In starting, it may help to give a brief review of general entomology and of the relations between man and the insect world. The lowly insect impresses man by apparently possessing an uncanny wisdom out of all proportion to its size. Both early and modern man have been impressed and awe-stricken by the ability of insects to do many of the things that man does, often with greater efficiency; hence the tendency to speak of insects in anthropomorphic terms.

There are insects that raise crops, and others that herd insect cattle that the "milk" of a sweet liquid (ants and aphids). There are insect architects that construct living quarters so intricately engineered that they achieve year-around air conditioning. There are insect carpenters (bees, ants), papermakers (wasps), slave-raiders (ants), and undertakers (beetles). Some insects live in social organizations that are considerably more complex, or at least more rigidly organized, than our own-and are favorite specters of various commentators on the human scene who hold up to us the horrible example of the beehive or the ant hill as the ultimate fate of man if he does not learn to curb his own numbers. But let us remember-while the brains of these social insects are tiny, they are larger in proportion to body size, than those of many vertebrates.

While man has formed an uneasy alliance with such useful insects as the honey-bees, he has never formed any real affection for the insect world as he has for many of his fellow vertebrates. Maurice Maeterlink wrote:

"Something in the insect seems to be alien to the habits, morals, and psychology of this world, as if it had come from some other planet, more monstrous, more energetic, more insensate, more atrocious, more infernal, than our own."

This alien character of the insect and the insect world is understandable when we reflect that the insects are, after all, about 400 times as old as man in terms of the fossil record and incredibly more numerous. Everyone who estimates the total number of described species of insects comes up with a different figure, but it is conservative to use 1,000,000 as a safe number. So, the insects are not only 400 times as old, but about a million times as varied as man who is but a single species with a short history.

The long geologic history of insects and the tremendous extent of their speciation has given them ample opportunity to develop extremely intricate automatic behavior which, perhaps for want of a better name, is often called instinct. Although the nervous systems of insects are highly complicated, and serves their owners well, they appear to be extremely rigid and incapable of allowing their possessors to exercise the kind of judgement that man habitually uses.

Insects have had the time to adapt to almost every conceivable ecological niche in, on, or above the earth. They are all around man, sharing his houses, puncturing his skin, consuming his food and clothing, contesting the harvest of his fields, and filling the summer air around him with a veritable cloud of moving particles. There is scarcely a place on the earth that is not home to at least one kind of insect. Insects have been found in deep underground caves and a termite was captured in a trap attached to an airplane flying at 19,000 feet. Over 40 kinds of insects live in the bleak Antarctic. Bumblebees, beetles, moths, and butterflies live as far beyond the Arctic Circle as flowering plants grow. Mosquitoes and other biting insects penetrate the polar regions as far as do the warm blooded animals they feed one. Insects are abundant in the driest of deserts and in torrential streams. A few even live on the surface of the ocean beyond sight of land. In the Himalayas, insects are found amid rock and snow 20,000 feet above sea level.

Wherever they live, insects seem to endure with a unique kind of indestructibility. Many are adapted to withstand winter temperatures more than 30° below zero Fahrenheit. Others inhabit hot springs where temperatures reach 120°F. Still others can survive in as great a vacuum as man has the power to create. Larvae of the brine fly, Ephydra, live in almost pure salt. Their relative, the petroleum flies, spend their immature stages in pools of crude oil around well-heads in Southern California and are not found living anywhere else. A grain weevil may live for hours in pure CO₂ (carbon dioxide). This gas, immediately poisonous to man and most other animals, acts as an anesthetic to the weevil, and the inactive insect can survive on the reserve oxygen in its breathing tubes. Many insects can endure long periods without water. They have stored food reserves from which they can produce so-called metabolic water. Then the stored carbohydrates are broken down by digestion into H₂O and CO₂ is eliminated but the H₂O is retained.

