Insects, Their Ways and Means of Living/Chapter III
Chapter III
Roaches and Other Ancient Insects
We used to speak quite confidently of time as something definite, measurable by the clock, and of a year or a century as specific quantities of duration. In this present age of relativity, however, we do not feel so certain about these things. Geologists calculate in years the probable age of the earth, and the length of time that has elapsed since certain events took place upon it, but their figures mean only that the earth has gone around the sun approximately so many times during the interval. In biology it signifies nothing that one animal has been on the earth for a million years, and another for a hundred million, for the unit of evolution is not a year, but a generation. If one animal, such as most insects, has from one to many generations every year, and another, such as man, has only four or five in a century, it is evident that the first, by evolutionary reckoning, will be vastly older than the second, even though the two have made the same number of trips with the earth around the sun. An insect that antedates man by several hundred million years, therefore, is ancient indeed.
The roach scarcely needs an introduction, being quite well known to all classes of society in every inhabited part of the world. That he has long been established in human communities is shown by the fact that the various nations have bestowed different names upon him. His common English name of "cockroach" is said to come from the Spanish, cucaracha. The Germans call him, rather disrespectfully, Küchenschabe, which signifies "kitchen louse." The ancient Romans called him Blatta, and on this his scientific family name of Blattidae is based. A small species of Europe, named by the entomologists
Fig. 49. The four species of common household roaches
A, the German roach, or Croton bug, Blattella germanica (length 9⁄16 inch). B, the American cockroach, Periplaneta americana (length 1⅜ inches). C, the Australian cockroach, Periplaneta australasiae (length 1¼ inches). D, the wingless female of the Oriental roach, Blatta orientalis (length 1⅛ inches). E, the winged male of the Oriental roach (length 1 inch)
Blattella germanica, which is now our most common American roach, received the nickname of "Croton bug" in New York, because somehow he seemed to spread with the introduction of the Croton Valley water system, and this appelation has stuck to him in many parts of the country.
The Croton bug, or German roach (Fig. 49 A), is the smallest of the "domestic" varieties of roaches. It is that rather slender, pale-brown species, about five-eighths of an inch in length, with the two dark spots on the front shield of its body. This roach is the principal pest of the kitchen in the eastern part of the United States, and prob-
Fig. 50. Egg cases of five species of roaches. (Twice natural size)
A, egg case of the Australian roach (fig. 49 C). B, that of the American roach (fig. 49 B); the other three are made by out-of-door species
ably the best support of the trade in roach powders. Several other larger species are fortunately less numerous, but still familiar enough. Among these are one called the American roach (Fig. 49 B), a second known as the Australian roach (C), and a third as the Oriental roach (D, E). These four species of cockroaches are all great travelers and recognize no ties of nationality. They are equally at home on land and at sea, and, as uninvited passengers on ships, they have spread to all countries where ships have gone.
Besides the household roaches, there are great numbers of species that live out of doors, especially in warm and tropical regions. Most of these are plain brown of various shades, or blackish, but some are green, and a few are spotted, banded, or striped. Different species vary much in size, some of the largest reaching a length of four inches, measured to the tips of the folded wings, while the smallest are no longer than three thirty-seconds of an inch in length. They nearly all have the familiar flattened form, with the head bent down beneath the front part of the body, and the long, slender antennae projecting forward. Most species have wings which they keep closely folded over the back. In the Oriental roach, the wings of the female are very short (Fig. 49 D), a character which gives them such a different appearance from the males (E) that the two sexes were formerly supposed to be different species.
The roach, of course, was not designed to be a household insect, and it lived out of doors for ages before man constructed dwellings, but it happens that its instincts and its form of body particularly adapt it to a life in houses. Its keen sense, its agility, its nocturnal habits, its omnivorous appetite, and its flattened shape are all qualities very fitting for success as a domestic pest.
Many kinds of roaches give birth to living young; but most of our common species lay eggs, which they inclose in hard-shelled capsules. The material of the capsule is a tough but flexible substance resembling horn, and is produced as a secretion by a special gland in the body of the female opening into the egg duct. The capsule is formed in the egg duct, and the eggs are discharged into it while the case is held in the orifice of the duct. When the receptacle is full its open edge is closed, and the eggs are thus tightly sealed within it. The sealed border is finely notched, and transverse impressions on the surface of the capsule indicate the position of the eggs within it.
