Insects, Their Ways and Means of Living/Chapter X
CHAPTER X
MOSQUITOES AND FLIES
Thoughtful persons are much given to pondering on what is to be the outcome of our present age of intensive mechanical development. Thinking, the writer holds, is all right as a means of diverting the mind from other things, but those who make a practice or a profession of it should follow the example of that famous thinker of Rodin's, who has consistently preserved a most commendable silence as to the nature of his thoughts. We can all admire thinking in the abstract; it is the expression of thoughts that disturbs us. So it is that we are troubled when the philosophers warn us that the development of mechanical proficiency is not synonymous with advancement of true civilization. However, it is hot for an entomologist to enter into a discussion of such matters, because an observer untrained in the study of human affairs is as likely as not to get the impression that only a very small percentage of the present human population of the world is devoted to efficiency in things mechanical or otherwise.
There is no better piece of advice for general observance than that which admonishes the cobbler to stick to his last, and the maxim certainly implies that the entomologist should confine himself to his insects. However, we can not help but remark how often parallelisms are to be discovered between things in the insect world and affairs in the human world. So, now, when we look to the insects for evidence of the effect of mechanical perfection, we observe with somewhat of a shock that those very insect species which unquestionably have gone farthest along the road of mechanical efficiency have produced little else commendable. In this class we would place the mosquitoes and the flies; and who will say that either mosquitoes or flies have added anything to the comfort or enjoyment of the other creatures of the world?
Reviewing briefly the esthetic contributions of the major groups of insects, we find that the grasshoppers have produced a tribe of musicians; the sucking bugs have evolved the cicada; the beetles have given us the scarab, the glow-worm, and the firefly; the moths and butterflies have enriched the world with elegance and beauty; to the order of the wasps we are indebted for the honeybee. But, as for the flies, they have generated only a great multitude of files, amongst which are included some of our most obnoxious insect pests.
However, in nature study we do hot criticize; we derive our satisfaction from merely knowing things as they are. If out subject is mosquitoes and flies, we look for that which is of interest in the lives and structure of these insects.
FLIES IN GENERAL
The mosquitoes and the flies belong to the same entomological order. That which distinguishes them principally as an order of insects is the possession of only one pair of wings (Fig. ?67). Entomologists, for this reason, call the mosquitoes and files and all related insects the Diptera, a word that signifies by its Greek components "two wings." Since nearly all other winged insects have four wings, it is most probable that the ancestors of the winged insects, including the Diptera, had likewise two pairs of wings. The Diptera, therefore, are insects that have become specialized primarily during their evolution by the loss of one pair of wings.
We shall now proceed to show that the evolution of a two-winged condition from one of four wings bas been a
INSECTS progress toward greater efficiency in the mechanism of flight, and that the acme in this line has been attained by the files and mosquitoes. The truth of this contention will become apparent when we compare the relative development of the wings and the manner or effectiveness of flight in the several principal orders of insects.
Fig. 167. A robber fly, showing the typical structure of any member of the order Diptera
The flies are two-winged insects, the hind wings being reduced to a pair of knobbed stalks, the halteres (Hl)
It is most probable that when insects first acquired wings the two pairs were alike in both size and form. The termites (Fig. 168 A) afford a good example of insects in which the two pairs of wings are still almost identical. Though the termites are poor flyers, their weakness of flight is hot necessarily to be attributed to the form of the wings, because their wing muscles are partially degenerate. The dragonflies (Fig. 58) are particularly strong flyers, and with them the two pairs of wings are but little different in size and form; but the dragonflies AND FI.IES
are provided with sets of highly developed wing muscles which are much more effective than those of other insects. From these examples, therefore, we can hot well judge of the mechanical eflîciency of two pairs of equal wings moved by the equipment of muscles possessed by most
?" E1 C ??
Fro. 168. Evolution of the wings of insects A, wings of a termite, approximately the same in size and shape. B, wings of a katydid, the hind wings are the principal organs of flight. C, wings of a beetle, the fore wings changed to protective sheIIs, elytra (El), covering the hind wings. D, wings of a hawk moth, united by the spine (f), which is held in a hook on under surface of fore wing. E, wings of the honeybee, held together by hooks (h) on edge of hind wing. F, wing of a blowfly, and the rudimentary hind wing, or halter (HI)
[317]
INSECTS insects; but it is evident that the majority of insects have found it more advantageous to have the fore and hind wings different in one way or another.
In the grasshoppers, it was observed (Fig. 63), the hind wings are expanded into broad membranous fans, while the fore wings are slenderer and of a leathery texture. The same is true of the roaches (Fig. 53), the katydids (Fig. ?168 B), and the crickets, except in special cases where the fore wings are enlarged in the male to form musical organs (Fig. 39). In all these insects the hind wings are the principal organs of flight. When not in use they are folded over the body beneath the fore wings, which latter serve then as protective coverings for the more delicate hind wings. In the beetles ?(Figs. ?137, 168?C) the hind wings are much larger than the fore wings, and, as with the grasshoppers and their kind, they take the chief part in the function of flight. The beetles, however, have carried the idea of converting the fore wings into protective shields for the hind wings a little farther than have the grasshoppers; with them the fore wings are usually hard, shell-like flaps that fit together in a straight line over the back (Fig. 137 A), forming a case that completely conceals, ordinarily, the membranous hind wings folded beneath them. Neither the grasshoppers nor the beetles are swift or particularly efficient, but they appear to demonstrate that the ordinary insect mechanism of flight is more effective with one pair of wings than with two.
The butterflies and the moths use both pairs, of wings in flight; but with these insects, it is to be noted, the front wings are always the larger (Fig. 168 D). The buttertties, with four broad wings, fly well in their way and are capable of long-sustained flight, though they are comparatively slow goers. Some of the moths do much better in the matter of speed, but it is round that the faster flying species have the fore wings highly devcloped at the expense of the hind wings; and that the two wings on each side, furthermore, are voked together in such a manner as to insure their acting as a single wing (D). The moths clearly show, therefore, as do the grasshoppers and the beetles, the effciency of a single pair of flight organs as opposed to two. The moths, however, have attacked from a different angle the problem of converting their inherited equipment of four wings into a two-wing mechanism—instead of suppressing the flight function in one pair of wings, they have given a mechanical unity to the two wings of each side, thus attaining functionally a two-winged condition.
The wasps (Fig. 133) and bees, likewise, have evolved a two-winged machine from a four-wing mechanism on the principle of uniting the two wings on each side. The bees have adopted a particularly efficient method of securing the wings to each other, for each hind wing is fastened to the wing in front of it by a series of small hooklets on its anterior vein that grasp a marginal thickening on the rear edge of the front wing (Fig. 168 E). Moreover, the bees have so highly perfected the unity in the design of the wings that only on close inspection of it to be seen that there are actually two wings on each side of the body.