Insects have a tremendous range in size, probably greater than that of any other major group of animals. The smallest insects are smaller than some single-celled protozoa. The largest ones are larger than such mammals as mice and shrews. The smallest North American insect is a beetle about 1/1000 of an inch long. Among the largest insects in the world today are the Atlas moth of India and the Ornithoptera (bird wing) butterflies of the South Pacific, both of which may have wing spans exceeding a foot from tip to tip. Representations of the fossil Meganeura monyi, the ancestor of a dragonfly living in the Pennsylvanian (Carboniferous-Paleozoic), had a wing spread of about 29".

The success of the insects as a group is due, according to one authority, to their having at least six major assets in the endless struggle for survival: (1) flight, (2) adaptability, (3) external skeleton, (4) small size, (5) metamorphosis and (6) a specialized system of reproduction.

First, the most obvious endowment that sets insects apart from all other living things except birds and bats is flight. With wings, insects were able to spread all over the globe. If conditions became unfavorable at one place, they could take to the air and search for another place. Flight has given them an advantage over land-bound animals in making them able to search actively for their mates, to forage widely for food, and to make good their escape from enemies. Be it noted, insects developed wings in addition to legs; birds and bat wings are modified existing appendages.

Second, no other form of animal life has been able to adapt to such extremes of living conditions. The things insects feed upon provide just one example of their endless adaptability. Not only does one insect or another feed on every kind of higher plant, some also feed on paint brushes, on wine bottle corks, on mummies, on tobacco, pepper, and opium. Their feeding tools, too, are as varied as their appetites. What began presumably as pairs of jointed legs in primitive insects, have evolved into the retractable siphon of the butterfly, the skin-piercing tools of the mosquito, the vise-like jaws of the beetle, the hypodermic syringe of the aphid, the marvelous extensible grappling hood of the dragonfly nymph.

Third, the insect has its skeleton on the outside of its body in the form of a cylinder, basically and within certain size limits the strongest possible construction for a given amount of material. The external skeleton is formed by the hardening of secretions from the animal's true outer skin and is a remarkable protective armor. Chitin is a nitrogenous polysaccharide with the formula (C₃₂H₅₄N₄O₂₁)x. This material is flexible, light-weight, tough, and resistant to most chemicals. By definition it is insoluble in water, alcohol, dilute acids and alkalines. It is not attacked by the digestive enzymes of mammals or fishes but is broken down by certain bacteria. Clearing with alkali (as with KOH) removes the coloring and hardening substances but produces no visible change in the essential structure. Sections if the exoskeleton that have no need for flexibility are further strengthened by a complex substance formed from the protein component of the cuticle by the action of quinones called sclerotin or cuticulin, which is somewhat similar in composition to human fingernails. In addition, the entire exoskeleton is coated with waxes that not only keep wetness out, but also keep it in and prevent the insect from drying up inside.

Fourth, the small size of insects is of great advantage in survival. Their individual demands from the environment are meager. The speck of food that is a feast for an insect may be too small to be noticed by a larger animal. A dew drop quenches its thirst, a pebble in the desert provides shade.

The science fictioneer is often fond of peopling the plots of his stories with insects grown as large as humans. This is, of course, a mechanical impossibility. The engineering of an insect's body is very efficient for its small size, but there are obvious limitations. One is the strength of its skeleton. As the hollow cylinder increases in size, it grows progressively weaker, its mechanical advantage diminishes. More important is the system of respiration characterizing most insects and limiting their size. Insects breathe through a maze of microscopic air tubes or tracheae that bring oxygen to all parts of their bodies. Air seeps into these tubes by the diffusion of individual molecules of gas, a method that works only over short distances. That is the reason very few insects have bodies more than three-quarters of an inch thick. Much thicker than that, the insect would suffer from oxygen starvation and become too lethargic to survive. Nearly all exceptionally large insects are tropical species, possibly because gasses diffuse more rapidly at high than low temperatures.

Fifth, most insects receive survival benefits from the life pattern involving metamorphosis. With this type of life cycle the immature stage of the insect is able to exploit one food supply while the winged adult stage may nourish itself on something completely different or need none. We humans spend about a fifth of our expected life in developing to maturity. A typical insect, however, spends most of its life in its immature stages, which often are inconspicuous. During this time, it develops a multitude of adaptations for coping with problems of survival; and the adult stage, often quite noticeable and vulnerable to enemies, need survive only long enough for reproduction to take place.

