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 flies, 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 our 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. 167). 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 has been a 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 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 not well judge of the mechanical efficiency of two pairs of equal wings moved by the equipment of muscles possessed by most
Fig. 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 shells, 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 (Hl)
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 flyers, 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 butterflies, 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 found that the faster flying species have the fore wings highly developed at the expense of the hind wings; and that the two wings on each side, furthermore, are yoked 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 efficiency 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 master stroke by completely eliminating the second pair of wings from the mechanism of flight. The flies 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, however, these flies are extremely annoying pests, and, where abundant, they must make the lives of animals 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 (Fig. 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 fly injected into the wound dissolves the muscles of the victim, and the predigested solution is then completely sucked out.
As was shown in Chapter VIII, on metamorphosis, whenever the adult form of an insect is highly specialized for a particular kind of life, 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 not 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 not only wingless but legless as well (Fig. 171). The legs, however, as the wings, are represented by internal buds, which, when they enter the period of growth during the early stage of metamorphosis, are turned inside out to form the legs of the adult fly.
The lack of legs gives a cylindrical simplicity of form to most fly larvae, which not only makes these insects look like worms, but has caused many of them to live the life of
Fig. 171. Structure of a fly larva, or maggot
An, anus; ASp, anterior spiracle; DTra, dorsal tracheal trunk; LTra, lateral tracheal trunks; mh, mouth hooks; PSp, posterior spiracle
a worm and to adopt the ways of a worm. In compensation for the loss of legs, the fly 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 soft-bodied, wormlike creature can stretch itself by muscular contraction. It must be remembered, however, that the body of the larva is filled with soft tissues, many of which are but loosely anchored, and that the spaces between the organs are filled with a body liquid. The creature is, therefore, capable of performing movements by making use of its structure as a hydraulic mechanism; a contraction of the rear part of the body, for example, drives the body liquid and the soft movable organs forward, and thus extends the anterior parts of the body. A contraction of the lengthwise muscles then pulls up the rear parts, when the movement 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 correlated with the manner of breathing. In most insects, as we have learned (Fig. 70), there is a row of breathing pores, or spiracles, along each side of the body, which open into
Fig. 172. Rat-tailed maggots, larvae of the drone fly, which live submerged in water or mud and breathe at the surface through a long, tail-like respiratory tube
Upper figure, resting beneath a small floating object; lower, feeding in mud at the bottom
lateral tracheal trunks. In the fly larva, however, these spiracles are closed and are not opened for respiration until the final change of the pupa to the adult.
The fly larva is provided with one or two pairs of special breathing organs situated at the ends of the body. Some species have a pair of these organs at each end of the body (Fig. 171, ASp, PSp), and some a pair at the posterior end only. The anterior organs, when present (Fig. 171, Asp), consist of perforated lobes on the first body segment, the pores of which communicate with the anterior 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 fly larva can live submerged in water, 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. 172), 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 foul 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 fly and the adult necessarily involves much reconstruction 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 of most files, however, while it has the general structure of the adult fly (Fig. 182 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 developed. 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 not seem necessarily to demand it, suggests, furthermore, that the pupae of the ancestors of such species lived in the same medium as the larvae.
If our supposition is correct, we may see a reason for the apparent exception in the flies to the general rule that the pupa presents the adult structure and discards the peculiarly larval characters. The pupae of some flies whose
Fig. 173. Larva (A) and pupa (B) of a horsefly, Tabanus punctifer (about 1½ times natural size)
An, anus; H, head; PSp, posterior spiracle; Sp, spiracle
larvae live in the water, however, revert at once to the adult system of lateral spiracles (Fig. 173 B, Sp). With such species, the larva comes out of the water just before pupation time and transforms in some place where breathing is possible by the ordinary respiratory organs. This is the general rule 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 in 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 associated with the welfare of man or of his domesticated animals. Such species include the mosquitoes, the house fly, the blowfly, the stable fly, the tsetse fly, the flesh flies, and related forms.
Mosquitoes
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 given 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 animals, 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 suffering 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 not 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. Modern 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 rainwater barrel.
The supposed ancestry of the horsehair snake is now an exploded myth, but the advance of science has unfortunately not altered the fact that wigglers turn into mosquitoes, except in so far as the spread of applied sanitation has brought it about that fewer of them than formerly succeeded 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 mosquito larvae, since that is what they are.
The rainwater barrel never told us how those wiggling 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 life. Now, of course, we understand that it is a very simple matter for a female
Fig. 174. Life stages of a mosquito, Culex quinquefasciatus
A, the adult female. B, head of an adult male. C, a floating egg raft, 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
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" mosquitoes, species of the genus Culex or of related genera; second, the yellow-fever mosquito, Aëdes aegypti; and third, the malaria-carrying mosquitoes, which belong to the genus Anopheles.
The common Culex mosquitoes (Fig. 174 A) lay their eggs in small, flat 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 buoyed up from below by a film of air beneath the eggs.
