Page:EB1911 - Volume 19.djvu/459

 G. Enderlein. The eleventh abdominal segment which carries the short jointed cerci (fig. 1, 𝑏, 𝑐) may remain in a reduced condition distinct from the tenth. There are only six or eight Malpighian tubes—contrasting with the large number of these excretory organs found in the Orthoptera and Plecoptera.

The Embiidae are feeble, somewhat soft-skinned insects with the prothorax small and the mesothorax and metathorax elongate. The feelers are long and simple, and the wings are very narrow, each with a sub-costal, a radial, a median and a cubital nervure; the branches of the median and the cubital, however, as well as the anal nervures, are vestigial, and there are a few short cross-bars between the radial and the median. Some Embiidae are entirely wingless in the adult state, and it has been suggested that this is always the condition in the female sex. According to the recent investigations of K. W. Verhoeff, the family contains only thirteen known species.

The Embiidae live in warm countries, and are very retiring in their habits, hiding under stones where they spin webs formed of silk produced by glands in the basal segments of the fore-feet.

The Termitidae (so-called “white ants”) are the other family of Isoptera. They are relatively shorter and broader insects than the Embiidae with large prothorax and long wings, which have a transverse line of weakness at the base and are usually shed after the nuptial flight. The Termitidae are numerous in species in warm countries. The vast majority of individuals in a community consist of wingless forms—“workers” and “soldiers,” which are undeveloped members of either sex. Their economy is fully described in a special article on.

Order Corrodentia.

The insects included in this order differ from those of the two preceding orders in their more condensed abdomens which bear no cerci, while the number of Malpighian tubes is reduced to four. In the absence of cerci the Corrodentia are more specialized than the Isoptera and Plecoptera, but some of them show a more primitive character in the retention of vestigial maxillulae—the minute pair of jaws that are found behind the mandibles in the (q.v.). A large proportion of the Corrodentia are wingless. When wings are present the front pair are much larger than the hind pair, and the neuration is remarkable for the concresence of the median with the cubital trunk, and the zigzag course of many of the branches. All the insects of this order are of small size and the cuticle is imperfectly chitinized, so that the body as a whole is soft. The name Corrodentia was first used by H. Burmeister (1832) and has reference to the biting habits of the insects. Originally, however, the Corrodentia included the order which Enderlein has recently separated as Isoptera (see above). As at present restricted, the Corrodentia include two distinct sub-orders.

Copeognatha.—This sub-ordinal name has been applied by Enderlein to the “book-lice.” These frail insects, the majority of

which have wings of the type described above, are further characterized by the presence of minute but distinct maxillulae, while the inner lobe (lacinia) of the first maxilla is an elongate, hard structure (the “pick,” fig. 3, 𝑒) and the outer lobe is convex and soft. The labial (second maxillary) palps are reduced to small, rounded prominences external to the still smaller prominences that represent the lobes (fig. 3, 𝑐). The feelers of these insects are elongate and thread-like, consisting of from a dozen to nearly thirty segments. The prothorax is very small.

The book-lice are familiar wingless insects, often found in houses running about among old papers and neglected biological collections. They belong to the family Psocidae which has a few score species—most of them winged—living out of doors on the bark of trees and among vegetable refuse. In some Psocidae the wings are in a vestigial state, and the fully winged species rarely if ever fly. H. A. Hagen observed that some genera possess wing-like outgrowths on the prothorax, comparable to those seen in certain insects of the Carboniferous Period. The Psocidae themselves have not been traced back beyond the Oligocene, in the amber of which period their remains are fairly numerous.

Mallophaga.—This term was first applied by C. L. Nitzsch (1818) to the degraded wingless parasites (fig. 4) commonly known as bird-lice or biting-lice, differing from the true lice (see, ) by their jaws adapted for biting (not for piercing or sucking). By their structure they are evidently allied to the Copeognatha. They are abundantly distinct, however, through the short feelers with only three to five segments and the conspicuous prothorax. The head is relatively very large, but the eyes are degraded and often absent. A remarkable feature is the frequent concrescence of mesothorax and metathorax and in some cases, even, their fusion with the anterior abdominal segments. The legs are stout and spiny, and well adapted for clinging to the hair or feathers of the host animal. It is usual to divide the Mallophaga into two families—the Liotheidae, possessing labial palps and two foot-claws, being fairly active insects, which are capable, on the death of their host, of seeking another, and the Philopteridae, without labial palps and with a single foot-claw modified for clasping (fig. 4) which never leave the host and perish themselves soon after its death.

This order includes the single family of the Ephemeridae or May-flies. The name, although quite recently proposed by A. E. Shipley, should be used rather than A. S. Packard’s older term Plectoptera on account of the great liability of confusion between the latter and Plecoptera. The May-flies are remarkably primitive in certain of their characters, notably the elongate cerci, the paired, entirely mesodermal genital ducts, and the occurrence of an ecdysis after the acquisition of functional wings. On the other hand, the reduced feelers, the numerous Malpighian tubes (40), the large complex eyes, the vestigial condition of the jaws, the excessive size of the fore-wings as compared with the hind-wings and their complex neuration with an enormous number of cross-nervules are all specializations. So in some respects is the life-history, with a true larval preparatory stage, unlike the parent form, and living an aquatic life, breathing dissolved air by means of a paired series of abdominal tracheal gills. Except for its aquatic adaptations, however, the ephemerid larva is wonderfully thysanuran in character, and possesses conspicuous and distinct maxillulae. See special article on.

Order Odonata.

The distinctness of the dragon-flies from other insects included in Linnaeus’s Neuroptera was long ago recognized by J. C. Fabricius, who proposed for them the ordinal name of Odonata (1775). They resemble the May-flies in their “hemimetabolous” life-history; the young insects are markedly unlike their parents, inhabiting fresh water and breathing dissolved air, either through tracheal gills at the tip of the abdomen, or by a branching system of air-tubes on the walls of the rectum into which water is periodically admitted. The winged insects resemble the May-flies in their short feelers and in the large number (50 to 60) of their Malpighian tubes, but differ most strikingly from those insects in their strong well-armoured bodies, their powerful jaws adapted for a predaceous manner of life, and the close similarity of the hind-wings to the fore-wings. All the wings are of firm, glassy texture, and very complex in their neuration; a remarkable and unique feature is that a branch of the radius (the radial sector) crosses the median nervure, while, by the development of multitudinous cross-nervules, the wing-area becomes divided into an immense number of small areolets. The tenth abdominal segment carries strong, unjointed cerci, while the presence of reproductive armature on the second abdominal segment