Sixth, a feature of the life cycle of most insects, is the ability of the winged adult stage to delay fertilization of the eggs after mating until the proper living conditions for the young have been found. Sperm may be stored in the female spermatheca and released for fertilization of the eggs only when favorable conditions are found. For instance, the queen bee may store as many as 4 million sperm cells in her spermatheca and these will last her for all of her life.

Of course, there are vulnerable points in the foregoing six items. Particularly, insects may be quite vulnerable to enemies during their period of molting or during their period of true metamorphosis.

The origin of insects remains a source of controversy among students of evolution. Some are inclined to seek the ultimate ancestors of the insects among the Trilobites, seeing resemblances between the extinct insect order Paleodictyopteran and the marine trilobites. Others consider it more likely that the insects originated from crustaceans which left the water, much as amphibians led the march to land for the vertebrates. It may be pointed out, however, that there are no links whatever to tie the earliest known fossil insects with either the trilobites or the crustacea, and apparently, current belief is inclined to favor the idea that insects and crustacea shared common ancestors but that the insect is not derived from the crustacean. A common ancestor with myriopods seems likely in some ways. A "baby" myriapod has 6 legs, segmented body, compound eyes.

What this means, among other things, is that the aquatic insects almost certainly are insects which re-invaded the water. In other words, true insects probably evolved on land from ultimate ancestors which were aquatic, but insects now found in water must have returned from terrestrial ancestors.

The fossil record of the Hexapoda is not particularly good, and is certainly highly incomplete. Insect fossils are not found in as many places as are fossils of some other groups because of special conditions needed to insure adequate preservation. Because of their small size, the delicacy of their parts, and the minute nature of characters useful in identification insect remains must be preserved in a medium of extremely fine texture, to provide a comparatively grainless matrix. Satisfactory materials are mud and volcanic ash resulting in shales, concretions, fine humus such as coal, and resin of coniferous trees giving amber. Insect fossil-bearing deposits have been found in scattered localities all over the world. Some in North America are productive of valuable additions to the fossil record. Along Mazon Creek near Morris, Illinois, are found iron nodules or concretions containing insects dating back to Pennsylvanian (Paleozoic) times. One of these fossils is the wing of a paleodictyopteran Lithoneura which is considered by some insect paleontologists to be a probable relative of one of the early mayflies. In Kansas, there are several deposits which contain large numbers of Permian insects, and in Colorado and Washington State there are deposits with fossils of Cenozoic age, from Alaska and Manitoba there are fossils in Cretaceous amber.

Now, while it is thought to be almost certain that the aquatic insects represent forms which reinvaded the aquatic habitat after terrestrial evolution, it may be noted that in the Pennsylvanian, which is represented today by about 1,500 species from its fossil record, orders ancestral to dragon flies and cockroaches were so abundant that this is often called "the age of cockroaches." Yet, it is thought that the nymphs of these early orders were aquatic or semi-aquatic and lived in the swamp pools which were extensive in many areas of the barely emergent continents. Actually, at no other time in geologic history have conditions been so ideal for insects. The climate was warm and humid, with neither winters nor dry sea-sons. Swamps, lagoons, estuaries were well forested and relatively widespread. Insects and amphibians dominated the swamps. Competition between these two groups must have been ferocious, for whereas the adult amphibians undoubtedly fed on insects, many of the large predaceous insect nymphs surely fed on the vulnerable amphibians. This competition, together with competition among the insects themselves, must have exerted a strong evolutionary pressure toward the development of large size in insects. The climate would not discourage this tendency but rather encourage it by providing uniform growing conditions throughout the year.

