Frenulum of the Lepidoptera, 125 
a trace of the retinaculum and are quite useless: they are, 
however, very interesting as showing the near relationship 
of these insects to the Heterocerous Lepidoptera. 
The Sphingidz exhibit the highest development of the 
frenulum, the spina of the male insect attaining its 
maximum of size and strength and the retinaculum being 
powerful and capable of holding with a firm grip. In 
the female insect too the spinule are strong, and their 
extremities converge to a strong fine point, thus forming 
a claw or hook, which catches the fasciculus of the fore- 
wing with considerable effect. These characters hold 
good as regards the majority of the species of this family, 
but some members of the subfamily Smerinthine form 
a noteworthy exception. Of one of these, Smerinthus 
popult, Giorna states in the paper referred to, that it 
is without the appliance. Such, however, can hardly be 
said to be the case; the male insect possesses, in the 
position occupied by the spina in other species, a process 
standing out from the margin of the wing, rounded in 
outline and in some few examples terminated by a 
minute point (Fig. 4). This process corresponds to the 
root or prominence which in other species carries the 
- spina. 
The female has a much more largely developed appa- 
ratus ; not only does she possess the root-process, but also 
a perfectly formed group of spinule (Fig. 5), which, how- 
ever, are too small to be of any practical use in connecting 
the wings; so that the organ in both sexes may be 
considered abortive. 
The retinaculum of the male is absent, as we might 
expect, and its place is simply indicated by a most 
inconspicuous group of scales. It will be evident on 
consideration that the peculiar rest-position always as- 
sumed by S. populi would be quite impossible in a species 
with fully developed spina and retinaculum, inasmuch as 
the humeral angle of the hindwing always projects before 
the costal margin of the forewing, and it would be necessary 
for the moth to withdraw the bristle from its loop every 
time that it composed its wings for rest. 
A similar rudimentary development of the part also 
occurs in greater or less degree in several other allied 
species, notably in the fine Australian species, Cagwosa 
triangularis, which has the spina very short and probably 
quite useless. Some other Smerinthinz, however, such 
