LEPJDOPTERA . 
217 
tnere exists near the base of the costal margin of the hind 
wings a strong bristle or bunch of bristles named the frenu¬ 
lum , or little bridle (Fig. 237,/). As the frenulum projects 
forward under the fore wing it tends to depress the hind 
wing when the fore wing is depressed, thus insuring the act- 
ing together of the two pairs of wings. Usually the frenulum 
consists of two or more bristles in females and of a single 
stronger bristle in males. The difference is due to the fact 
that in males the bunch of bristles have grown together into 
single strong bristle. There is also another sexual differ¬ 
ence. In the males the tip of the frenulum fits into a mem¬ 
branous hook borne on the lower surface of the fore wing, 
thus firmly tying together the two wings (Fig. 237, /. h). 
This frenulum hook is rarely found in females. In certain 
moths there is, besides the frenulum hook, a tuft of hairs 
projecting forwards from just behind the cubitus of the fore 
wing near its base, which tends also to keep the frenulum in 
place. 
With some moths and with all skippers and all butterflies 
the base of the costal portion of the hind wings, the humeral 
angle as it is termed, is largely developed, so that it projects 
far under the fore wing (Fig. 241). This overlapping of the 
two wings at the base to so great an extent insures their act¬ 
ing together without the aid of the frenulum ; and, conse¬ 
quently, there being no use for a frenulum, this organ has 
disappeared. In other words, the frenulum has been super¬ 
seded by the large development of the humeral angle. But 
as we believe that these moths, skippers, and butterflies have 
descended from forms which had a frenulum, we class them 
with the moths that still possess this organ under the sub¬ 
order Frenatse. 
A more easily observed character which serves to distin¬ 
guish members of this suborder is a striking difference in 
the venation of the two pairs of wings, the hind wings hav¬ 
ing fewer veins than the fore wings. 
