada appears in great num- bers, the daily choruses of the mmes leave an impression long remembered in the neighborhood; and, curiously, the sound appears to become increasingly louder ?n retrospect, until, after the lapse of years, each hearer is convinced it was a deafening clamor that almost deprived him of his senses. Fortunately the cicadas are day- time performers and are seldom heard at night. The song of the periodical species has no resemblance to the shrill, undulating screech of the annual cicadas so coin- mon every summer in August and September. All the notes of the more common large form of the seventeen- year race are characterized by a burr sound, and at least four different utterances may be distinguished; the quality of three of the notes probably depends on the age of the individual insect, the fourth is an expression of fright or anger. The simplest notes to be heard are sort purring sounds, generally made by solitary insects sitting low in the bushes, probably individuals that have but recently emerged from the ground. The next is a longer and louder note, characterized by a rougher burr, lasting about rive seconds, and always given a falling inflection at the close. This sound is the one popularly known as the "Pharaoh" song, because of a fancied resemblance to the name if the first syllable is sufficiently prolonged and the second allowed to drop off abruptly at the end. It
[ 2.10 ]
THE PERIODICAI. CICADA
is repeated at intervals of from two to rive seconds, and is given always as a solo by individuals sitting in the bushes or on lower branches of the trees. Males singing the Pharaoh song, therefore, are easily observed in the act of performing. With the beginning of each note, the singer lifts his abdomen to a rigid, horizontal position, thus opening the cavity beneath the lower drumheads and letting out the full volume of the sound. Toward the end of the note, the abdomen drops again to the usual somewhat sagging position, appearing thus to give the abrupt falling inflection at the close. The grand choruses, by which the periodical cicada is chiefly known and remembered, are given by the fully matured mmes of the swarm, always high in the trees where the singers may seldom be closely observed while performing. The individual notes are prolonged bur-r-r-r- like sounds, repeated all day and day after day, but all single voices are blended and lost in the continuous hum of the multitude. The fourth note of the larger form of the cicada is uttered by males when they appear to be surprised or frightened'. On such occasions, as the insect darts away, he makes a loud, rough sound, and the same note is often uttered when a male is picked up or otherwise handled. "l'he notes of the small form of the seventeen-year race of the cicada have an entirely different character from those of his larger relative. The regular song of the little toiles much more resembles that of the annual summer cicadas, th?mgh it is not so long and is less continuous in tone. It opens with a feu, short chirps; then follows a series of strong, shrill sounds like z?'ing, za:'i?g, zaaing, and so on, closing again with a number of chirps. The whole song lasts about fifteen seconds. Several of these males kept in cages for observation sang this song re-. peatedly and no other. It is common out of doors, but always heard in solo, never in chorus. When handled
[211 ]
I or otherwise disturbed, the small males utter a succession of sharp chirps very suggestive of the notes of some miniature wren angrily scolding at an intruder. Never does the small form of the cicada utter notes having the burr tone of those of the larger species, and the vocal differences of the two varietms are strikingly evident when several males of both kinds are caged together. When disturbed, each produces his own sound, one the burr, the other the chirp; and there is never any suggestion of similarity or of gradation between them.
Egg Laying
The cicadas lay their eggs in the twigs of trees and shrubs and frequently in the stalks of deciduous plants. They show no particular choice of species except that conifers are usually avoided.
The eggs are not stuck into the wood at random, but are carefully placed in skillfully constructed nests which the female excavates in the twigs with the blades of her ovipositor (Plate 8). These nests are perhaps always on the under surfaces of the twigs, unless the latter are vertical, and usually there are rows of from half a dozen to twenty or more of them together.
Egg laying begins in the early part of June, and by the tenth of June it is at its height. The female cicadas can easily be watched at work, taking flight only from actual interference. They usually select twigs of last year's growth, but often use older ones or green ones of the same season. In the majority of cases the female works outward on the twig; but if this is a rule, it is a very loosely observed one, for many work in the opposite direction.
