MOTHS AND THEIR CLASSIFICATION. 291 
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 mis- 
leading; 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 Lepi- 
doptera 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 mem- 
branous 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 Hepialide and the 
Micropterygide, 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 Hepialide, 
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 Lepi- 
doptera; these veins could not have been evolved from non- 
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