Doc Leonard's Lecture Notes: Introduction to the Insects and the Aquatic Orders
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.
W. J. Holland, the author of the classic works on moths, ended his volume with:
"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.
Leonard's lecture notes converted to html: 16 January 2001 (EB)
Page last edited: January 24, 2005