Each nest is double; that is, it consists of two chambers having a common exit, but separated by a thin vertical partition of wood (Plate 8, D, F). The eggs are placed on end in the chambers in two rows, with their head ends PLATE 8
Egg punctures and the eggs of the periodical cicada
A, B, C, twigs of dogwood, oak, and apple containing rows of cicada egg nests. D, cross-section of a twig through an egg nest, showing the two chambers, each containing a double row of eggs. E, vertical lengthwise section through two egg nests, showing the rows of slanting eggs and the frayed lip of the nest opening. F, horizontal section showing each chamber filled with a double row of eggs. G, several eggs (much enlarged)
downward and slanted toward the door. Generally there are six or seven eggs in each row (E), making twenty-four to twenty-eight eggs in the whole nest, but frequently there are more than this. The wood fibers at the entrance are much frayed by the action of the ovipositor and make a fan-shaped platform in front of the door (A, B, C). Here the young shed their hatching garments on emerging from the nest. The series of cuts in the bark eventually run together into a continuous slit, the edges of which shrink back so that the row of nests comes to have the appearance of being made in a long groove. This mutilation kills many twigs, especially those of oaks and hickories, the former soon showing the attacks of the insects by the dying leaves. The landscape of oak- covered regions thus becomes spotted all over with red- brown patches which often almost cover individual trees from top to bottom. Other trees are not so much injured directly, but the weakened twigs often break in the wind and then hang down and die.
An ovipositing female (Plate 7) finishes each egg nest in about twenty-five minutes; that is, she digs it out and fills it with eggs in this length of rime, for each chamber is filled as it is excavated. A female about to oviposit alights on a twig, moves around to the under surface, and selects a place that suits her. Then, elevating the abdomen, she turns her ovipositor forward out of its sheath and directs its tip perpendicularly against the bark. As the point enters it goes backward, and when in at full length the shaft slants at an angle of about forty-five degrees.
In a number of cases females were frightened away at different stages of their work, and an examination o(the unfinished nests showed that each chamber is filled with eggs as soon as it is excavated; that is, the insect completes one chamber first and fills it with eggs, then digs out the other chamber which in turn receives its quota of eggs, and the whole job is done. The female now moves
[213 ]
INSECTS
forward a few steps and begins work on another nest, which is completed in the same fashion. Some series consist of only three or four nests, while others contain as many as twenty and a few even more, but perhaps eight to twelve are the usual numbers. When the female bas finished what she deems sufficient on one twig, she flies away and is said to make further layings elsewhere, till she has disposed of her 4c?o to 6c?o eggs,, but the writer made no observations covering this point. Probably the cicada feels it saler not to intrust all her eggs to one tree, on the principle of not putting all your money in the same bank.
DEATH OF THE ADULTS The din of music in the trees continues with monot- onous regularity into the second week of June, by which time the mating season is over. Soon thereafter the per- formers lose their vitality; large numbers of them drop to the earth where many perish from an internaI fungus disèase that eats off the terminal rings of the body; others are mutilated and destroyed by birds, and the rest perhaps just die a naturaI death. Beneath the trees, where a great swarm has but recently given such abundant evidence of lire, the ground is now strewn with the dead or dying. A large percentage of the living are in various stages of disfigurement--wings are torn off, abdomens are broken open or gone entirely, mere fragments crawl about, stiI1 alive if the head and thorax are intact. In the ma]es often the great muscle coIumns of" the drums are exposed and visibly quivering, and many of" the insects, gaine to the end, even in their diIapidated condition stiII utter purring remnants of their song. From now on tiI1 the latter part of JuIy, the onIy evi- dence of the Iate swarm of noisy visitors wiI1 be the scarred twigs on the trees and bushes that have received the eggs and the red-brown patches of dying Ieaves that every- where disfigure the oaks and hickories.
THE PERIODICAL CICADA
TI-:E BROODS
The two races of the periodical cicada, the seventeen- year and the thirteen-year, together occupy most of the eastern part of the United States, except the northern part of New England, the southeastern corner of Georgia, and the peninsula of Florida. The western limits extend into the eastern part of Nebraska, Kansas, Oklahoma, and Texas. In general, the seventeen-year race is north- ern, and the thirteen-year race is southern, but, though the geographic line between the two races is remarkably distinct, there is considerable overlapping. While the two cicada races are distinguished from each other by the length of their lire cycle, the members of each race do not all appear in the adult stage in any one year. Both the seventeen-year race and the thirteen-year race are broken up into groups of individuals that emerge in different years, and these groups are known as "broods." Each brood bas its definite year of emergence, and in general a pretty well-defined territorv. The territories of the different broods, however, overlap, or the range of a small brood may be included in that of a larger one. Hence, in any particular locality, there is not always an interval of thirteen or seventeen years between the ap- pearance of the insects; and it may happen that members of a thirteen-year brood and of a seventeen-year brood will emerge in the same year at the same place. The emergence years of the principal cicada broods have now been recorded for a long time, and the oldest record of a swarm is that of the appearance of the "locusts" in New England two hundred and ninety-five years ago. A full account of the broods of both races of the periodical cicada, their distribution, arAd the dates of their emergence, is given in Dr. C. L. Marlatt's Bulletin, already cited, and the following abstract is taken from this source: Wherever a well-defined cicada brood appears in a certain year, it is generally observed that a few individuals
INSECTS
come out the year before or the year after. This fact has suggested the idea that the various broods established at the present rime had their origin from individuals of a primary brood that, as we might say, got their dates mixed, and came out a vear too soon or a year too late, the multiplying descendants of these individuals thus founding a new brood dated a year in advance or a year behind the emergence time of the parent stock. In this way, it is conceivable, the seventeen-year race might come to appear on each of seventeen consecutive years, and the thirteen-year race on each of thirteen consecutive years. Individuals emerging on the eighteenth or fourteenth year, according to the race, would be reckoned as a part of the first brood of its race.