Finally, the flies, including all members of the order Diptera, have boldly executed the toaster stroke by completely eliminating the second pair of wings from the mechanism of flight. The files are literally two-winged insects (Figs. 167, 168 F). Remnants of the hind wings, it is true, persist in the form of a pair of small stalks, each with a swelling at the end, projecting from behind the bases of the wings (Figs. 167, 168 F, Hl). These stalks are known as "balancers", or halteres, and in their structure they preserve certain features that show them to be rudiments of wings.
The giving over of the function of flight to the front pair of wings has necessarily involved a reconstruction in the entire framework and musculature of the thorax, and a study of the fly thorax gives a most interesting and instructive lesson in the possibilities of adaptive evolution, showing how a primitive ancestral mechanism may be entirely remodeled to serve in a new capacity. If the flies had been specially “created” and not evolved, their structure could have been much more directly fitted to their needs.
It is not only in the matter of wings and the method of flight that the flies show they are highly evolved insects;
Fig. 169. The black horsefly, Tabanus atratus.
A, the entire fly. B, facial view of the head and mouth parts. Ant, antenna;
E, E, compound eyes; Lb, labium; Lbr, labrum; Md, mandible; Mx, maxilla;
MxPlp, maxillary palpus.
they are equally specialized in the structure of their mouth parts and in their manner of feeding. The flies subsist on liquid food. Those species that can satisfy their wants from liquids freely accessible have the mouth parts formed for sucking only. Unfortunately, however, as we all too well know, there are many species that demand, and usually obtain, the fresh blood of mammals, including that of man, and such species have most efficient organs for piercing the skin of their victims.
The most familiar examples of flies that “bite” are the mosquitoes and horseflies. The horseflies (Fig. 169 A), some of which are called also gadflies and deer flies, belong to the family Tabanidae. An examination of the head of the common large black horsefly (Fig. 169 B) will show the nature of the feeding organs with which these flies are equipped. Projecting downward from the lower part of the head are a numbeof appendages; these are the mouth parts. They correspond in number and in relative position with the mouth appendages of the grasshopper (Fig. 66), but they differ from the latter very much in form because they are adapted to quite a different manner of feeding. The horsefly does hot truly bite; it pierces the skin of its victim and sucks up the exuding blood.
By spreading apart the various pieces that compose the group of mouth parts of the horsefly, it will be seen that there are nine of them in all. Three are median in position, and therefore single, but the remaining six occur in duplicate on the two sides, forming thus three sets of paired structures. The large club-shaped pieces, however, that lie at the sides of the others, are attached at their bases to the second paired organs and constitute a part of the latter, so that there are really only two sets of paired organs. The anteriormost single piece is the labrum (Fig. 169 B, Lm); the first paired organs are the mandibles (Md); the second are the maxillae (Mx); the second median piece is the hypopharynx (not seen in Fig. 169 B); and the large, unpaired, hindmost organ is the labium (Lb). The lateral club-shaped pieces are the palpi of the maxillae (MxPlp).
The labrum is a strong, broad appendage projecting downward from the lower edge of the face (Figs. 169 B, 170 A, Lm). Its extremity is tapering, but the tip is blunt; its under surface is traversed by a median groove extending from the tip to the base but closed normally by the hypopharynx (Fig. 170 D, Hphy), which fits against the under side of the labrum and converts the groove into a tube. The upper end of this tube leads directly into the mouth, a small aperture situated between the base of the labrum and the base of the hypopharynx and opening into a large, stiff-walled, bulblike structure (Fig. 170 A, Pmp) which is the mouth cavity. The anterior wall of the bulb is ordinarily collapsed, but it can be lifted by a set of strong muscles (Mcl) arising on the front wall of the head (Clp). This bulb is the sucking pump of the fly, and it will be
Fig. 170. Mouth parts of a horsefly, Tabanus atratus
A, the labrum (Lm) and mouth pump (Pmp), with dilator muscles of the pump (Mcl) arising on the clypeal plate (Clp) of the head wall. The mouth is behind the base of the labrum
B, the left mandible
C, the left maxilla, consisting of a long piercing blade (Lc), and a large palpus (Plp)
D, the labium (Lb) terminating in the large labella (La), and the hypopharynx (Hphy) showing the salivary duct (SlD) and its syringe (Syr), discharging into a channel of the hypopharynx (Hphy) that opens at the tip of the latter
seen that it is very similar to that of the cicada (Fig. 122, Pmp). In the fly, however, the liquid food is drawn up to the mouth through the labro-hypopharyngeal tube instead of through a channel between the appressed maxillae.
The mandibles of the horsefly (Fig. 170 B, Md) are long, bladelike appendages, very sharp pointed, thickened on the outer edges and thin on the knifelike inner edges. They appear to be cutting organs, for each is articulated to the lower rim of the head by its expanded base in such a manner that it can swing sidewise a little but can not be protruded and retracted as can the corresponding organ of the cicada. The maxillae (C) are slender stylets, each supported on a basal plate attached to the head; this plate carries also the large, two-segmented palpus (Plp). The maxillae are probably the principal piercing tools of the horsefly's mouth-part equipment.
The median hypopharynx (Fig. 170 D, Hphy) is a tapering blade somewhat hollowed above, normally appressed, as just observed, against the under surface of the labrum to form the floor of the food canal. The hypopharynx itself is traversed by a narrow tube which is a continuation from the salivary duct (SlD). The latter, however, just before it enters the base of the hypopharynx, is enlarged to form an injection syringe (Syr). The salivary syringe in structure is a small replica of the mouth pump (A, Pmp), and its muscles arise on the back of the latter. The saliva of the fly is injected into the wound from the tip of the hypopharynx. By reason of this fact, the bite of a fly may be the source of infection to the victim, for it is evident that the injection of saliva affords a means for the transfer of internal disease parasites from one animal to another.
