TRANSFORMATIONS OF INSECTS. 
1 56 
of incubation ; but in some species closely allied to the silkworm 
two or three broods are obtained every year, and the embryonic 
state is then very short. 
How different to this metamorphosis is that of the Sesia 
apiformis , the caterpillar of which lives for two years, and then 
changes into a chrysalis which has some power of progression ; and 
how different is the physical condition of the perfect Sesia to the 
slow-moving Bombyx. The goat moth larva, like its fellow tree- 
dweller the Sesia , has a long life, and is metamorphosed into 
a more or less active chrysalis, and these long-living caterpillars 
change their skins during growth, but no essential difference is 
noticed as regards their ornamentation during the successive 
moults. All the supposed causes of the metamorphosis into the 
chrysalis state act over and over again upon these long-lived 
caterpillars, and cold winter, genial spring, and hot summer, 
succeed each other without the change taking place. 
The hybernation of such caterpillars as those of (Enectra 
pilleriana and Lasiocampa querciis, which go into hiding imme- 
diately after hatching, proves that there is something more 
required to produce the metamorphosis besides want of food and 
heat. The hybernation of well-fed caterpillars, like those of the 
Anthrocera already mentioned, must be considered with these 
proofs that metamorphosis is not a condition to be determined 
by simple physical influences, but that it often refers back to 
ancestral peculiarities. The cocoon making and burying in the 
height of the warm weather, when placed in relation with the 
occurrence of two or more broods during the season, in some 
closely allied species, the last being hatched late, are incompre- 
hensible on the simple explanation that the silken and stony 
shelters are absolutely required for the preservation of the insect 
during its longer or shorter inclusion. The case of the Reapers 
—which undergo metamorphosis both in summer and in autumn — 
is complicated by the fact that the last brood, if it is very late, 
hybernate as caterpillars, and not in their admirably protected 
natural shelters. Here is a direct proof that something more 
than cold and absence of food is required to determine the 
first metamorphosis of the Lcpidoptera , and that it cannot take 
place until the development of the caterpillar has advanced to 