The facts known concerning the emergence of the cicadas seem to confirm the above theory, for members of the seventeen-vear race appear somewhere every year within the lim'its of their range, and the emergence of members of the thirteen-year race bas been recorded for at least eleven out of the possible thirteen years. All the individuals of a brood are not, of course, descendants of a single group of ancestors, nor do they necessarily occur together in a restricted area—they are simply individuals that coincide in the year of their emergence. However, at least thirteen of the broods of the seventeen-year race are well defined groups, for the most part with definitely circumscribed territories, though overlapping in many cases. The broods of the thirteen-vear race are not so well developed.
The broods are conveniently designated by Roman numerals. According to the system of brood numbering proposed by Doctor Marlatt, and now generally adopted, the brood of the seventeen-year race that appeared last in 1927 is Brood I. This is not a large brood, but it has representatives in Pennsylvania, Maryland, District of Columbia, Virginia, West Virginia, North Carolina, Kentucky, Indiana, lllinois, and eastern Kansas. Brood II, 1928, lives in the Middle Atlantic States, with a few PERIODICAI? CICADA
scattering colonies farther west. Brood III, 1929, is mostly confined to lowa, Illinois, and ,?iissouri. The largest of the broods is X, covering almost the entire range of the seventeen-year race. This brood made its last appearance m 1919, and is due next, therefore, in 1936. The series of broods as numbered thus follows the suc- cessive years to Brood XVI I, the last brood of the seven- teen-year race, which will return next in I943. The small and uncertain broods of the seventeen-year race are Vil, NII, XV, XVI, and XVII. The cicadas that emerge in the vears, corresponding with these num- bers represent incipient broods, being probably the descendants of a few individuals that sometime became separated from the larger broods of the vears preceding or following. One of the smallest of the seventeen-vear broods is XI, but since its colonies occur in Massa?hu- setts, Connecticut, and Rhode Island, it is likely that it was more numerous in individuals in former times than at present. The brood with the oldest recorded history is XIV. This is a large brood extending over much of the range of the seventeen-vear race, with colonies in eastern Massachusetts on Cape Cod and near Plymouth, the emergence of which was observed by the early settlers probably in I634. The broods of the thirteen-vear race are numbered flore XVIII to XXX, Brood ?VIII being that which appeared last in I9I 9. But there are only two important broods of this southern race, XIX, which emerged in I92O , and XXII, which emerged in ?924 . In most of the other years the shorter-lived race is represented by only a few individuals that emerge here and there over its range; and none at all are known to appear during the years corresponding with the numbers XXV and XXVIII. Trie H.?.TCHIN¢ Or THE EGGS Five weeks have elapsed since the departure of the cicada swarms. It is nearly six weeks since egg laying [ :?17]
INSECTS
was at its height, and the eggs are now due to hatch almost any rime. When studying the cicadas of Brood X near Washington in I9X9, the writer found the first evidence of hatching on the twenty-fourth of July. Perhaps the normal rime of hatching had been delayed somewhat by heavy tains that fell almost continuously during the ten days previous, for many eggs examined during this time were found tobe dead and turning brown, though the percentage of these was small. The twenty-fifth was hot and bright all day. The trees were inspected in the afternoon. Their twigs had been bare the day before. Now, at the entrance holes of the egg nests were little heaps of shriveled skins, thousands in all, and each so light that the merest breath of air suflàced to blow it off; so, if according to this evidence thousands of nymphs had hatched and gone, the evidence of as many more must have been carried away by the winds. An examination of many egg nests themselves showed that over half con- tained nothing but empty shells. Whole series were thus deserted, and usually all or nearly all, of the eggs in any one series of nests would be either hatched or unhatched. But often the eggs of one or more nests would be un- hatched or mostly so in a series containing otherwise only empty shells. Delay appeared to go by nests rather than by individual eggs. As a very general fuie the eggs nearest the door of an egg chamber are the ones that hatch first, the others following in succession, though not in absolute order. But unhatched eggs, if present, are always found at the bottom of the nest, with the tlsual exception of one or two farther forward. Only occasionally an empty shell occurs in the middle of an unhatched row. lfthe actual hatching of the eggs is observed in an opened nest, several nymphs are usually seen coming out at the same time, and in nearly all cases they are in neighboring eggs, though not always contiguous ones. So this rule of hatching, like most rules, is general but not binding.
