SOME PRIMITIVE GROUPS
Insects go back in geologic history to the middle of the Paleozoic age, and their remains are numerous and much differentiated in Carboniferous rocks, when the orders Aptera, Orthoptera, Neuroptera, and Hemiptera (the last represented in the Silurian by ancestral forms of the bedbug and the cockroach—the oldest fossils yet discovered) were flourishing. The beetles and ants first appear in the Trias, the true flies, in the Jurassic, and the butterflies and moths, wasps, and bees not until the Tertiary. This indicates an evolutionary progress in structure with advancing time, as elsewhere in biology. The most primitive type (Aptera) is still with us in the skipping silver fish and snow fleas, or spring-tails, that annoy us in various situations. They are wingless, very simple in organization, and without any larval metamorphosis. Not much better are the Mayflies, or dayflies (Ephemeridæ), that sometimes in early summer arise in enormous numbers from lake shores and rivers, and then quickly disappear. Most of them live, in truth, only a single day (or night), a single one of the many American species surviving three weeks. During their brief life the female drops into water several hundred eggs where they presently hatch into swimming or crawling larvæ that next year, or perhaps not until the third spring, creep out on land, molt, and fly abroad in ephemeral crowds.
It is not a long step from these Mayflies to the dragon flies and damsel flies (Odonata), which also belong to the water spaces of the country, and are among the most interesting of all the insect tribes, and the most beautiful, as they dart and curvet over the surface of some glassy pool that reflects the steel-blue or peacock-green sheen of their long slender bodies, and the black bars that alone make their narrow and almost transparent wings visible. They are known by many ridiculous names, as "darning needles," "snake doctors," etc., but there is no harm in them; on the contrary they are to be encouraged, for they consume, especially in their larval stages in the water, a vast number of mosquitoes, gnats, and other troublesome "bugs." The adults capture their food on the wing, and are hawklike in the agility with which they turn and dodge in pursuit of their active prey. The actual catching is done with the feet, which curve far forward, and are studded with spines that give a sure grip on anything caught between them; they assist, too, in clinging to plants, but the legs are ill-adapted to walking. The wings are very powerful; are of a glassy texture, and never folded; they are crossed by a great many veins, breaking the surface into innumerable small squarish areas, and bear markings that distinguish each of the two or three hundred North American species.
Dragon flies, and their cousins, the smaller and more graceful, low-flying damsel flies, pair as a rule in flight. In some of the families the female descends below the surface of the water, and is able by special apparatus to insert her eggs beneath the skin of a plant; others place them in plant stems above the water, or simply drop them at the surface, whence they sink to the bottom. The "nymphs," as aquatic larvæ like this, with incomplete metamorphosis, are termed, go about preying on anything they can seize and eat, and possess some very peculiar temporary adaptations to their underwater career. After a time the nymph (which is the "dragon" in dragon fly) changes from a rather slender to a broad and flattened creature and crawls out of the water. Soon its skin splits, and an adult dragon fly emerges.
Closely allied to the dragon flies are the stone flies, or alder flies (Percoptera), whose ugly and predacious nymphs are so well known to anglers as "dobsons," "crawlers," and by many other local and opprobrious names, because they make excellent bait for still-water fishing. The adult is that great, thin-winged creature called "hellgrammite" (Corydalis cornuta), with a wing spread of four inches, and possessed in the female of powerful biting jaws, which, as in all insects, work horizontally and not up and down as among vertebrates. In the male the jaws are extended into long, curved, piercing organs which cross when at rest, and which are fully an inch in length, but fortunately they are not used as jaws, but for holding.
Related to these is a group of well-known insects belonging to the old order Neuroptera, but now placed in separate orders, all with lacelike wings and an incomplete metamorphosis. They include the ant lions, the useful aphis lions, the scorpion flies (Panorpa), the lovely lace-winged flies, and the caddis flies, which make larval cases of bits of stick, or of shells or fragments of stone, in the bottoms of rapid streams. From somewhere in this group, probably, the ancestral Lepidoptera branched off to develop into the butterflies and moths of the present day. Next to them are the earwigs (Dermaptera), beetlelike insects very conspicuous in Europe, but little noticed in this country.