How Insects Live

E. O. Essig.

For some 250 million years insects have been able to flourish on land and water, in arctic barrens and tropic jungles, in deserts and grassy prairies because they have developed special and wonderful adaptations to meet all the varied conditions of this earth. Not just a few but literally thousands of species, representing practically every order, live together in nearly every ecological niche.

They have survived so long without being greatly altered in size and form or reduced in numbers. In direct competition with all other higher forms of life on land, they stand supreme in numbers of species and individuals. Only some lower microscopic forms like bacteria may outnumber them.

It is easier to describe the remarkable adaptations of insects than it is to explain the reasons for them. It is difficult to know whether the body structures and complicated life histories or the environmental factors were the most important. Many other factors may have entered into the long, slow process of change and adjustment.

Consider first as a factor of survival the dominant position of the female insect. She is the foundation, the perpetuation, the multiplication of the species. In many species she is the all; males do not exist. The termite queens of the fungus-feeding tropical species may live for a half century. Certain Australian queen termites may lay as many as 360 eggs in an hour, 8,640 in a day, 3,153,000 in a year. She may continue without interruption for 25 to 50 years.

Parthenogenesis, or reproduction without fertilization of the female, is not unusual among insects. The most common examples are among the aphids. Certain species have in their life cycle both parthenogenetic and sexual forms. In the tropical and warmer regions no males appear, however, and the females continue year after year producing only their own sex. A similar phenomenon occurs among coccids, or scale insects, and among many weevils, sawflies, gall wasps, bethylid wasps, certain bees, and hymenopterous parasites. Among honey bees, unfertilized females may produce only males. In certain parasites, one species may produce only males and another only females. A South African race of the common honey bee is said to produce parthenogenetically not only males and workers but even queens. Some ants also reproduce by unfertilized eggs. Thus we find that parthenogenetic reproduction results only in males in many of the insects although females are not uncommon, while among aphids and coccids only females are produced.

Polymorphism is a condition in which there are adults of two or more distinct forms of the same sex. There also are polymorphic larval forms. To illustrate, in the aphid genus Periphyllus there are as many as 17 distinct recognizable forms, some of which are so different from the others that they have been mistaken for separate species. Among social insects, especially among termites and ants, polymorphism reaches its peak in the insect world. As many as 12 distinct series of castes and forms occur in the species of Eutermes.

Among the many qualities of insect fitness are the hard, elastic, tough exoskeleton with its powers of renewal and its resistance to corrosive chemicals ; the many protective devices, such as rugosities, hairs, spines, and scales, as well as the folded wings; the many legs; the ability to lose and even to regenerate certain appendages without greatly interfering with life and reproductive processes; the protective coloration and devices for mimicry; the excretion of protective waxes, resins, and offensive glandular materials; poisonous body fluids and gases; stinging hairs and other devices; the specially constructed living quarters in plant tissues, in water, soil, debris; the enveloping and protecting cases of wood, earth, waxes, paper; the webs, cocoons, spittle, nests, galls; the internal parasitic habits on other hosts; the innumerable other means of protection and of escaping natural enemies through the complicated processes of development; the methods of escape by protective coloration and mimicry, death feigning, jumping, snapping, and flight; the aggressiveness exemplified by ants, mosquitoes, bees, wasps; the ability to bite and sting; the ability to reproduce in such numbers as to overcome almost every opposing factor, even including larger animals and human beings.

Those are the weapons with which insects counter man's inroads on forests and native vegetation and other habitats of insects, the cultivation of onetime grasslands, the draining of aquatic breeding places, and the devices of chemical and biological control. That is why insects are rarely exterminated. In fact, many species, benefited by the immense acreages of special farm crops, have actually increased because of man's agricultural activities.

As I said, insects generally have occupied only land surfaces and freshwater areas. On land, they occupy all areas except the permanent arctic and antarctic icecaps. They live in the soil, in fresh and brackish water, and in and on all conceivable kinds of animal and plant materials. Insects, even the aquatic forms, are always associated with the flora and fauna of every region. The associations may be somewhat detrimental to plant and other animal life, or they may be mutually beneficial to both the hosts and the insects; despite the damage by locusts, armyworms, weevils, and other serious pests, plants have not been greatly hampered by them. On the other hand, it is difficult to ascertain to what degree plants may be benefited by insects. Assuredly, we do know that the pollination of many plants can only be accomplished by certain ants, bees, butterflies, moths, flies, beetles, and other insects.