Behind all the parts thus far described is the median
labium (Fig. 170 D, Lb), a much larger organ than any of the others, consisting of a thick basal stalk and two great terminal lobes (La). The sort, membranous under surfaces of the lobes, which are known as the labella, are marked by the dark lines of many parallel, thick-walled
grooves extending crosswise. These grooves may be channels for collecting the blood that exudes from the wound, or they may also distribute the saliva as it issues from the tip of the hypopharynx between the ends of the labella. The effect of the saliva of the horsefly on the
blood is not known, but the saliva of some files is said to prevent coagulation of the blood. Some of the smaller horseflies will give us an unsolicited sample of their biting powers, and in shaded places along roads they often make themselves most vexatious to the foot traveler just when he would like to sit down and enjoy a quiet test. To horses, cattle, and wild mammals, how- ever, these files are extremely annoying pests, and, where abundant, they must make the lives of animais almost unendurable; for the sole means of protection the latter have against the painful bites of the flies is a swish of the tail, which only drives the insects to make a fresh attack on some other spot. There is another family of "biting" flies, known as the robber flies, or Asilidae (]Tig. 167) , the members of which attack other insects. They are strong flyers and take their victims on the wing, even bees falling prey to them. The robber flies have no mandibles, and the strong, sharp- pointed hypopharynx appears to be the chief piercing implement. The saliva of the tir injected into the wound dissolves the muscles of the victim, and the predigested solution is then completely sucked out. As was shown in Çhapter VIII, on metamorphosis, ? whenever the adult form of an insect is highly specialized for a particular kind of lire, it is usually round that the young is also specialized but in a way of its own to adapt it to a manner of living quite different from that of its parent. This principle is particularly true of the flies, for, if the adult flies are to be regarded as in general the most highly evolved in structure of all the adult insects, there can be no doubt that the young fly is the most highly specialized of all the insect larvae. The files belong to that large group of insects which do hot have external wings in the larval stage, but with the flies the suppression of the body appendages includes also the legs, so that their larvae are hot only wingless but legless as well (Fig. ?7I). The legs, however, as the wings,
, IOS. QL ITOES AND FLIES
are represented by internal buds, which, when they enter the period of growth during the early stage of metamor- phosis, are turned inside out to form the legs of the adult tir. The lack of legs gives a cylindrical simplicity of form to most fly larvae, which hot only makes these insects look lik, e worms, but bas caused many of them to lire the lire of
/ ml? LTpa An
Fro. 171. Structure of a fly larva, or maggot anus; .ISp, anterior spiracle; DTra, dorsal tracheal trunk; LTra, lateral tracheal trunks; mb, mouth hooks; PSp, posterior spiracle
a worm and to adopt the wavs of a worm. In compensa- tion for the loss of legs, the fli" larvae are provided with an intricate system of muscle fibers lying against the inner surface of the body wall, which enables them to stretch and contract and to make all manner of contortionistic twists. At first thought it seems remarkable that a soh-bodied, wormlike creature can stretch itsel( by muscular contrac- tion. It must be remembered, however, that the body of the larva is filled with soif tissues, many of which are but loosely anchored, and that the spaces between the organs are fil'led with a body liquid. The creature is, therefore, capable of pe.rforming movements by making use of its structure as a hvdraulic mechanism; a contraction of the rear part o( the body, for example, drives the body liquid and the soit movable organs forward, and thus extends the anterior parts of the bodv. A contraction of the length- wise muscles then pulls up the rear parts, when the more-
[325]
INSECTS
ment of extension may be repeated. In this fashion the soft, legless larva moves forward; or, by a reversal of the process when occasion demands, it goes backward. A special feature in the construction of fly larvae is the arrangement of their breathing apertures, which is cor- re!ated with the malmer of breathing. In most insects, as we have learned (Fig. 70), there is a row of breath]ng pores, or spiracles, along each side of the body, which open into .3 -- , Fro. ?7 ?E, Rat-tailed maggots, larvae of the drone ?y» whkh lire submerged in water or mud and breathe ?t the surface through a long» tail-like respiratory tube Upper figure, resting beneath a small ?oating obiect; lower, feeding in mud at the ?ottom lateral tracheal trunks. In the tir larva, however, these spiracles are closed and are hot opened for respiration until the final change of the pupa to the adult. The tir larva is provided with one or two pairs of special breathing organs situated at the ends of the bodv. Some species have a pair of these organs at each end of t?e body (Fig. 171 , .J.ç?, P«ç?), and some a pair at the pos- terior end only. The anterior organs, when present (Fig. 171 , «J.ç?), consist of perforated lobes on the first body segment, the pores of which communicate with the an- terior ends of a pair of large dorsal tracheal trunks (DTra). The posterior organs (PSp) consist of a pair of spiracles on the rear end of the body, which open into the posterior ends of the dorsal tracheae. By means of this respiratory arrangement, the tir larva can lire submerged in water, [3261
MOSQUITOES AND FLIES
or buried in mud or any other sort medium, so long as it keeps one end of the body out for breathing. The rat-tailed maggot (Fig. I72), which is the larva of a large fly that looks like a drone bee, has taken a special advantage of its respiratory system; for the rear end of its body, bearing the posterior spiracles, is drawn out into a long, slender tube. The creature, which lives in foui water or in mud, can by this contrivance hide itself beneath a floating object and breathe through its tail, the tip of which may come to the surface of the water at a point some distance away. The end of the tail is provided with a circlet of radiating hairs surrounding the spiracles, which keeps the tip of the tail afloat and prevents the water from entering the breathing apertures. The great disparity of structure between the larva of a tir and the adult necessarily involves much reconstruc- ti'on during the period of transformation, and probably the inner processes of metamorphosis are more intensive in the more highly specialized Diptera than in any other group of insects. The pupa of an insect, as we have seen in Chapter VIII (page 254), is very evidently a preliminary stage of the adult, the larval characters being usually discarded with the last molt of the larva. The pupa ofmost files, however, while it has the general structure of the adult fly (Fig. ?82 A, F), retains the special respiratory scheme of the larva and at least a part of the larval breathing organs. The fact that the larvae breathe through special spiracles located on the back suggests that the primitive fly larvae lived in water or in sort mud, and that it was through an adaptation to such an environment that the lateral spiracles were closed and the special dorsal spiracles de- veloped. The retention by many fly pupae of the larval method of breathing and of at least a part of the larval respiratory organs, though their habitat would hot seem necessarily to demand it, suggests, furthermore, that the
[3OE7 ]
INSECTS
pupae of the ancestors of such species lived in the same medium as the larvae.. Ifour supposition is correct, we may see a reason tor the apparent exception in the files to the general rule that the pupa presents the adult structure and discards the pecu- liarly larval characters. The pupae of some flies whose
Fro. ?73- Larva (A) and pupa (B) ofa horsefly, Tabanus puncti- fer (about 1 ? rimes natural size) ?/n, anus; H, head; PSp, posterior spiracle; Sp, spiracle larvae lire in the water, however, revert at once to the adult system of lateral spiracles (Fig. ?73 B, Sp). With such species, the larva comes out of the water just before pupauon time and transforms in some place where breathing is possible by the ordinary respiratory organs. This is the general fuie with other insects whose larvae are aquatic. The order of the Diptera is a large one, and we might go on indefinitely describing interesting things about flies m general. Such a course, however, would soon fill a larger book than this; hence, since we are already in the last chapter, a more practical plan will be to select for special consideration a few species that have become closely as- sociated with the welfare of man or of his domesticated animais. Such species include the mosquitoes, the bouse fly, the blowfly, the stable fly, the tsetse fly, the flesh files, and related forms.