Almost 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 water 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 incubation 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 life 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 (Fig. 175, b), a pair of short antennae (Ant), and, on the ventral surface in front of the mouth, a pair of large brushes of hairs curved inward (a). From the sides of 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 lower branch. The
Fig. 175. Structure of a Culex mosquito larva
a, mouth brushes; Ab abdomen; Ant, antenna; b, eye; c, respiratory tube; d, terminal lobes; H, head; PSp, posterior spiracle; Th, thorax; Tra, dorsa tracheal trunks
upper branch (c), however, is really a long tube projecting dorsally and backward from the next to the last segment. The lower branch is the true terminal segment of the body and bears the anal opening of the alimentary canal at its extremity. On the end of this segment four long, transparent flaps project laterally (d), two groups of long hairs are situated dorsally, and a fan of hairs ventrally (Fig. 174 E).
The principal characteristic of the mosquito larva is the specialization 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 inner 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 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 thorax, the mouths of which open above the water when the pupa comes in contact with the surface. The pupa, of course, does not feed, but it is almost as active as the larva, for it must avoid its enemies. When disturbed it rapidly swims downward by quick movements of the abdomen, the extremity of which is provided with two large swimming flaps. The duration of the pupal stage in midsummer is about two days.
The adult mosquito issues from the pupal skin through a split in the back of the latter. We now see why the pupa
Fig. 177. Aëdes atropalpus, male, a mosquito related to the yellow fever mosquito and similar to it in appearance
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. 174 A) has the general features of any other two-winged fly, but it is distinguished from nearly all other flies by the presence of scales on its wings and on parts of its head, body, and appendages. The mouth parts of the adult mosquito are of the piercing and sucking type, and are similar 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. 176 A, Prb). The male and the female mosquitoes are readily distinguishable by the character of the antennae, these organs in the male being large and feathery (Fig. 174 B), while those of the female are 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 position, do not appear as separate pieces, as do those of the horsefly. The various elements, except the palpi, are compressed into a beak that projects forward and downward from the lower part of the head (Fig. 176 A, Prb). The length of the beak varies in different kinds of mosquitoes; it is particularly long in the large South American species shown in Figure 176.
When the beak of the female mosquito is dissected (Fig. 176 B), the same equipment of parts is revealed as is possessed by the female horsefly (Fig. 169 B), namely, a labrum (Lm), two mandibles (Md), two maxillae (Mx), a hypopharynx (Hphy), 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 groove in its upper surface.
The labrum (Fig. 176 B, Lm) 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 mandibles of the mosquito (Md) are very slender, delicate bristles; those of the species figured are so weak that it would seem they can be of little use to the insect. The maxillae (Mx) are thin, flat 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 probably serve to keep the mouth parts fixed in the puncture as the piercing labrum is thrust deeper into the flesh. The palpi (MxPlp) arise from the bases of the maxillae. The hypopharynx (Hphy) is a slender blade with a median rib which is traversed by the channel of the salivary duct. Its upper surface is concave and, in the natural 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 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) serves
Fig. 178. Mosquito larvae
A, Aëdes atropalpus. B, Anopheles punctipennis, the malaria mosquito larva
c, respiratory tube; d, terminal lobes; e, stellate groups of hairs that hold the larva at the surface of the water (fig. 181 A); f, spiracular area; PSp, spiracle
principally as a sheath for the other organs. It ends in two small lateral lobes, the labella, between which projects a weak, median tonguelike process. When the mosquito pierces its victim the base of the labium bends backward as the other bristlelike members of the group of mouth parts sink into the wound.
Mosquitoes of both sexes are said to feed on the sap of 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 notorious 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 mosquito and the subsequent irritation and swelling probably result from the injection of the secretion from the salivary glands of the insect into the wound. It is said that the saliva of the mosquito prevents coagulation of the blood.
Because of the short time necessary for the completion of the life cycle from egg to adult during summer, there are many generations of mosquitoes from spring to fall. The winter is passed both in the adult and in the larval stage. Fertile females may survive cold weather in protected 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 of warmth.
The yellow-fever mosquito, now known as Aëdes aegypti but at the time of the discovery of its relation to yellow lever generally called Stegomyia fasciata, is similar in its habits during the larval and pupal stages to the Culex mosquitoes. It lays 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 ringed with white; the black abdomen is conspicuously cross-banded with white on the basal half of each segment. The male has large plumose antennae and long maxillary palpi. The female has a strong beak, but small palpi, and her antennae are of the short-haired form usual with female mosquitoes. The species of Aëdes shown in Figure 177 much resembles the yellow-fever mosquito, but it is a more northern one common about Washington, D. C., where it breeds in rock pools along the Potomac River.
The larva of Aëdes (Fig. 178 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
Fig. 179. Mosquito pupae in natural position resting against the under
surface of the water
A, Aëdes atropalpus. B, Anopheles punctipennis
surface for air. In its search for food it noses about in the refuse at the bottom of the water and voraciously consumes dead insects and small crustaceans. The pupa likewise (Fig. 179 A) does not 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 respiratory tubes in the manner in which the pupae of various species are often figured.