The Croton bug, or German roach (Fig. 49 A), makes a small fiat tabloid egg case, which the female usually carries about with her for some time projecting from the end of her body, and sometimes the eggs hatch while she is still carrying the case. The American and Australian roaches (Fig. 49 B, C) make egg cases much resembling miniature pocketbooks or tobacco pouches, about three-eighths or half an inch in length, with a serrated clasp along the upper edge (Fig. 50 A, B). The cases of some of the smaller species of roaches are only one-sixteenth of an inch long
Fig. 51. Young of the German roach, or Croton bug (fig. 49 A), in various stages just before and after hatching
A, the young roach in the egg just before hatching. B, the young roach just after hatching, shedding its embryonic covering membrane. C, young roach after shedding the embryonic covering. D, the same individual half an hour old
(C), while larger species may make a case three-quarters of an inch in length (E).
The embryo roaches mature within the eggs, and when they are ready to hatch they emerge inside the egg case. By some means, the roughened edge of the case where it was last closed is opened to allow the imprisoned insects to escape. Small masses of the tiny creatures now bulge out, and finally the whole wriggling contents of the capsule is projecting from the slit. First one or two individuals free themselves, then several together fall out, then more of them, until soon the case containing the empty eggshells is deserted.
When the young roaches first liberate themselves from the capsule, they are helpless creatures, for each is contained in a close-fitting membrane that binds its folded legs and antennae tightly to the body and keeps the head pressed down against the breast (Fig. 51 A). The inclosing sheath, however, a film so delicate as to be almost invisible, is soon burst by the struggling of the little roach anxious to be free—it splits and rapidly slides down over the body (B), from which it is at last pushed off. The shrunken, discarded remnant of the skin is now such an insignificant flake that it scarce seems possible it so recently could have enveloped the body of the insect.
The newly liberated young roach dashes off on its slim legs with an activity quite surprising in a creature that has never had the use of its legs before. It is so slender of figure (Fig. 51 C) that it does not look like a roach, and it is pale and colorless except for a mass of bright green material in its abdomen. But, almost at once, it begins to change; the back plates of the thorax flatten out, the body shortens by the overlapping of its segments, the abdomen takes on a broad, pear-shaped outline, the head is retracted beneath the prothoracic shield, and by the end of hall an hour the little insect is unmistakably a young cockroach (D).
The roaches have a potent enemy in the house centipede, that creature of so many legs (Fig. 52) that it looks like an animated blur as it occasionally darts across the living-room floor or disappears in the shades of the basement before you are sure whether you have seen something or not, but which is often trapped in the bathtub, where its appearance is likely to drive the housewife into hysteria. Unless you are fond of roaches, however, the house centipede should be protected and encouraged. The writer once placed one of these centipedes in a covered glass dish containing a female Croton bug and a capsule of her eggs which were hatching. No sooner were the young roaches running about than the centipede began a feast which ended only when the last of the brood had been devoured. The mother roach was not at the time molested, but next morning she lay dead on her back, her head severed and dragged some distance from the body, which was sucked dry of its juices—mute evidence of the tragedy that had befallen sometime in the night, probably when the pangs of returning hunger stirred the centipede to renewed activity. The house centipede does not confine itself to a diet of live roaches, for it will eat almost any kind of food, but it is never a pest of the household larder.
Fig. 52. The common house centipede, Scutigera forceps (natural size), a destroyer of young roaches
Most species of roaches have two pairs of well-developed wings, which they ordinarily keep folded over the back, for in their usual pursuits the domestic species do not often fly, except occasionally when hard pressed to avoid capture. The front wings are longer and thicker than the hind wings, and are laid over the latter, which are thin and folded fanwise when not in use. In these characters the roaches resemble the grasshoppers and katydids, and their family, the Blattidae, is usually placed with these insects in the order Orthoptera.
The wings of insects are interesting objects to study. When spread out flat, as are those of the roach shown in Figure 53, they are seen to consist of a thin membranous tissue strengthened by many branching ribs, or veins, extending outward from the base. The wings of all insects are constructed on the same general plan and have the primary veins; but, since the great specialty of insects is flight, in their evolution they have concentrated on the wings, and the different groups have tried out different styles of venation, with the result that now each is distinguished by some particular pattern in the arrangement of the veins and their branches. The entomologist can thus not only distinguish by their wing structure the various orders of insects, as the Orthoptera, the dragonflies, the moths, the bees, and the flies, but in
Fig. 53. Wings of a cockroach, Periplaneta, showing the vein pattern characteristic of the roach family
many cases he can identify families and even genera. Particularly are the wings of value to the student of fossil insects, for the bodies are so poorly preserved in most cases that without the wings the paleontologist could have made little headway in the study of insects of the past. As it is, however, much is known of insects of former times, and a study of their fossil remains has contributed a great deal to our knowledge of this most versatile and widespread group of animals.