When the Pennsylvanian gave way to the Permian, a tremendous change took place in the world. Extensive mountain making (represented by the Appalachian Revolution) brought about great changes in the climate, resulting in widespread cold and acid conditions. This change following on the warm, moist, climate of the Pennsylvanian doomed many specialized stocks, especially those that had developed toward gigantism. The surviving forms were mostly the smaller, generalized stems which had the inherent potential of becoming adapted to new environments. Judging by Permian fossils, these insects were smaller, and many of the old archaic forms had practically disappeared. It is in the Permian that are found the first representatives of many modern orders-the beetles (Coleoptera), the scorpion flies (Mecoptera), true dragonflies (Odonata), mayflies (Ephemeroptera), and very primitive bugs (Hemiptera) and Orthoptera. Later in the Permian, other orders appear, including Neuroptera and Diptera. However, it is important to note that four orders with complete metamorphosis were developed during this period. Some writers consider that this was in response to seasons of drought rather than to cold winters, although both factors may have played a selective role.

In summary, winged insects of many kinds swarmed through the coal-age jungles of 300-400 million years ago, but most of the insect orders that lived then became extinct. Their wings, perhaps, had become a liability. Most of the wings were large in size, and their predecessors lacked the means for folding them. Today, only a few insects that cannot fold their wings - notably the dragonflies (Anisoptera) - still survive. By some 225 million years ago, at the end of the Permian, nearly all today's many insect orders had become established or at least existed in prototype. As the earth entered the age of reptiles, a wide variety of insect types familiar today swarmed through the air-grasshoppers, crickets, mayflies, dragonflies, cockroaches, cicadas, leafhoppers, beetles, and others. A few important ones were missing. Only when flowering plants arose some 130 million years ago, (Cretaceous) did the insects that specialize in pollinating them-butterflies, moths, bees, wasps, and the "higher" flies (Diptera)-flourish.

Thus, for over a third of a billion years, insects have consistently displayed their adaptability. They took advantage of the rise of flowering plants; they developed such complex social lives as those of the pollinating honey bees. They assumed a number of curious forms that protected them against enemies or made them more successful at finding prey-flowers that fly, twigs that walk, thorns that climb stems.

"When the moon shall have faded out from the sky, and the sun shall shine at noonday a dull cherry-red, and the seas shall be frozen over, and the ice-cap shall have crept downward to the equator from either pole, and no keels shall cut the waters, nor wheels turn in mills, when all cities shall have long been dead and crumbled into dust, and all life shall be on the very last verge of extinction on this globe; then on a bit of lichen, growing on the bald rocks beside the external snows of Panama, shall be seated a tiny insect, preening its antennae in the glow of the worn-out sun, representing the sole survival of animal life on this our earth,--a melancholy "bug."

Holland's universe ended with a whimper rather than a bang. Our sun may become a red giant and go through violent pyrotechnics fatal to all earthly life before it burns out. But the idea of the "bug's" lasting qualities is not a bad one.

Vocabulary you should learn:

It is worthwhile here to attempt some generalizations on insect anatomy and morphology on which more detailed modifications of each aquatic order can be fitted. The cuticle of an insect, as stated before, forms a more or less hardened exoskeleton and, although perfectly continuous over the whole body, it remains flexible along certain definite, and usually transverse, lines. In the latter positions, the cuticle becomes infolded and is membranous in character. Consequently, the body of an insect presents a joined structure-referred to as segmentation-and is divided into a series of successive rings variously known as segments, somites, or metameres. The flexible infolded portion of the cuticle between adjacent segments is the intersegmental membrane whose function is to allow for free movement of the body.

The cuticle exhibits localized areas of hardening termed sclerites, which meet one another along certain lines of union known as sutures. In the case of movable sclerites, their membranous continuity may be concealed, but if the cuticle of an insect be distended, many of the sclerites will be forced apart and it is then seen that they are connected by membrane along the lines of the sutures. Others of the sclerites may be rigidly fixed, and inseparable in this manner. The sutures in these cases being little more than linear impressions (this situation will be found most often in some of the higher orders, especially Coleoptera). In certain regions, the sclerites may not come into apposition along sutures, and appear instead like islands of cuticle surrounded by membrane. Complete fusion of adjacent sclerites is common, particularly among the higher orders and all traces of sutures may be lost.