[ 3?-8 ]
MOS( 2UITOES AND FLIES
,?|OSQUITOES The mosquitoes, perhaps more than any other noxious insect, impel us to ask the impertinent question, why pests were made to annoy us. It would be well enough to answer that they were g:lven as a test of the efficiency of our science in learning how to control them, if it were not for the other creatures, the wild animais, whose existence must be at times a continual torment from the bites of insects and from the diseases transmitted by them. Such creatures must endure their tortures without hope of relief, and there is ample evidence of the sufl'ering that insects cause them. In earlier and more primitive days the rainwater barrel and the town watering trough took the place of the course in nature study in our present-day schools. While the lessons of the water barrel and the trough were perhaps hot exact or thoroughly scientific, we at least got our learning from them at first hand. We all knew then what "wigglers" and "horsehair snakes" were; and we knew that the former turned into mosquitoes as surely as we believed that the latter came from horsehairs. ?\lodern nature study has set us upon the road to more exact science, but the aquarium can never hold the mysteries of the old horse trough or the marvels of the raJnwater barrel. The supposed ancestry of the horsehair shake is now an exploded myth, but the advance of science has unfortu- nately hot altered the fact that wigglers turn into mos- quitoes, except in so far as the spread of applied sanita- tion has brought it about that fewer of them than for- merly succeed in doing so. And now, as we leave the homely objects of our first acquaintance with "wigglers" for the more convenient apparatus of the laboratory, we will call the creatures mosqu#o larvae, since that is what they are. OEhe rainwater barrel never told us how those wiggling
[3?9 ]
INSECTS
mosquito larvae got into it--that was the charm of the barrel; we could believe that we stood face to face with the great mystery of the origin of lire. Now, of course, we understand that it is a very simple matter for a female
FIO. I74. Lire stages of a mosquito, Culex quinquefasciatus A, the adult female. B, head of an adult male. C, a floating egg fart, with four eggs shown separately and more enlarged. D, a young larva suspended at the surface of the water. E, full-grown larva. F, the pupa resting against the surface film of the water
[ 33 ° 1
MOSQUITOES AND FLIES
mosquito to lay her eggs upon the surface of the water, and that the larvae come from the eggs. There are many species of mosquitoes, but, from the .standpoint of human interest, most of them are included in three groups. First there are the "ordinary" mos- quitoes, species of the genus Culex or of related genera; second, the yellow-fever mosquito, dëdes aegypti; and third, the malaria-carrying mosquitoes, which belong to the genus dnopheles. The common Culex mosquitoes (Fig. I74 A) lay .their eggs in small, fiat masses (C) that float on the surface of the water. Each egg stands on end and is stuck close to its neighbors in such a manner that the entire egg mass has the form of a miniature raft. Sometimes the eggs toward the margin of the raft stand a little higher, giving the mass a hollowed surface that perhaps decreases the chance of accidental submergence, though the raft is buoved up from below by a film of air beneath the eggs. A]most any body of quiet water is acceptable to the Culex mosquito as a receptacle for her eggs, whether it be a natural pond, a pool of rainwater, or ?vater standing in a barrel, a bucket, or a neglected tin can. Each egg raft contains two or three hundred eggs and sometimes more, but the largest raft seldom exceeds a fourth of an inch in longest diameter. The eggs hatch in a very short time, usually in less than twenty-four hours, though the in- cubation period may be prolonged in cool weather. The young mosquito larvae come out of the lower ends of the eggs, and at once begin an active lire in the water. The body of the young mosquito larva is slender and the head proportionately large (Fig. 174 D). As the creature becomes older, however, the thoracic region of the body swells out until it becomes as large as the head, or finally a little larger (E). The head bears a pair of lateral eyes iFig. 175, b), a pair of short antennae (dnt), and, on the ventral surface in front of the mouth, a pair of large brushes of hairs curved inward (a). From the sides of
[331 ]
INSECTS
the body segments project laterally groups of long hairs, some of which are branched in certain species. The rear end of the body appears to be forked, being divided into an upper and a ]ower branch. The
Fm. ?7?- Structure of a Culex mosquito larva a, mouth brushes; abdomen; Ant, antenna; b» eye; c, respiratory tube; d, terminal lobes; H, head; " P8p, posterior spiracle; TA, thorax; Tra, dorsa tracheal trunks
upper branch (c), however, is really a long tube projecting dor- sally and backward from the next to ?he last segment. The lower branch is the true terminal seg- ment of the body and bears the anal opening of the alimentary canal at its extremitv. On the end of this segment ?ur long, trans- parent flaps project laterally (d), t?so groups of long hairs are situ- ated dorsally, and a fan of hairs ventrally (.Fig. 174 E. The principal characteristic of the mosquito larva is the speciali- zation of its respiratory system. The larva breathes through a single large aperture situated on the end of the dorsal tube that projects from the next to the last segment of the body (Fig. 175, PSp). This orifice opens by two mner spiracles into two wide tracheal trunks (Tra) that extend forward in the body and give off branches to all the internal organs.
The mosquito larva, therefore, can breathe only when the tip of its respiratory tube projects above the surface of the water, and, though an aquatic creature, it can be drowned by long submergence. Yet the provision for breathing at the surface has a distinct advantage: it renders the mosquito larva independent of the aeration of the water it inhabits, and allows a large number of larvae to thrive
[33 OE ]
MOSQUITOES in a small quantity of water, provided the latter contains sufficient food material.
The tip of the respiratory tube is furnished with five small lobes arranged like the points of a star about the central breathing hole. When the larva is below the surface, the points close over the aperture and prevent the ingress of water into the tracheae; but as soon as the tip of the tube comes above the surface, its points spread apart. Not only is the breathing aperture thus exposed, but the larva is enabled to remain indefinitely suspended from the surface film (Figs. 174 D, 181 B). In this position, with its head hanging downward, it feeds from a current of water swept toward its mouth by the vibration of the mouth brushes. Particles suspended in the water are caught on the brushes and then taken into the mouth. Any kind of organic matter among these particles constitutes the food of the larva. Larvae of Culex mosquitoes, however, feed also at the bottom of the water, where food material may be more abundant.
The body of the mosquito larva has apparently about the same density as water; when inactive below the surface, some larvae slowly sink, and others rise. But the mosquito larva is an energetic swimmer and can project itself in any direction through the water by snapping the rear half of its body from side to side, which characteristic performance has given it the popular name of "wiggler." The larva can also propel itself through the water with considerable speed without any motion of the body. This movement is produced by the action of the mouth brushes. Likewise, while hanging at the top of the water, the larva can in the same manner swing itself about on its point of suspension, or glide rapidly across the surface.