The paleontological history of life on the earth shows us that the land has been inhabited successively by different forms of animals and plants. A particular group of creatures appears upon the scene, first in comparative insignificance; then it increases in numbers, in diversity of forms, and usually in the size of individuals, and may become the dominant form of life; then again it falls back to insignificance as its individuals decrease in size, its species in numbers, until perhaps its type becomes extinct. Meanwhile another group, representing another type of structure, comes into prominence, flourishes, and declines. It is a mistake, however, to get the impression that all forms of life have had this succession of up and down in their history, for there are many animals that have existed with little change for immense periods of time.
The history of insects gives us a good example of permanence. The insects must have begun to be insects somewhere in those remote periods of time before the earliest known records of animals were preserved in the rocks. They must have been present during the age when the water swarmed with sharks and great armored fishes; they certainly flourished during the era when our coal beds were being deposited; they saw the rise of the huge amphibians and the great reptilian beasts, the Dinosaurus, the Ichtyosaurus, the Plesiosaurus, the Mosasaurus, and all the test of that monster tribe whose names are now familiar household words and whose bones are to be seen in all our museums. The insects were branching out into new forms during the time when birds had teeth and were being evolved from their reptile ancestors, and when the flowering plants were beginning to decorate the landscape; they were present from the beginning of the age of mammals to its culmination in the great fur-bearing creatures but recently extinct; they attended the advent of man and have followed man's whole evolution to the present time; they are with us yet—a vigorous race that shows no sign of weakening or of decrease in numbers. Of all the land animals, the insects are the true blue-blood aristocrats by length of pedigree.
The first remains of insects known are found in the upper beds of the rocks laid down in the geological period of the earth's history known as the Carboniferous. Dur-
Fig. 54. A group of common Carboniferous plants reaching the size and proportions of large trees. (From Chamberlin and Salisbury, drawn by Mildred Marvin from restorations of fossil specimens.) Courtesy of Henry Holt & Co.
Of the two large trees in the foreground, the one on the left is a Sigillaria, that on the right a Lepidodendron; of the two large central trees in the background the left is a Cordaites, the right a tree fern; the rail stalks in the outermost circle are Calamites, plants related to our horsetail ferns
ing Carboniferous times much of the land along the shores of inland seas or lakes was marshy and supported great forests from which our coal deposits have been formed. But the Carboniferous landscape would have had a strange and curious look to us, accustomed as we are to an abundance of hard-wood, leafy trees and shrubs, and a multitude of flowering plants. None of these forms of vegetation had yet appeared.
Much of the undergrowth of the Carboniferous swamps was composed of fernlike plants, many of which were, indeed, true ferns, and perhaps the ancestors of our modern brackens. Some of these ancient ferns grew to a great size, and rose above the rest in treelike forms, attaining a height of sixty feet and more, to branch out in a feathery crown of huge spreading fronds. Another group of plants characteristic of the Carboniferous flora comprised the seed ferns, so named because, while closely resembling ferns in general appearance, they differed from true ferns in that they bore seeds instead of spores. The seed ferns were mostly small plants with delicate, ornate leaves, and they have left no descendants to modern times.
Along with the numerous ferns and seed ferns in the Carboniferous swamps, there were gigantic club mosses, or lycopods, which, ascending to a height sometimes of much more than a hundred feet, were the conspicuous big trees in the forests of their day (Fig. 54). These lycopods had long, cylindrical trunks covered with small scales arranged in regular spiral rows. Some had thick branching limbs starting from the upper part of the trunk and closely beset with stiff, sharp-pointed leaves; others bore at the top of the trunk a great cluster of long slender leaves, giving them somewhat the aspect of a gigantic variety of our present-day yucca, or Spanish bayonet. The bases of the larger trees expanded to a diameter of three or four feet, and were supported on huge spreading underground branches from which issued the roots—a device, perhaps, that gave them an ample foundation in the soft mud of the swamps in which they grew.