In the majority of adults as well as in many of the larvae, the body wall of a typical segment is divisible into four definite sclerotized regions: a dorsal region or tergum, a ventral region or sternum, and a lateral region or pleuron on each side of the body. Each of these regions may be differentiated into separate sclerites. In this case, the sclerites composing the tergum are known as tergites, those of the sternum as sternites, and those constituting each pleuron as pleurites. In certain instances, one may see small detached plates between adjacent segments. Such sclerites are often called intersegmentalia and belong partly to the segment in front and partly to the one behind them. According to their position, they may be termed intertergites, interpleurites, or intersternites.

Typical appendages are joined tubes invested with a dense cuticle. Between each pair of joints or segments, the cuticle remains membranous and becomes infolded to form the articular membrane. One account of its jointed structure, the whole or part of an appendage is movable by means of its muscles. An insect appendage consists typically of a limb base and a shaft which represents the endopodite of Crustacea. There is no conclusive evidence of a biramous condition among the appendages in any insects.

In addition to true appendages, numerous other outgrowths of the body wall are found in various insects. Unlike true appendages, processes of the body wall are by no means invariably represented by embryonic counterparts. They may or may not be segmentally arranged, they may be originally paired or unpaired, and more than a single pair is sometimes borne on a segment. They differ from cuticular processes in containing a definite extension of the body cavity and in some cases, they are freely movable. The principal types of organs which come under this category are pseudopods which are characteristic of many true fly larvae, but often are referred to as prolegs; gills, which are found in most of the immature stages of aquatic insects; and wings, confined to the meso- and meta-thorax and which attain their full development in adult insects.

The head capsule is a hard, compact case formed of several sclerites fused together. To take a typical generalized head we would find at the top a Y-shaped epicranial suture. The stem of the Y forms a median line and the two arms diverge anteriorly. The frons is the unpaired sclerite which lies between the arms of the epicranial suture. It bears the median ocellus and its distal limit is marked on either side by the invaginations or small pits which form the anterior arms of the tentorium. The clypeus lies just anterior to the frons and is often fused to it owing to a com-plete or partial obliteration of the suture which separates them (clypeo-frontal suture). The labrum of an unpaired sclerite ususally movable articulated with the clypeus by means of the clypeo-labral suture.

In general, the epicranium forms the whole of the upper region of the head from the frons to the neck. That portion of the epicranium which lies immediately behind the frons and between the compound eyes is referred to as the vertex. The vertex usually carries the paired ocellae and the antennae but is not differentiated as a separate sclerite. The occiput is the hinder part of the epicranium between the vertex and the neck. It is rarely present as a distinct sclerite. The gena forms the lateral area below and behind the eyes on each side. At its junction with the clypeus, it bears a facet for articulation of the mandible. The mouthparts of insects consist typically of the labrum or upper lip, the labium, or lower lip, an upper pair of jaws called mandibles, and a lower or posterior pair of jaws called the maxillae. Arising from the floor of the mouth is a median tongue-like structure called the hypopharynx and associated superlinguae. Mouth parts vary widely in form, this variation being correlated with the method of feeding and other uses to which they are put. Examination of the mouth parts may, therefore, give a clue to the method of feeding and frequently the nature of the food of an insect.

We will have to consider the mouth parts separately for each of the Orders as we come to them because of their tremendous modification from Order to Order and because of their significance in taxonomy. The labrum is a simple plate hinged to the clypeus and capable of a limited amount of up and down movement. It forms the roof of the buccal cavity.

The mandibles or true jaws each represent the basal joint or coxopodite of the typical arthropod limb. They undergo tremendous modification in the various groups. The maxillae are composed of: the cardo or hinge, which is the first or proximal piece, and in many insects the only part directly attached to the head; the stripes or footstalk which articulates which(?) the distal border of the cardo and bears an outer scerite called the palpifer (and sometimes an inner sclerite, the subgalea or parastipes). The palpifer carries a maxillary palpus which is the most conspicuous appendage of the maxillae. It if 1- to 7- jointed and sensory in function. Distally, the maxilla is composed of two lobes: an outer one or alea and an inner one, or lacinia. The former is two-jointed, frequently, and it often partially overlaps the lacinia. The lacinia or blade as a rule, is spined or toothed on its inner border.