Not all insects are plant feeders. Fleas, lice, gnats, midges, mosquitoes, and some bugs, beetles, thrips, neuropterons, strepsipterons, ants, wasps, and parasitic forms prey on animals. As carriers of diseases to man and to domestic and wild animals, they are of great concern and have spread death and misery over the world through the ages.

The varying conditions of topography, climate, and other forms of life with which they are associated and upon which they subsist undoubtedly also, have influenced their variability of structure and habits. Although the ancestors of insects are thought to have been aquatic, many entomologists believe insects first evolved on land. Another general belief is that insects did not arise until after plants appeared on earth. R. J. Tillyard has said that the earliest fossils have been found in the lower part of the Upper Carboniferous period in North America, and that at a somewhat higher horizon, in the upper part of the Upper Carboniferous, insects occurred abundantly both in North America and in Europe. These insects were aquatic and terrestrial. If insects were ever oceanic, they did not remain so because up to now, at least, they have never invaded the ocean to any extent. Few, if any, truly oceanic aquatic insects feed, for instance, on the abundance of aquatic plants along the shores of all the great oceans. This warehouse of plant food appears not to have been the objective of insect adaptability.

In fresh water, insects are quite at home and often develop in tremendous numbers, as, for example, the mayflies in certain of the Great Lakes. The abundance of dragonflies, stoneflies, caddisflies, water bugs, water beetles, mosquitoes, gnats, and other insects testify to their perfect conformity to an aquatic life. Many of the land and aerial forms also have remarkable aquatic adaptations, such as walking on water and swimming with the wings that ordinarily they use for flight. An example is certain minute hymenopterous species, which parasitize the eggs of water insects.

Probably many more years of study will be needed to explain all of the complex adaptations and interrelations of insects to other insects and animals, plants, and lower organisms. The more general relationships have been known for many years, but only in recent times has the relationship of insects to viruses and other micro-organisms been revealed.

Small size is not the only important asset of insects in their struggle to survive. Their varied methods of locomotion are admirably adapted to their needs. The younger stages of all insects and all stages of many species are wingless. Thus early in life they learn to walk, crawl, run, jump, and hop. Except among the more primitive wingless forms, flying is attained in the mature stages. In some, like aphids, wingless (apterous) and winged (alate) forms appear in the same generations. Then the alates are the dispersing and migrating forms, and the apterae remain more or less fixed in the immediate surroundings of their birth. Among many other insects, only one of the sexes ( the male in some coccids and aphids, for example) may be winged. Throughout practically the whole insect group we find both the winged and wingless condition in the adult forms, as exemplified by such well-developed groups as ants, parasitic hymenoptera, flies, moths, and beetles. In the lower orders, Protura, Thysanura, Aptera, and Collembola, all stages are wingless. Nevertheless, representatives of nearly all of them occur throughout the whole world. Among the higher insects, all members of the orders Mallophaga, Anoplura, and Siphonaptera are wholly apterous. In nearly all the other orders, wingless adults occur. The remarkable thing is the phenomenal degree of variation from any definite standard in their anatomical, physiological, and ecological constitution.

Practically all insects walk or crawl. Six legs enable the adults to move along at a rather even and rapid rate. Certain of the running forms, such as silverfish, cockroaches, psocids, bird lice, bugs, many beetles, flies, and ants may move very fast.

The larvae may be legless (apodous) or have either the normal three pairs of legs, or these and additional prologs, as among caterpillars, sawflies, and horntails. Larvae of weevils, flies, and other members of the order Diptera and some other specialized parasitic forms are apodous. Many of the weevil larvae appear to get about as easily as caterpillars; for them, pseudopods serve as legs.

Certain running insects, like silverfish, cockroaches, and ants, travel almost like lightning and are gone in a flash. Other forms move with a slow and often wobbly gait. Insects that are more or less attached to the host plant, like wingless aphids, move about slowly and depend upon the winged forms for dispersal.

Such forms as the female armored scales can crawl only in the first stage of development. Males eventually develop into fully legged and also winged adults, which seek out and mate with the immobile females.

Many insects jump. Those that do usually also walk and run. Jumping may be a means of locomotion and of escaping from enemies, as exemplified by springtails, crickets, grasshoppers, leafhoppers, fleas, and others.