Aëdes 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 not yet definitely known, though strong evidence has been adduced to show that it is one of the minute, non-filterable organisms called spirochetes. The virus will not develop in the mosquitoes at a temperature below 68° F., and Aëdes aegypti will not breed
Fig. 180. The female malaria mosquito, Anopheles punctipennis
in latitudes much beyond the possible range of yellow fever. Yellow fever, therefore, is a disease ordinarily confined to the tropics and warmer parts of the temperate zones. Seasonal outbreaks of it that have occurred in northern cities have been caused probably by local infestations of infected mosquitoes brought in on ships from some southern port.
The malaria mosquitoes belong to the genus Anopheles, a genus represented by species in most temperate and tropical regions of the world, which are prevalent wherever malaria occurs. Our most common malaria species is Anopheles punctipennis (Fig. 180), characterized by a pair of dull white spots on the edges of the wings. The Anopheles females lay their eggs singly on the surface of the water, where they float, each buoyed up by an air jacket about its middle.
The larvae of Anopheles (Fig. 178 B) differ conspicuously from those of Culex and Aëdes both in structure and habits. Instead of a respiratory tube projecting from near the end of the body, as in Culex (Figs. 174 E, 175), there is a concave disc (Fig. 178 B, f) on the back of the next to 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
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. 181 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 domestic 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), live until they are ready for transformation. 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 flies to stable manure and the destruction of maggots living in it.
The eggs of the house fly (Fig. 182 B) are small, white, elongate-oval objects, about one twenty-fifth of an inch in length, each slightly curved on one side and concave on the other. The female fly begins to lay eggs in about ten days after having transformed to the adult form, and she deposits from 75 to 150 eggs at a single laying. She repeats the laying, however, at intervals during her short productive period of about twenty days, and in all may deposit over 2,000 eggs. Each egg hatches in twenty-four hours or less.
The larva of the house fly, in common with that of many other related files, is a particularly wormlike creature, and is commonly called a maggot (Fig. 182 D). Its slender white body is segmented, but, in external appearance, it is legless and headless. On a flat area at the rear end of the body are located two large spiracles (PSp), 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 has disappeared, which serves the maggot as a mouth, two clawlike hooks project (mh), 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 off to a secluded place, generally in the earth beneath its manure pile, where it enters a resting condition. Its skin now hardens and contracts until the creature takes on the form of a small, hard-shelled, oval capsule, called a puparium (Fig. 182 E).
Fig. 182. The house fly, Musca domestica
A, the adult fly (5½ times 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.
One of the essential differences between flies of the house fly type and the mosquitoes and horseflies is in the structure of the mouth parts. The house fly lacks mandibles 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 ordinarily folded beneath the head, but which is extended downward when in use (Fig. 183 A, Prb).
The labium (Fig. 183 B, 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 (Lm). Against the labial wall of 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 functional 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 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 aperture 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 proboscis; Plp, maxillary palpi (the maxillae are lacking); Prb, proboscis
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 pore (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.
During recent years we have become so well educated concerning the ways of the house fly, its disgusting habits of promiscuous feeding, now in the garbage can or somewhere worse, and next at our table or on the baby's face, and we have learned so much about its menace as a possible carrier of disease, that it is scarcely necessary to enlarge here upon the fly's undesirability as a domestic companion.
The most serious accusation against the house fly is that, owing to the many kinds of places it frequents without regard to sanitary conditions, and to its 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 fever, tuberculosis, and dysentery. It has been demonstrated that files can carry germs about with them which
Fig. 184. Head of the stable fly, Stomoxys calcitrans
Ant, antenna; Plp, maxillary palpus; Prb, proboscis
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 flies 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 accusations of biting. And yet we hear it often asserted by persons of unquestioned veracity that they have been bitten by house flies. The case is one of mistaken identification and not of imagination on the part of the plaintiff; the insect that inflicts the bite is not the house fly, but another species closely resembling the common domestic fly in general appearance, though a little smaller. If the culprit is caught, there may be seen projecting from its head a long, hard, tapering beak (Fig. 184, Prb), an organ quite different from any part of the mouth equipment of the true house fly (Fig. 183). This biting fly is, in fact, the stable fly, a species known to entomologists as Stomoxys calcitrans. 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 fermenting vegetable matter, the larvae being found principally 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 of the horns of cattle, where great numbers of individuals often assemble. But the horns of the animals 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 animals because of the severity of its bite, but it is a deadly menace by reason of its being the carrier of the parasite of African sleeping sickness of man, and that of the related disease called nagana in horses and cattle.
African sleeping sickness is caused by a protozoan parasite of the genus Trypauosoma that lives 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 found as parasites in many vertebrate animals, but most of them do not produce disease 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 distributions and produce each a distinct variety of the disease.
Fig. 185. A tstse fly, Glossina palpalis, male (about five times natural size)
One is confined to the tropical parts of Africa, the other is more southern. The southern form of the disease is said to be much more severe than the tropical form, claiming its victims in a matter of months, while the other may drag along for years. The sleeping sickness 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 fly (Fig. 185) is a 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 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
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.