The labium (or second maxillae), is formed by the fusion of a pair of appendages which are serially homologous with the maxillae. The labium is divided into two primary regions; a proximal postmentum and a distal prementum, the line of division between the two being the labial suture. In some orders, just behind the labial suture is a distal sclerite called the mentum and the proximal area of the original postmental plate will then be termed the submentum. Near the base of the prementum is the palpiger which carries the labial palpus and often resembles a basal joint of the latter. The labial palps are composed of one to four joints and function as sensory organs. Arising from the distal margin of the prementum are two pairs of lobes which collectively form the ligula. These are: an outer pair of paraglossae and an inner pair of glossae.

Sometimes some small sclerites exist in the neck region. These are called, logically enough, cervical sclerites. They are found in most of the orders of insects but are best developed in the more primitive groups, among the aquatic groups, chiefly the Odonata.

The thorax is composed basically of three segments: prothorax, mesothorax, and metathorax. In almost all insects, each segment bears a pair of legs and in the majority of adult insects, both the meso- and the meta-thorax carry a pair of wings. In all cases where the legs are wanting, their absence is due to atrophy. This condition is rare among adults but is the rule among larvae of the Diptera and certain families of Coleoptera. The absence of wings may be an ancestral character in the Apterygota. But among the Pterygota it is always an acquired feature, due to the atrophy of pre-existing organs. The thorax is exhibited in its simplest form in the Thysanura and in the larvae of many Orders. In these instances the segments differ but little in size and proportions, but usually with the acquisition of wings, a correlated specialization of thoracic structure results. The meso- and meta-thorax become more or less intimately welded together, and the union is often so close that the limits of those regions can only be ascertained with difficulty. In Orders where the wings are of about equal area, these two thoracic segments are of equal size. This is true particularly in the Odonata. Where the forewings are markedly larger than the hind pair, there is a correspondingly greater development of the meso-thorax. (A good example is the Diptera where the hind wings are absent.) The prothorax never bears wings and is variable in the degree of its development (it may be noted that some of the fossil ancestors of the stoneflies had a structure on the margins of the pro-thorax which some paleontologists have interpreted as a third pair of wings).

To introduce a few more terms common in the literature - in many immature stages and also in the adults of the more generalized insects, the tergum of each segment is a simple undivided plate or notum. In the wing-bearing segments of most adults, the tergum is composed of large anterior plate or notum, already mentioned, and a narrower posterior plate or post-notum, which has a rise in the inter-segmental membrane, the scutum, and the scutellum. These are illustrated on the blackboard, the example being the thorax of a cranefly.

The side of the thorax is termed the pleuron and is composed of sclerites called pleurites. There is an anterior sclerite called the epimeron, the two separated by the pleural suture. These sclerites may be variously modified, fused, or further subdivided. In many insects, including both Corydalis, Tipula and Tabanus the episternum is divided into upper and lower halves. Similar division of the epimeron may be found in some forms. The sclerites forming the sternum also are subject to various subdivision, fusion, or other modification generally.

Each thoracic segment typically bears one pair of legs. A generalized typical insect leg consists fundamentally of a basal segment, tibia the coxa, which joins the body and is usually surrounded by the epimeron and episternum. Next comes a small segment called the trochanter, then the heaviest (usually) segment, the femur, followed by the tibia, and finally by the tarsus, which may have anywhere from one to five joints.

In various of the aquatic Orders the coxa is subject to some modification, as are most of the other leg segments. The trochanter, for instance, is divided into two subsegments in the Odonata. The femur undergoes wide modification in some of the aquatic insects. The tibia is almost always slender and usually equal to or exceeding the femur in length. It often bears tibial spurs. The tarsus consists primitively of a single element, as seen in the Protura. More usually, however, it is divided into subsegments, typically five in number. It terminates in a claw or claws which are subject to wide variation and often have taxonomic value.