The Carboniferous lycopods furnished most of our coal, and then, in later times, their places were taken by other types of vegetation. But their race is not yet extinct, for we have numerous representatives of them with us today in those lowly evergreen plants known as club mosses, whose spreading, much-branched limbs, usually trailing on the ground, are covered by rows of short, stiff leaves. The most familiar of the club mosses, though not a typical species, is the "ground pine." This humble little shrub, so much sought for Christmas decoration, still in some places carpets our woods with its sort, broad, frondlike stems. In the fall when its rich dark green so pleasingly contrasts with the somber tones of the season's dying foliage, it seems to be an expression of the vitality that has preserved the lycopod race through the millions of years which have elapsed since the days of its great ancestors. The "resurrection plant," often sold to housekeepers under false or exaggerated claims of a marvelous capacity for rejuvenation, is also a descendant of the proud lycopods of ancient times.
In our present woodlands, along the banks of streams or in other moist places, there grows also another plant that has been preserved to us from the Carboniferous forests—the common "horsetail fern," or Equisetum, that green, rough-ribbed stalk with the whorls of slender branches growing from its joints. Out equisetums are modest plants, seldom attaining a height of more than a few feet, though in South American countries some species may reach an altitude of thirty feet; but in Carboniferous times their ancestors grew to the stature of trees (Fig. 54) and measured their robust stalks with the trunks of the lycopods and giant ferns.
Aside from the numerous representatives of these several groups of plants, all more or less allied to the ferns, the Carboniferous forests contained another group of treelike plants, called Cordaites, from which the cycads of later times and out present-day maidenhair tree, or ginko, are probably descended. Then, too, there were a few representatives of a type that gave origin to out modern conifers.
It is probable that a visitor to those days of long ago might give us a more complete account of the vegetation that grew in the Carboniferous swamps than can be known from the records of the rocks, but the paleobotanist has a wealth of material now at hand sufficient to give us at least a pretty reliable picture of the setting in which the earliest of known insects lived and died.
And now, what were the insects like that inhabited the forests of those early times? Were they, too, strangely fashioned creatures, fit denizens of a far-off fairyland? No, nothing of the sort, at least not in appearance or structure, though "fit" they probably were, from a physical standpoint, for insects are fitted to live almost anywhere. In short, the Carboniferous insects were principally roaches! Yes, those woods and swamps of millions of years ago were alive with roaches little different from our own familiar household pests, or from the numerous species that have not forsaken their native habitats for life in the cities.
Whoever looks to the geological records for evidence of the evolution of insects is sorely disappointed, for even in the venation of the wings those early roaches (Fig. 55) were almost identical with our present species (Fig. 53). As typical examples of the Carboniferous roaches, the species shown in Figure 55 serve well, and anyone can see, even though the specimens lack antennae and legs, that the creatures were just common roaches. Hence, we can easily picture these ancient roaches scuttling up the tall trunks of the scaly lycopods, and shuffling in and out among the bases of the close-set leaf stems of the tree ferns, and we should expect to find an abundant infestation of them in the vegetational refuse matted on the ground. Insects of those days must have been comparatively free from enemies, for birds did not yet exist, and all that host of parasitic insects that attack other insects were not evolved until more recent times.
Though by far the greater number of the Carboniferous insects known are roaches, or insects closely related to roaches, there were many other forms besides. Some of these are of particular interest to entomologists because, in some ways, they are more simple in structure than are
Fig. 55. Fossil cockroaches from Upper Carboniferous rocks A, Asemoblatta mazona, found in Illinois, length of wing one inch. (From Handlirsch after Scudder.) B, Phyloblatta carbonaria, found in Germany. (From Handlirsch)
any of the modern insects, and in this respect they apparently stand closer to the hypothetical primitive insects than do any others that we know. And yet, the characters by which these oldest known insects, called the Paleodictyoptera, differ from modern forms are so slight that they would scarcely be noticed by anyone except an entomologist; to the casual observer, the Paleodictyoptera would be just insects. Their chief distinguishing marks are in the pattern of the wing venation, which is more symmetrical than in other winged insects, and, therefore, probably closer to that of the primitive ancestors of all the winged insects. These ancient insects probably did not fold the wings over the back, as do most present-day insects, showing thus another primitive character, though not a distinctive one, since modern dragonflies (Fig. 58) and mayflies (Fig. 60) likewise keep the wings extended when at rest.
