Page:The Zoologist, 4th series, vol 2 (1898).djvu/323

Rh familiar that to most British entomologists all deviations from it seem artificial and unnatural.

In a sound system of classification the various groups must be capable of actual diagnostic definition in words, or the system is unworkable; they must be defined by structure, or it is misleading; and the system must be based on the study of the phylogeny (or scheme of ancestral descent), or it is artificial. Of late years some progress has been made in classifying the Lepidoptera in accordance with these principles, and as the general lines of such a classification are now fairly well established, it may be of interest to give a sketch of the results already reached, as illustrating principles of universal application.

Four-winged insects usually have some contrivance for holding together the two wings on one side, and ensuring their common action. Thus in the Hymenoptera (bees, &c.) there is a row of hooks and eyes; in the Trichoptera (caddis-flies) there is a membranous process (jugum) from the dorsum of the fore wings near the base, which projects beneath the edge of the hind wings, whilst the following part of the dorsum extends above it; again in the Lepidoptera there is normally a stout bristle or group of bristles (frenulum) rising from the edge of the hind wings near the base, and passing under a catch on the under side of the fore wings. Some Lepidoptera do not possess this frenulum, and in such cases the basal angle of the hind wings is made more prominent so as to project beneath the base of the fore wings and prevent dislocation. But some five years ago Prof. Comstock made the discovery that in two families, the Hepialidæ and the Micropterygidæ, instead of the usual lepidopterous structure, there is a jugum, quite as in the Trichoptera. There is no difficulty in seeing this structure, at any rate in the Hepialidæ, some of which are very large insects of five or six inches expanse of wing, and it remained undiscovered so long simply because no one had thought of looking for it; a striking instance of the ease with which characters of the highest importance can be over* looked by competent observers, unless their attention is specially directed towards them. Now it was known previously that these same two families agree together in possessing several additional veins in the hind wings, which are not found in any other Lepidoptera; these veins could not have been evolved from non-