The larvae of Culex mosquitoes reach maturity in about a week after hatching, during the middle of summer; but the larval period is prolonged during the cooler seasons of spring and fall. The larva passes through three stages, and then becomes a pupa.
The mosquito pupa (Fig. 174 F) also lives in the water, but is quite a different looking creature from the larva. The thorax, the head, the head appendages, the legs, and the wings are all compressed into a large oval mass from
Fig. 176. Mouth parts of a female mosquito, Joblotia digitata A, the head with the proboscis (Prb) in natural position. B, the mouth parts separated, showing the component pieces of the proboscis
Ant, antenna; E, compound eye; Hphy, hypopharynx; Lb, labium; Lm, labrurn; Md, mandibles; Mx, maxillae; MxPlp, Plp, maxillary palpi; Prb, proboscis
which the slender abdomen hangs downward. The pupa, owing to air sacs in the thorax, is lighter than water and, when quiet, it rises to the surface where it floats with the back of the thorax against the surface film. The pupa has lost the respiratory tube and the posterior spiracles of the larva, but has acquired two large, trumpetlike breathing tubes of its own that arise from the anterior part of the AND FLIES
thorax, the rnouths of which open above the water when the pupa comes in contact with the surface. The pupa, of course, does hot feed, but it is almost as active as the larva, for it rnust avoid its enemies. When disturbed it rapidly swirns downward by quick rnovernents of the abdomen, the extrernity of which is provided with two large swirn- rning flaps. The duration of the pupal stage in midsurnrner is about two days. The adult rnosquito issues flore the pupal skin through a split in the back of the latter. We now see why the pupa
is made lighter than water--it must float at the surface in order to allow the adult to escape into the air. The full-fledged mosquito (Fig. ?74 A) bas the general fea- tures of any other two-winged fly, but it ?s distinguished from nearly all other flies by the presence of scales on its wings and on parts of its head, body, and ap- pendages. The rnouth parts of the adult
/ \ / \_ / \
F?c. 177. ?lëdes atropalpus, male, a mosquito re- lated to the yellow lever mosquito and similar to it in appearance m.osq.uito are of the plerclng and sucking type, and are sirnilar in structure to those of the horsefly, except that the individual pieces are longer and slenderer, and together constitute a beak, or proboscis, extending forward and downward from the head (Fig. I76 A, Prb). The rnale and the fernale rnos- quitoes are readily distinguishable by the character of the antennae, these organs in the rnale being large and feathery (Fig. ?74 B), while those of the female are
[335 ]
" INSECTS
threadlike and provided with comparatively few short hairs (A). The sexes differ also in the mouth parts, for, as in the horseflies, the males lack mandibles. The mouth parts of the mosquito, in the natural posi- tion, do hot appear as separate pieces, as do those of the horseflv. The various elements, except the palpi, are com- pressed into a beak that projects forward and downward from the lower part of the head (Fig. 176 A, Prb). "l'he length of the beak varies in different kinds of mosquitoes; it is particularly long in the large South American species shown in Figure ?76. When the beak of the female mosquito is dissected (Fig. ?76 B), the same equipment of parts is revealed as is possessed by the female horsefly IFig. ?69 B), namely, a [abrum (Lin), two mandibles (.(ld), two maxil[ae (.1Ix), a hypopharynx (Hph.v), and a labium (Lb). It is the labium that forms most of the visible part of the beak, the other pieces being concealed within a deep gro«?ve in its upper surface. The la/rivera (Fig. 176 B, Lin) is a long median blade, concave below, terminating in a hard, sharp point; it is probably the principal piercing tool of the mosquito's outfit. The mamtib/es of the mosquito (:lld) are very slender, delicate bristles; those of the species figured are so weak that it would seem they can be oflittle use to the insect. The maxi//ae (Mx) are thin, fiat organs with thickened bases, each terminating in a sharp point armed on its outer edge with a row of backward-pointing, saw- like teeth which probabl.y serve to keep the mouth parts fixed in the puncture as the p?erclng lal?rum is thrust deeper into the flesh. "Fhe pa/pi I.IlxP/p) arise from the bases of the maxillae. The h)'popha??mx (lqpk),) is a slender blade with a median rib which is traversed by the channel of the salivary duct. |ts upper surface is con- cave and, in the naturai position, is closed against the concave lower side of the labrum, the two apposed pieces thus forming between them a tube which leads up to the
[ 336 ]
blOS(? UITOES AND FLIES
mouth opening. The saliva of the mosquito is injected into the wound from the tip of the hypopharynx, and the blood of the victim is sucked up to the mouth through the labro-hypopharyngeal tube. The labium (Lb} serres
F,c. 178. Mosquito larvae A, Atë'des atropalpus. 13, .¢nopheles punctipennis, the malaria mosquito larva c, respiratory tube; d, terminal Iobes; e, stellate groups of hairs that hold the larva at the surface of the water (fig. 181 A); f, spiracular area; P8p, spiracle
principally as a sheath for the other organs. It ends in two small lateral lobes, the labella, between which pro- ]ects a weak, median tonguelike process. When the mos- quito pierces its victim the base of the labium bends back- ward as the other bristlelike members of the group of mouth parts sink into the wound. blosquitoes of both sexes are said to feed on the sap of
[ 337 ]
INSECTS
plants, which they extract by puncturing the plant tissues; they will also feed on the exuding juices of fruit, or on any sort vegetable matter. The females, however, are notori- ous for their propensity for animal blood, and they by no means limit their quest for this article of food to human beings. The male mosquitoes, apparently, very rarely depart from a vegetarian diet. The pain from the bite of a female mosqmto and the subsequent irritation and swelling prob?bly result from the injection of the secre- tion from the salivary glands of the insect into the wound. It is said that the salira of the mosquito prevents coagula- tion of the blood. Because of the short time necessary for the completion of the lire cycle from egg to adult during summer, there are many generations of mosquitoes from spring to fall. The win?er is passed both in the adult and in the larval stage. Fertile females may survive cold weather in pro- tected places; and larvae round in large numbers, frozen solid in the ice of ponds, have become active on being thawed out, and capable of development when given a sufficient degree o? warmth. The vellow-fever mosquito, now known as .?lë'des aeg.vpti but at the rime of the discovery of its relation to yellow lever generally called ,çt«gom),ia.[asdata, is similar in its habits during the larval and pupal stages to the Culex mosquitoes. It lavs its eggs singly, however, and they float unattached on the surface of the water. The adult mosquito may be identified by its decorative markings. On the back of the thorax is a lyrelike design in white on a black ground; the joints of the legs are ringedwith white? the black abdomen is conspicuously cross-banded with white on the basal half of each segment. The male has |arge plumose antennae and |ong maxillary palpi. The female has a strong beak, but small palpi, and her an- tennae are of the short-haired form usual with female mosquitoes. The species of .iëdes shown in Figure 177 much resembles the vellow-fever mosquito, but it is a
[338]
MOS. QUITOES AND FLIES
more northern one common about Washington, D. C., where it breeds in rock pools along the Potomac River. The larva of Aëdes (Fig. ?78 A) resembles a Culex larva, but it feeds more habitually at the bottom of the water and may spend long periods below without coming to the
Fro. 179. Mosquito pupae in natural position resting against the under surface of the water A, .4ëdes atropalpus. B, .4nopheles punctipennis
