STATES OF INSECTS. (Larva.) 235 
others about two or three days,—after a certain interval 
it casts its last skin, which is usually suffered to remain in 
the cocoon (but which one moth, Drepana lacertinaria, 
ejects through an opening purposely left in its bottom), 
and the pupa makes its appearance?. This interval is 
exceedingly various. Most larvee assume the pupa state 
within a few days after they have formed their cocoons ; 
but some not for several weeks, or even months. The 
caterpillar of Episema ceruleocephala, according to Rosel, 
lies three weeks in the cocoon before this change is ef- 
fected; those of many Pupzvora and Diploleparie Latr., 
according to Reaumur, six months® ; that of Margaritia 
urticata nine months‘; and that of Cimbex lutea, accord- 
ing to De Geer, sometimes eighteen months*. Brahm 
observes, that such larvee of the double-brooded moth, 
Apoda Testudo, as form their cocoons in autumn, do not 
become pupze until the following spring; while those 
which form them in summer undergo this change ina 
few days*. From this fact it might be conjectured, that 
the degree of heat prevailing at the time the insect in- 
closes itself determines the period of the pupa’s appear- 
ance; but this supposition seems contradicted by what 
Reaumur observed of a brood of the larvae of Marga- 
ritia urticata, just mentioned, which, though they form- 
ed themselves cocoons in September, did not become 
pupe till the June following‘. Iam unable, therefore, 
to assign any plausible cause for these extraordinary 
variations. ‘The difficulty of comprehending how ani- 
mals before so voracious can live so long without food 
* De Geer i. 339—. b Reaum. ii. 423, and iii. 497. 
* Thid.i. 605. * De Geer ii. 941. 
° Brahm Insek, 72. * Reaum. ubi supra. 