The abdomen is subject to a wide range of variation among the immature and adult stages of the various aquatic Orders. The abdomen frequently bears gills or other appropriate appendages in the immature stages. The abdomen also bears the reproductive organs and so-called accessory genitalia which are of tremendous importance in taxonomic work, especially as regards male specimens. A generalized abdominal tip for a typical male insect may be sketched on the board and you will soon see that it is very close to the form assumed by living mayflies. Almost every morphological feature of the insect body has been employed at one time or another by one worker or another as showing taxonomic significance. At first glance, mouth parts may seem too complicated to "learn" and wing venation itself may present a seemingly bewildering maze. However, just as in learning to recognize makes of automobiles, soon you will be spotting insects not only to higher category but even to the species level on the basis of a quick observation.

The Aquatic Insects: An Overview

One never ceases to be amazed at the tremendous variety, abundance, and out and out color of the insect life that teems in freshwater. The surface film is a sort of trampoline on which many species of insects appear to defy gravity as they variously leap or scull or slide across it. A handful of bottom mud washed through a sieve may resolve itself into about two-thirds mud and one-third insect larvae. At appropriate times of the year and day, the air seems to be filled with damselflies and dragonflies, with the rustling wings of millions of mayflies in flight, with dancing swarms of craneflies. And, of course, this insect abundance in aquatic situations is one of the key elements in the trophic cycle of other aquatic organisms including amphibians, reptiles, and, of course, fishes.

Although only a few kinds of insects spend both their adult and immature lives in the water, the aquatic immature stages of the several orders inhabit every type of freshwater environment. At least one or another species has managed to establish itself whenever there is water. Some will tolerate hot springs, others live in streams flowing out of the face of glaciers. One of the chief problems of the mosquito control worker is the speed with which various species of mosquitoes can start to breed in temporary water catchment areas - rain troughs, collections of old rubber tires, empty coconut shells, axils of leaves. Indeed, the bromeliads which are such a conspicuous feature of the tropical flora generally support an entire fauna of insects which is peculiarly associated with them and which numbers species all the way from mosquitoes up to much larger forms such as dragonflies. Aquatic insects have numerous advantages over their terrestrial relatives. They have largely escaped competition and crowding from the hordes of land insects. They have eluded many of the terrestrial predators, although they have exchanged these for fish, frogs, and other aquatic predators. Certainly, they are much less troubled than terrestrial forms by sudden variations in temperature. Some will say that this is one of the reasons why such forms as the dragonflies, which appear to have been one of the first to return from land to water, have changed so little since the earliest fossils appear (this argument also makes the assumption that highly evolved forms, such as the aquatic Diptera, did not return to the water until after a much longer period of terrestrial evolution, however, it is recognized that Diptera as an order evolved in swampy or very wet habitats to which most families are still tied).

It would seem that, of all the challenges that water insects must meet, that of obtaining their air supply is the most pressing. The ancestors of insects lost their gills long ago when they left the sea, and they developed new apparatus for breathing atmospheric rather than water-dissolved oxygen. This apparatus, which is common to all insects today, consists of a network of extremely small air tubes that branch through the interior tissues and open to the outer atmosphere through a row of small holes, called spiracles, on each side of the body. Air reaches the cells of the body by entering these spiracles, and this flow is encouraged by expansion and contraction of the insects body, which acts like a pump. The tiny air tubes are pre-vented from collapsing by being supported with a spiral of had material much as the rubber hose of a vacuum cleaner is reinforced.

It has been estimated that today roughly three percent of all insect species are aquatic for part or most of their lives. These three percent, then have had to develop methods for continuing to utilize oxygen after turning to an aquatic habitat. Some aquatic insect larvae have become fully aquatic by developing gills which enable them to utilize dissolved oxygen, just as do fish. More commonly, larvae and aquatic adults continue to breathe gaseous oxygen, and these forms have developed a number of special ingenious ways for taking an air supply with them under the water. A variety of methods has been developed by totally unrelated species.