surface for air. In its search for food it noses about m the refuse at the bottom of the water and voraciously con- sumes dead insects and small crustaceans. The pupa like- wise (Fig. I79 A) does hOt differ materially from a Culex pupa. When quiet it floats at the surface of the water with the entire back of its thorax against the surface film and the tips of its breathing tubes above the surface. Probably no mosquito pupa hangs suspended from its resp.iratory tubes in the manner in which the pupae of vanous species are often figured. ./ëdes aegypti is the only known natural carrier of the virus of yellow lever from one person to another. The disease can be taken only from the bite of a mosquito of this species that has become infected by previous feeding on the blood of a yellow-fever patient. The organism that produces yellow fever is perhaps hOt yet definitely known, though strong evidence has been adduced to show
[ 339 1
INSECTS
that it is one of the minute, non-filterable organisms called spirochetes. The virus will hot develop m the mosquitoes at a temperature below 68 ° F., and ,tëdes
aegypti will not breed /" in latitudes much be- "'\), ,,// yond the possible range of yellow lever. -,,??/ Yellow fever, there- fore, is a disease ordi- narily confined to tbe ? , tropics and warmer j/??Ù.???-?\????? parts of the te tope r- ate zones. Season- . al outbreaks of it that have occurred in // ?\ northern cities have been caused probably by local infestations // \ of infected mosqui- / / ] toes brought in on Fie.. 18o. The female malaria mosquito, ships from some .4nopheles punctipennis SOU thern port. The malaria mos-
quitoes belong to the genus .tnopheles, a genus repre- sented by species in most temperate and tropical regions of the world, which are prevalent wherever malaria oc- curs. Our most common malaria species is «qnopheles punctipennis (Fig. ?8o), characterized by a pair of dull white spots on the edges of the wings. The .tnopheles females lay their eggs singly on the surface of the water, where they float, each buoved up by an air jacket about its middle. The larvae of Anopheles (Fig. ?78 B) differ conspicu- ously from those of Culex and Aëdes both in structure and habits. Instead of a respiratory tube projecting from nea? the end of the body, as in Culex (Figs. 174 E ?75), there is a concave disc (Fig. ?78 B,f) on the back of the next to
[ 34 ° ]
MOSQUITOES the last segment, in which the posterior spiracles (PSp) are located. The larva floats in a horizontal position just below the surface film of the water (Fig. 181 A), from which it is suspended by a series of floats (Fig. 178 B, e) consisting of starlike groups of short hairs arranged in pairs along the back. The spreading tips of the hairs pro-
 | An image should appear at this position in the text. If you are able to provide it, see Wikisource:Image guidelines and Help:Adding images for guidance. |
Fig. 181. Feeding positions of Anopheles and Culex mosquito larvae
A, Anopheles larva suspended horizontally beneath the surface film, and feeding at the surface with its head inverted. B, Culex larva hanging from the respiratory tube
ject slightly above the water surface and keep the larva afloat. In the floating position, the respiratory disc breaks through the surface film, and its raised edges leave a dry area surrounding the spiracles. The long hairs that project from the sides of the thorax and the first three body segments are mostly branched and plumose.
The Anopheles larva (Fig. l81 A) feeds habitually at the top of the water. When disturbed it shoots rapidly across the surface in any direction, but goes downward reluctantly. In order to feed in its horizontal position, it turns its head completely upside down and with its mouth brushes creates a surface current toward its mouth.
The pupa of Anopheles (Fig. 179 B) is not essentially different from that of Culex or Aëdes. Its most distinctive character is in the shape of the respiratory tubes, which are very broad at the ends.
The parasitë of malaria is not a bacterium but a microscopic protozoan animal named Plasmodium. There are several species or varieties that correspond with the different varieties of the disease. The malaria Plasmodium has a complicated life cycle and is able to complete its life only when it can spend a part of it in the body of a mosquito and the other part in some vertebrate animal. In the human body the malaria parasites live in the red corpuscles of the blood. Here they multiply by asexual reproduction, producing for a while many other asexual generations. Eventually, however, certain individuals are formed that, if taken into the stomach of an Anopheles mosquito, develop there into males and females. In the stomach of the mosquito, these sexual individuals unite in pairs, and the resulting zygotes, as they are called, penetrate into the cells of the stomach wall. Here they lire for a while and multiply into a great number of small spindle-shaped creatures, which go through the stomach wall into the body cavity of the mosquito and at last collect in the salivary glands. If now the mosquito, with its salivary glands full of the Plasmodium parasites in this stage, bites some other animal, the parasites are almost sure to be injected into the wound with the saliva. If they are not at once destroved by the white blood corpuscles, they will quickly enter the red blood corpuscles, and the victim will soon show symptoms of malaria.
The House Fly and Some of Its Relations
Our familiar domestic pest, the house fly, may be taken as the type of a large group of flies, and in particular of those belonging to the family Muscidae, which is named from its best known member, Musca domestica, the house fly—musca being the Latin word for fly.
The house fly (Fig. 182 A), though particularly a domesUITOES AND FLIES
tic pest to people that live indoors, is intimately associated with the stable. Its favorite breeding place is the manure pile. Here the female fly lays her eggs (B), and here the larvae, or maggots (Ç), l?ve until they are ready for trans- formation. It is estimated that fully ninety-five per cent of our house files have been bred in horse manure. A few may come from garbage cans, or from heaps of vegetable refuse, but such sources of fly infestation are comparatively unimportant. Measures of fly control are directed chiefly to preventing the access of files to stable manure and the destruction of maggots living in it. The eggs of the bouse fly IFig. ?82 B) are small, white, .elongate-oval objects, about one twenty-fifth of an inch m length, each slightly curved on one side and concave on the other. The female tir begins to lay eggs in about ten days after having transformed to the adult form, and she deposits from 75 to ?5o eggs at a single laying. She re- peats the laying, however, at intervals during her short productive period of about twenty days, and in all may deposit over 2,ooo eggs. Each egg hatches in twenty-four hours or less. The larva of the house tly, in common with that of many other related files, is a particularly wormlike creature, and is commonly called a maggot IFig. 182 D). Its slender white body is segmented, but, in external appearance, it is legless and headless. On a fiat area at the rear end of the body are located two large spiracles (P,çp), which the novice might mistake for eyes. The tapering end of the body is the head end, but the true head of the maggot is withdrawn entirely into the body. From the aperture where the head bas disappeared, which serves the maggot as a mouth, two clawlike hooks project (?,?h), and these hooks are both jaws and grasping organs to the maggot. The larva sheds its skin twice during the active part of its life, which is very short, usually only two or three weeks. Then it crawls of? to a secluded place, generally in the earth beneath its manure pile, where it enters a resting condi-
[ 343 ]
INSECTS tion. Its skin now hardens and contracts until the creature takes on the form of a small, hard-shelled, oral capsule, called a Dl@arium (Fig. 182 E).
