3l6 LEPIDOPTERA chap. 



wing always has a large shoulder, or area, anterior to its point 

 of insertion. In most moths this shoulder is absent, but in its 

 place there are one or more stiff bristks projecting forwards and 

 outwards, and passing under a- little membranous flap, or a tuft 

 of thick scales on the under face of the front wing ; the bristle is 

 called the " frenulum," the structure that retains it a " retinaculum." 

 In C'astnia (Fig. 162) and in some Sphingidae there is the un- 

 usual condition of a highly-developed shoulder (s) coexisting with 

 a perfect frenulum (/) and retinaculum (r). The frenulum and 

 retinaculum usually differ in structure, and the retinaculum in 

 position, in the two sexes of the same moth ; the male, which 

 in moths has superior powers of flight, having the better retaining 

 organs. Hampson says " the form of the frenulum is of great 

 use in determining sex, as in the males of all the forms that 

 possess it, it consists of hairs firmly soldered together so as to 

 form a single bristle, whilst in nearly all females it consists of 

 three or more bristles which are shorter than that of the male ; 

 in one female Cossid I have found as many as nine. Also in the 

 large majority of moths the retinaculum descends from the costal 

 nervure in the male, while in the female it ascends from the 

 median nervure." ^ This sexual difierence in a structure for the 

 discharge of a function common to the two sexes is a very re- 

 markable fact. There are a few — very few — moths in which the 

 bases of the hind wings are notweU coadapted with the front 

 wings, and do not possess a frenulum, and these species possess 

 a small more or less free lobe at the base of the front wing that 

 droops towards the hind wing, and may thus help to keep up an 

 imperfect connexion between the pair ; this lobe has been immed 

 a jugum by Professor Comstock. Occasionally there is a jugum 

 on the hind as well as on the front wing. There is usually a 

 very great difference between the front and the hind wings ; for 

 whereas in the front wing the anterior portion is doubtless of 

 great importance in the act of flight and is provided with 

 numerous veins, in the hind wing, on the other hand, the corre- 

 sponding part has not a similar function, being covered by the 

 front wing ; hence the hind wing is provided with fewer nervures 

 in the anterior region, the divisions of the subcostal being less 

 numerous than they are in the front wing. In the moths 

 possessing a jugum the two wings differ but little from one 



' Fauna of British India, Moths, i. 1892, p. 6. 