la*8]
THE PER1ODICAI. CICADA
The procedure of the female in placing the eggs leaves no doubt that the first-laid ones are those at the bottom of the cell, showing that the order of laying has no relation to the order of hatching, except that it is mostly the reverse. I t seems hardly reasonable to suppose that the eggs nearest the door are affected by greater heat or by a fresher sup- ply of air, so it is suggested that the order of hatching may be due simply to the successive release of pressure along the tightly packed rows, giving the compressed embryos a chance to squirm and kick enough to split the inclosing shells. \¥hen hatching once commences it pro- ceeds very rapidly through the whole nest, showing that the eggs are all at the bursting point when the rupture of the first takes place. In each lateral compartment of an egg nest the eggs (Plate 8, E, F) stand in two rows with their lower or head ends slanted toward the door. (It must be re- membered that the punctures are made on the lower sides of the twigs, so that the eggs are inverted in their natural position in the nests.) On hatching, each egg splits ver- tically over the head and about one-third of the length along the back, but for only a short distance on the ventral side. As soon as this rupture occurs, the head of the young cicada bulges out; and then, by a bending of the body back and forth, the creature slowly works its way out of the shell, which, when empty, remains behind in its original place. The nymphs nearest the door have an easy exit, but those from the depths of the cell find themselves still in a confined space between the project- ing ends of the empty shells ahead of them and the chamber wall, a passage almost as narrow as the egg itself, through which the delicate creatures must squirm to freedom. A newlv-hatched or a newly-born aphid, as we have seen in C?apter VI, is done up in a tight-fitting garment with neither sleeves nor legs, but nature has been more considerate in the case of the young cicada, lt, too, comes out of the egg clothed in a skin-tight jacket, but
[?I91
1NSECTS
g. Free nymph
Fro. I @
The egg, the newly-hatched nymph shedding the embryonic skin, and the free nymph of the periodical cicada
2.20]
THE this garment is not a mere bag: it is provided with special pouches for the appendages or a part of them (Fig. ? 25, 2). The incased antennae and the labrum project backward as three small points lying against the breast. The front legs are free to the bases of the femora, though so tightly held in their narrow sleeves that their joints have no independent motion. The middle and hind legs are also incased in long, slim sheaths, but they always adhere close to the sides of the body. Thus the cicada nymph newly-hatched much resembles a tiny fish provided only with two sets of ventral fins, but when it gets into action its motions are comparable with the clumsy flopping of a seal stranded on the beach and trying to get back into the water (3).
The infant cicada knows it is not destined to spend its life in the narrow cavern of its birth, or at least it has no desire to do so. With its head pointed toward the exit, it begins at once contortionistic bendings of the body, which slowly drive it forward. By throwing the head and thorax back, the antennal tips and the front legs are made to project so that their points may take hold on any irregularity in the path. Then a contractile wave running forward through the abdomen brings up the rear parts of the body as the front parts are again bent back, and the "flippers" grasp a new point of support. As these motions are repeated over and over again, the tiny, awkward thing painfully but surely moves forward, perhaps helped in its progress by the inclined tips of the flexible eggshells pressing against it, on the same principle that a head of barley automatically crawls up the inside of your sleeve.