PSp
F
Fro. 182. The house fly, .?lusca domestica A, the adult fly (5 ? rimes natural size). B, the house fly egg (greatly magnified). C, larvae, or maggots, in manure. D, a larva {.more enlarged). E, the puparium, or, hardened larval skin which becomes a case in which the larva changes to a pupa. F, the pupa
[ 344 ]
MOSQUITOES AND FI.IES
Within the puparium, the larva sheds another skin, and then transforms to the pupa. The pupa (Fig. ?82 F) is thus protected during its transformation to the adult by the puparial skin of the larva, which serres in place of a cocoon. When the adult is fully formed, it pushes off a circular cap from the anterior end of its case, and the fly emerges. The length of the entire cycle from egg to adult vanes according to temperature conditions, but it is usually from twelve to fourteen days. The adult flies are short-lived in sunanaer, thirty days, or not more than two months, being their usual span of life. In cooler weather, however, when their activities are suppressed, they lire ronger, and a few survive the winter in protected places. One of the essential differences between flies of the house tir type and the mosquitoes and horseflies is in the structure of the mouth parts. The house fly lacks mandi- bles and maxillae, but it retains the median members of the normal group of mouth-part pieces, which are the labrum, the hypopharynx, and the labium. These parts are combined to form a sucking proboscis that is ordi- narily folded beneath the head, but which is extended downward when in use (Fig. 183 A, Prb). The labium (Fig. 18.3 ]?, Lb) is the principal component of the proboscis of the house fly, and its terminal lobes, or labella (La), are particularly well developed. From the base of the labium there projects forward a pair of palps (Plp), which are probably the palpi of the maxillae, though those organs are otherwise lacking. The anterior surface of the labium is deeply concave, but its trough- like hollow is closed by the labrum (Lin). Against the labial wall çf the inclosed channel lies the hypopharynx (Hphy). When the lobes of the labium are spread out, the anterior cleft between them is closed except for a small central aperture (a). This opening becomes the func- tional mouth of the fly, though the true mouth is situated, as in other insects, between the bases of the labrum and the hypopharynx, and opens into a large sucking pump
[ 345 ]
INSECTS having the same essential structure as that of the horsefly (Fig. 170 A).
The house fly has no piercing organs; it subsists entirely on a liquid diet. The food liquid enters the aper- ture between the labella, and is drawn up to the true
Fig. 183. Head and mouth parts of the house fly.
A, lateral view of the head with the proboscis (Prb) extended. Ant,
antenna; E, compound eye; La, labella, terminal lobes of the pro-
boscis; Plp, maxillary palpi (the maxillae are lacking); Prb, pro-
boscis
B, the proboscis of the fly, as seen in three-quarter front view and
from below. The proboscis consists of the thick labium (Lb), ending
in the labellar lobes (La), between which is a small pote (a) leading
into the food canal (FC) of the proboscis. The food canal contains
the hypopharynx (Hphy), and is closed in front by the labrum (Lm)
mouth through the food canal in the labium between the labrum and the hypopharynx. The fly, however, is not dependent on natural liquids; it can dissolve soluble substances, such as sugar, by means of its saliva. The saliva is ejected from the tip of the hypopharynx, and probably spreads over the food through the channels of the labial lobes. These same channels, perhaps, also collect the food solution and convey it to the labellar aperture. AND FLIES
During recent years we have become so well educated concerning the ways of the bouse fly, its disgusting habits of promiscuous feeding, now in the garbage can or some- where worse, and next at our table or on the baby's face, and we have learned so much about its menace as a pos- sible carrier of disease, that it is scarcely necessary to en- large here upon the flv's undesirability as a domestic companion. The most serious accusation against the bouse fly is that, owing to the many kinds of places it frequents with- out regard to sanitary conditions, and toits indiscriminate feeding habits, there is always a chance of its feet, body, mouth parts, and alimentary canal being contaminated with the germs of disease, particularly those of typhoid lever, tuberculosis, and dysentery. It bas been demon- strated that files can carry germs about with them which
will grow when given a proper medium, and likewise that files taken at large may be covered with bacteria, a single fly sometimes being loaded with millions of them. The wisdom of sanitary measures for the protection of food from contamination by files can not, therefore, be questioned. There is one form of insect villainy, however, of which the house fly is hot guilty; the structure of its mouth parts clears it of all accusa- tions of biting. And yet we hear it often asserted by per-
FIG. I8 4. Head of the stable fly, Stomoxys calcitrans .4nt, antenna; PIp, maxillary pal- pus; Prb, proboscis
sons of unquestioned veracity that they have been bitten by bouse files. The case is one of mistaken identification and hot of imagination on the part of the plaintiff; the
[ 347 ]
INSECTS
insect that inflicts the bite is not the house fly, but another species closely resembling the common domestic fly in gen- eral appearance, though a little smaller. If the culprit is caught, there may be seen projecting from its head a long, hard, tapering beak (Fig. ?84, Prb), an organ quite differ- ent from any part of the mouth equipment of the true house fly (Fig. ?83). This biting fly is, in fact, the stable ff.v, a species known to entomologists as Stomoxvs calci- trans. It belongs to the same family as the housefly, and while it sometimes comes about houses, it is particularly a pest of horses and cattle. The stable fly lives in most parts of the inhabited world. Both sexes have blood-sucking habits, and probably feed on any kind of warm-blooded animal, though the species is most familiar as a frequenter of stables and as a pest of domestic stock. The stable fly breeds mostly in fer- menting vegetable matter, the larvae being found prin- cipally under piles of wet straw, hay, alfalfa, grain, weeds, or any vegetable refuse. Cattle are afflicted by another pestiferous fly called the horn fly, or Haematobia irritans. The species gets its common name from the fact that it is usually observed about the bases o( the horns ofcattle, where great numbers of individuals often assemble. But the horns of the animais are merely convenient resting places. Haematobia is a biting fly like Stomoxys, and, because of its greater numbers, it often becomes a most serious pest of cattle. Through irritation and annoyance during feeding, it may cause loss of flesh in grazing stock, and a reduction of milk in dairy cows. The horn fly resembles the stable fly, but is smaller, being about one-half the size of the house fly. It breeds mostly in fresh manure of cattle dropped in the fields. Of all the biting flies there is none to compare with the tsetse fly of Africa (Fig. 185). Not only is this fly an intolerable nuisance to men and animais because of the severity of its bite, but it is a deadly menace by reason of
[ 348 ]
MOSQUITOES AND FI.IES
its being the carrier of the parasite of Affican sleeping sickness of man, and that of the related disease called nagana in horses and cattle. African sleeping sickness is caused by a protozoan para- site of the genus Trypauosoma that lires in the blood and other body liquids. Trypanosomes are active, one-celled organisms having one end of the body prolonged into a tail, or flagellum. They are round as'parasites in many vertebrate animais, but most of them do not produce dis- ease conditions. There are at least three African species, however, whose presence in the blood of their hosts means almost certain death. Two cause the sleeping sickness in man, and the other produces nagana in horses, mules, and cattle. The two human species have different distribu- tions and produce each a distinct variety of the disease.