The question of how insects acquired wings is always one of special interest, since, while we know perfectly well that the wing of a bird or of a bat is merely a modified fore limb, the nature of the primitive organ from which the insect wing has been evolved is still a mystery. The Paleodictyoptera, however, may throw light upon the subject, for some of them had small flat lobes on the lateral edges of the back plate of the prothorax, which in fossil specimens look like undeveloped wings (Fig. 56). The presence of these prothoracic lobes, occurring as they do in some of the oldest known insects, has suggested the
Fig. 56. Examples of the earliest known fossil insects, called the Paleodictyoptera, having small lobes (a) projecting like wings from the prothorax
A, Stenodictya lobata (from Brongniart). B, Eubleptus danielsi (drawn from specimen in U. S. Nat. Mus.): T1, T2, T3, back plates of three thoracic segments
idea that the true wings were evolved from similar flaps of the mesothorax and metathorax. If so, we must picture the immediate ancestors of the winged insects as creatures provided with a row of three flaps on each side of the body projecting stiffly outward from the edges of the thoracic segments. Of course, the creatures could not actually fly with wings of this sort, but probably they could glide through the air from the branches of one tree to another as well as can a modern flying squirrel by means of the folds of skin stretched along the sides of its body between the fore and the hind legs. If such lobes then became flexible at their bases, it required only a slight adjustment of the muscles already present in the body to give them motion in an up-and-down direction; and the wings of modern insects, in most cases, are still moved by a very simple mechanism which has involved the acquisition of few extra muscles.
It appears, however, that three pairs of fully-developed wings would be too many for mechanical efficiency. In the later evolution of insects, therefore, the prothoracic lobes were never developed beyond the glider stage, and in all modern insects this first pair of lobes has been lost. Furthermore, it was subsequently found that swift flight is best attained with a single pair of wings; and nearly all the more perfected insects of the present time have the hind pair of wings reduced in size and locked to the front pair to insure unity of action. The files have carried this evolution toward a two-winged condition so far that they have practically achieved the goal, for with them the hind wings are so greatly reduced that they no longer have the form or function of organs of flight, and these insects, named the Diptera, or two-winged insects, fly with one highly specialized and efficient pair of wings (Fig. 167).
The Paleodictyoptera became extinct by the end of the Carboniferous period, and their disappearance gives added support to the idea that they were the last survivors of an earlier type of insect. But they were by no means the primitive ancestors of insects, for, in the possession of wings alone, they show that they must have undergone a long evolution while wings were in the course of development; but of this stage in the history of insects we know nothing. The rocks, so far as has yet been revealed, contain no records of insect life below the upper beds of the Carboniferous deposits, when insects were already fully winged. This fact shows how cautious we must be in making negative statements concerning the extinct inhabitants of the earth, for we know that insects must have lived long before we have evidence of their existence. The absence of insect fossils earlier than the Carboniferous is hard to explain, because for millions of years the remains of other animals and plants had
Fig. 57. Machilis, a modern representative of ancient insects before the development of wings. (Length of body 5⁄16 inch)
been preserved, and have since been round in comparative abundance. As a consequence, we have no concrete knowledge of insects before they became winged creatures evolved almost to their modern form.
At the present time there are wingless insects. Some of them show clearly that they are recent descendants from winged forms. Others suggest by their structure that their ancestors never had wings. Such as these, therefore, may have come down to us by a long line of descent from the primitive wingless ancestors of all the insects. The common "fish moth," known to entomologists as Lepisma, and its near relation, Machilis (Fig. 57), are familiar examples of the truly wingless insects of the present time, and if their remote ancestors were as fragile and as easily crushed as they, we may see a reason why they never left their impressions in the rocks.
Along with the Carboniferous roaches and the Paleodictyoptera, there lived a few other kinds of insects, many of which are representative of certain modern 
Fig. 58. Dragonflies (Order Odonata), modern representatives of an ancient group of winged insects. The adults are strong fliers, catching other insects on the wing; when at rest the wings are held straight out from the body. The young live in the water (fig. 59)
groups. Among the latter were dragonflies, and some of these must have been of gigantic size, for insects, because they attained a wing expanse of fully two feet, while the largest of modern dragonflies do not measure more than eight inches across the expanded wings. But the length of wing of the extinct giant dragonflies does not necessarily mean that the bulk of the body was much greater than that of the largest insects living today. In general, the insects of the past were of ordinary size, the majority of them probably matching with insects of the present time.