Once out of the door no time is lost in discarding the encumbering garment, but it is never shed in the nest under normal conditions. If, however, the nest is cut open and the hatching nymph finds itself in a free, open space, the embryonic sheath is cast off immediately, often while the posterior end of the insect's body is still in the
egg, so that the skin may be left sticking in the open end of the shell. If the young cicada did not have to gain its liberty through that narrow corridor, it might be born in a smooth bag as are its relations, the aphids. Watching at the door of an undisturbed nest during a hatching day, we soon may see a tiny pointed head come poking out of the narrow hole. The threshold is soon crossed, but no more; this traveling in a bag is nota pleasure trip. A few contortions are always necessary to rupture the skin, and sometimes several minutes are consumed in violent twistings and bendings before it splits. When it does break, a vertical rent is formed over the top of the head, which latter bulges out until the cleft becomes a circle that enlarges as the entire head pushes through, followed rapidly by the body (Fig. 125, 47. The appendages come out of their sheaths like tingers out of a glove, turning the pouches outside in. The antennae are free first; they pop out and hang sti?y downward. Then the front legs are released and hang stiff and rigid but quivering with a violent trembling. In a second or so this has passed, the joints double up and assume the characteristic attitude, while they violently claw the air. Then the other legs and the abd?m?en come out and the embryo is a free young cicada (7). All this usually happens in less than a minute, and the new creature i's already off without so much as a backward glance at the cl?thes it has just removed or at the home of its in- cubation period. Sentiment has no place in the insect mind. As the nymphs emerge from the nest, one after an- other, and shed their skins, the glistening white mem- branes accumulate in a loose pile before the entrance, where they remain until wafted off on the breeze. Each discarded sheath has a goblet form (Vig. '25, 5, ?6), the upper stiff part remaining open like a bowl, the Iower part shriveling to a twisted stalk. The antennal and labral pouches project from the skin as distinct append-
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THE PERIODICAL CICADA
ages, but those of the legs are usually inverted during the shedding and disappear from the outside of the slough, though the holes where they were pulled in can be round before the membrane becomes too dry. The nymph (Figs. 125, 7; I26) usually runs about at first in the groove of the twig containing its egg nest and then goes out on the smooth bark. Here any current of air is likely to carry it off immediately, but many wander about for some time, usually going toward the tips of the twigs, some even getting clear out on the leaves. But only a few nymphs are ever to be round on twigs where piles of embryonic skins show that hundreds have recently hatched; so it is evident that the great majority either fall off or are blown away very shortly after emerg- ing. Many undoubtedly fall before the shedding of the egg membrane, for the inclosed creature has no possible way of holding on, and even the free nymph has but feeble clinging powers. Those observed on twigs kept indoors often fell helplessly from the smooth bark while appar- ently making rem efforts to retain their grasp. Their weak claws could get no grip on the hard surface. In- stead, then, of deliberately launching themselves into space in response to some mysterious call from below, the young cicadas simply fall from their birthplace by mere inability to hold on. But the same end is gained-- they reach the ground, which is all that matters. Nature is ever careless of the means, so long as the object is at- tained. Some acts of unreasoning creatures are assured by bestowing an instinct, others are forced by with- holding the means of acting otherwise. The cicada nymphs are at first attracted by the light. Those allowed to hatch on a table in a room will leave the twigs and head straight for the windows ten feet away. This instinct under natural conditions serres to entice the young insects toward the outer parts of the tree, where they have the best chance of a clear drop to earth; but even so, adverse breezes, irregularity of the
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INSECTS
trees, underbrush, and weeds can hOt but make their downward journey one of many a bump and slide from leaf to leaf before the earth receives them. The creatures are too small tobe followed with the eye as they drop, and so their actual course and their be- havior when the ground is reached are hOt recorded. But several hatched indoors were placed on loose earth packed
Fro. t ?.6. The young cicada nymph ready to enter the ground (greatly enlarged) fiat in a small dish. These at once proceeded to get be- low the surface. "l'h«v did not dig in, but simply entered the first crevice that they met in running about. If the first happened to terminate abruptly, the nymph came out again and tried another. In a few minutes all had round satisfactory retreats and remained below. The eagerness with wl?ich the insects dived into any opening that presents itself indicates that the call to enter the earth is instinctive and imperative ouce their feet have touched the ground. Note, then, how within a few minutes their instincts shift to opposites: on hatching, their first eff,?rt is to extricate themselves front the narrow confines of the egg nest, and it seems unlikely that enough light can penetrate the depths of this chamber to guide them to the exit; but once out and divested of their en- cumbering embrvonic clothes, the insects are irresistibly drawu in the diréction of the strongest light, even though this takes them upward--.?ust the opposite of their [ 2-241
THE PERIODICAI. CICADA
destined course. When this instinct has served its purpose and has taken the creatures to the port of freest passage to the earth, all their love of light is lost or swallowed up in the call to enter some dark creiice narrower eien than the one so recently left by such physical exertion. When the young cicadas have entered the earth we practically have to say good-bye to them until their return. Yet this recurring event is ever full of interest to us, for, much as the cicadas have been studied, it seems that there is still plenty tobe learned from them each time they make their visit to our part of the world.
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