One is confined to the tropic.al parts of Africa, the other ?s more southern. The southern form of the disease is said to be much more severe than the tropical form, claiming its vic- tims in a matter of months, while the other may dra? along for years. The sl'eepin? sick- ness and nagana trypanosomes are entirely dependent in nature on the tsetse files for their means of transport from one person or from one animal to another. The tsetse tir (Fig. ?Sç) is a
FIG. 18 S. A tsetse fly, Glossina ?al?al?s, male (about rive times natural size)
larger relation of the horn fly and the stable fly, having the same type of beak and an insatiable appetite for blood. The tsetse fly genus is Glossina. There are two species particularly concerned with the transportation of sleeping sickness, corresponding with the two species of trypanosomes that cause the two
[ 349 1
INSECTS forms of the disease. One is Glossina palpalis (Fig. 185), distributor of the tropical variety of the disease; the other is Glossina morsitans, carrier both of the southern variety of sleeping sickness and of nagana.
The stable fly, the horn fly, and the tsetse fly, we have said, belong to the same family as the house fly, namely, the Muscidae; and yet they appear to have mouth parts of a very different type. The differences, however, are of a superficial nature. All the muscid flies, biting and non-biting, have the same mouth-part pieces, which are the labrum (Figs. 183 B, 186 C, Lm), the hypopharynx (Hphy), and the labium (Lb). They lack mandibles and maxillae, though the maxillary palps (Plp) are retained. In the biting species, the labium is drawn out into a long, slender rod (Fig. 186 C, Lb), and its terminal lobes, the labella (La), are reduced to a pair of small, sharp-edged plates armed on their inner surfaces with teeth and ridges. In the natural position, the deflected edges of the labrum (Fig. 186 B, Lm) are held securely within the hollow of the upper surface of the labium (Lb), the two parts thus in-closing between them a large food canal (FC) at the bottom of which lies the slender hypopharynx (Hphy), containing the exit tube of the salivary duct.
The biting muscids, therefore, have a strong, rigid, beaklike proboscis formed of the same pieces that compose the sucking proboscis of the house fly (compare Fig. 183 A with Figs. 184 and 186 A), but the labium is so modified that it becomes an effective piercing organ. When one of these files bites, it sinks the entire beak into the flesh of its victims. The tsetse fly is said to spread its front legs apart when it alights for the purpose of feeding, and to insert its beak by several quick downward thrusts of the head and thorax. The insect then quickly fills itself with blood, with which it may become so distended that it can scarcely fly. The bulb at the base of the tsetse fly's labium (Fig. 186 C, b) is no part of the sucking apparatus; it is merely an enlargement for the accommodation of muscles. The true sucking organ lies within the head (Pmp), and does not differ in structure from that of other flies.
While our indictment of the flies has applied thus far only to the insects in the mature form, there are species which, though entirely innocent of any criminality in their
| An image should appear at this position in the text. If you are able to provide it, see Wikisource:Image guidelines and Help:Adding images for guidance. |
Fig. 186. Head and mouth parts of the tsetse fly, Glossina
A, lateral view of the head and proboscis of Glossina fusca (from Vogel), showing the food canal (FC) inclosed by the labrum (Lm) and labium (Lb), and containing the tubular hypopharynx (Hphy) through which the saliva is injected into the wound
C, mouth parts of Glossina palpalis, with the parts of the proboscis separated. b, basal swelling of labium; La, the labella, or terminal lobes of the labium used for cutting into the skin of the victim; Lb, labium; Lm, labrum; Plp, maxillary papus (the maxillae are lacking); Pmp, mouth pump
Well-known cases of animal myiasis are that of the botfly in horses and of the ox warble in cattle. The flies of both these species lay their eggs on the outside of the animals. The young larvae of the botfly are licked off and swallowed, and then live until full-grown in the stomach of the host. The young ox-warble larva burrows into the flesh of its host and lives in the body tissues until mature, when it bores through the skin on the back of the afflicted beast, drops out, and completes its transformation in the ground.
Not only animals but plants as well are subject to internal parasitism by fly larvae. Garden crops are attacked by leaf maggots and root maggots; orchardists in the northern States have to contend against the apple maggot, which is a relation of the olive fly of southern Europe and of the destructive fruit files of tropical countries. That notorious scourge of wheat fields, the Hessian fly, is a second or third cousin of the mosquito, and it is in its larva form that it makes all the trouble.
The special attention that has been given to pestiferous files must make it appear that the Diptera are a most undesirable order of insects. As a matter of fact, however, there are thousands of species of flies that do not affect us in any injurious way; while, furthermore, there are species, and many of them, that render us a positive service by the fact that their larvae live as parasites in the bodies of other injurious insects and bring about the destruction of large numbers of the latter.
Scientifically, the Diptera are most interesting insects, because they illustrate more abundantly than do the members of any other order the steps by which nature has achieved evolution in animal forms. An entomologist would say that the Diptera are highly specialized insects; and as evidence of this statement he would point out that the files have developed the mechanical possibilities of the common insect mechanism to the highest general level of efficiency attained by any insect and that they have carried out many lines of special modification, giving a great variety of new uses for structures originally limited to one mode of action. But when we say that any animal has developed to this or that point of perfection, we do not mean just what we say, for the creature itself has been the passive subject of influences working upon it or within it. A fundamental study of biology in the future will consist of an attempt to discover the forces that bring about evolution in living things.