The modern dragonflies (Fig. 58) are noted for their rapid flight and for the ability to make instantaneous changes in the direction of their course while flying. These qualities enable them to catch other insects on the wing, which constitute their food. Their wings are provided with sets of special muscles, such as other insects do not possess, showing that the dragonflies are descended along a line of their own from their Carboniferous progenitors. They still retain a character of their ancestors in that they are unable to fold the wings flat over the back in the manner that most other insects fold their wings when they are not using them. The larger dragonflies hold the wings straight out from the sides of the body when at rest (Fig. 58); but a group of slender dragonflies, known as the damselflies (Plate 1, Fig. 2.), bring the wings together over the back in a vertical plane.
The dragonflies are usually found most abundantly in the neighborhood of open bodies of water. Over the unobstructed surface of the water the larger species find a convenient hunting ground; but a more important reason for their association with water is that they lay their eggs either in the water or in the stems of plants growing in or beside it. The young dragonflies (Fig. 59) are aquatic and must have an easy access to water. They are homely, often positively ugly, creatures, having none of the elegance of their parents. They feed on other living creatures which their swimming powers enable them to pursue, and which they capture by means of grasping hooks on the end of their extraordinarily long underlip (Fig. 134 A), which can be shot out in front of the head (B). The great swampy lakes of Paleozoic times must have furnished an ideal habitat for dragonflies, and it is probable that the most ancient dragonflies known had a structure and habits not very different from those of modern species.
Fig. 59. A young dragonfly, an aquatic creature that leaves the water only when ready to transform into the adult (fig. 58)
Another very common insect of the present time, which appears likewise to be a direct descendant of Paleozoic ancestors, is the mayfly (Fig. 60). The young mayflies (Fig. 61) also live in the water, and are provided with gills for aquatic breathing, having the form of flaps or filaments situated in a row along each side of the body. The adults (Fig. 60) are very delicate insects with four gauzy wings, and a pair of long threadlike tails projecting from the rear end of the body. At the time of their transformation they often issue in great swarms from the water, and they are particularly attracted to strong lights. For this reason large numbers of them come to the cities at night, and in the morning they may be seen sitting about on walls and windows, where they find themselves in a situation totally strange to their native habits and instincts. The mayflies do not fold their wings horizontally, but when at rest bring them together vertically over the back (Fig. 60). In this respect they, too, appear to preserve a character of their Paleozoic ancestors; though it must be observed that the highly evolved modern butterflies close their wings in the same fashion.
The roaches, the dragonflies, and the mayflies attest the great antiquity of insects, for since these forms existed practically as they are today in Paleozoic times, the primitive ancestors of all the insects, of which we have no remains in the geological records, must have lived in times vastly more remote. However, though we may search in vain the paleontological records for evidence of the origin and early development of insects, the subsequent evolution of the higher forms of modern insects is clearly shown by the species preserved in eras later
Fig. 60. A mayfly, representative of another order of primitive winged insects having numerous relations in Paleozoic times. (Twice natural size)
than the Carboniferous. Such insects as the beetles, the moths, the butterflies, the wasps, the bees, and the flies are entirely absent in the older rocks, but make their appearance at later periods or in comparatively recent times, thus confirming the idea derived from a study of their structure that they have been evolved from ancestors more closely resembling the paleodictyopteran types of the Carboniferous beds.
The long line of descent of the roach, with almost no change of form or structure, furnishes material for a special lesson in evolution. If evolution has been a matter of survival of the fittest, the roach, judged by survival, must be a most fit insect. Its fitness, however, is of a general nature; it is one that adapts the roach to live successfully in many kinds of conditions and circumstances. Most other forms of modern insects have been evolved through an adaptation to more special kinds of habitats and to particular ways of living or of feeding. Such insects we say are specialized, while those exemplified in the roach are said to be generalized. Survival, therefore, may depend either on generalization or on specialization. Generalized forms of animals have a better chance of surviving through a series of changing conditions than has an animal which is specifically adapted to one kind of life, though the latter may have an advantage as long as conditions are favorable to it.
Fig. 61. A young mayfly, a water-inhabiting creature. (One-half larger than natural size)
The roaches, therefore, have survived to present times, and will probably live as long as the earth is habitable, because, when driven from one environment, they make themselves at home in another; but we have all seen how the specialized mosquito disappears when its breeding places are destroyed. From this consideration we can draw some consolation for the human race, if we do not mind likening ourselves to roaches; for, as the roach, man is a versatile animal, capable of adapting himself to all conditions of living, and of thriving in extremes.