The nymphs of mayflies (Ephemeroptera), stoneflies (Plecoptera), and dragonflies (Odonata) still retain the air tubes of land insects, but to them are attached gills that can strain out dissolved oxygen. These gills take on a wide variety of form which may, upon occasion, be useful in taxonomy; and they are attached in a wide variety of sites all the way from the head (the maxillary gills of the mayfly Isonychia) and thorax (in the case of the stoneflies) to the thoracic and abdominal segments in most of the other forms. In the Anisoptera dragonflies, gills are located at the rear of the digestive canal, and expansion and contraction of the body wall pumps water in and out. The gills in the rectal chamber work by simple diffusion of oxygen through their surface and into the air tubes; and the pumping of water in and out enables the dragonfly nymph to move very rapidly by what must be one of the first forms of jet propulsion. Mayfly nymphs utilize gills which are located along the sides of the abdomen. In stoneflies (Plecoptera), the gills are filamentous or fingerlike: the peculiar little fingerlike gill which occurs at the base of the mentum of the stonefly Isogenus; Pteronarcys gills look like tufts of teased-out cotton at the bases of the legs; capniid gills are retracted and barely visible without dissection.

Some of the most interesting respiratory systems have been developed in the aquatic Diptera. The respiratory mechanisms of the phantom midge Chaoborus are unique, forming hydrostatic organs. Mosquitoes of the genus Mansonia have the habit of inserting their highly specialized breathing tube into the tissues of the water hyacinth, usually just about at the point where the roots begin to emerge from the plant at the mud-water interface. The habit of many mosquito larvae of anchoring themselves to the surface film where they could breathe atmospheric oxygen was, of course, taken advantage of in some of the pre-DDT mosquito control methods. The attempt then was made to attain control by spreading thin oil films-usually Number 2 Diesel oil-over the water with the thought that the oil would penetrate the mosquito's breathing system and cause suffocation.

The larva of the soldier fly Stratiomyidae has a fan of hairs that form a complete circle around the end of its long breathing tube. When it comes to the surface, the hairs radiate outward like the points of a starfish, serving to anchor the larva to the surface film, and at the same time open the spiracles. When the larva dives below the surface again, the hairs of this fan curve inward and trap an air bubble which is taken down with the larva to act as a reserve supply.

The mud-inhabiting rattailed maggot Syrphidae has a breathing tube that can be extended almost six inches. This enables the owner to feed on the bottom and at the same time keep in touch with a surface air supply, similar to the air gathering mechanisms of the Hemiptera genera Nepa and Ranatra.

Another interesting aquatic respiratory modification is the so-called gaseous plastron. Those aquatic insects, especially riffle-beetles (Elmidae), which use this method carry a bubble of air around with them in contact with some of their spiracles, and this bubble actually serves as a sort of gill. As the insect consumes the oxygen in the bubble, the oxygen pressure in the bubble decreases until it becomes less than that of the oxygen dissolved in the water around it. At this point oxygen passes from the higher concentration to the lower concentration and replaces the oxygen in the bubble, which has been used up by the insect's respiration. With the oxygen supply continually renewed in this way, the insect obtains from its gaseous plastron, or bubble, many times the amount of oxygen it originally held. The amount of air that many species can take down may be sufficient for only about twenty minutes (many families of adult Coleoptera and Hemiptera), yet Elmids can stay down indefinitely because the supply in the bubble is constantly renewed from the surrounding water.

It should be noted that the buoyancy imparted by this air bubble makes it necessary for some insects which depend on this method to swim constantly to stay sub-merged or if they wish to rest under water, promptly anchor themselves to underwater plants or other submerged objects. Otherwise, they would promptly bob up to the surface.

There are many further adaptations insects have made to gain an advantage in the aquatic habitat which will be examined as we work through the major hexapodan Orders.

Leonard's lecture noted converted to html: 16 January 2001