236 STATES OF INSECTS. 



some species, that proceed so earnestly as though they 

 had not a moment to lose, is the work of a few hours, of 

 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, Geometra lacertinaria, 

 ejects through an opening purposely left in its bottom), 

 and the pupa makes its appearance a . This interval is 

 exceedingly various. Most larvae 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 Bombyx cceruleocephala, according to Rosel, 

 lies three weeks in the cocoon before this change is ef- 

 fected ; those of many Pujrivora and Diploleparioe Latr., 

 according to Reaumur, six months b ; that of Phalcena 

 urticata nine months c ; and that of Cimbex lutea, accord- 

 ing to De Geer, sometimes eighteen months d . Brahm 

 observes, that such larvae of the double-brooded moth, 

 Hepialus Testudo, as form their cocoons in autumn, do 

 not become pupae until the following spring ; while those 

 which form them in summer undergo this change in a 

 few days e . 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 Phalcena 

 urticata^ just mentioned, which, though they formed 

 themselves cocoons in September, did not become pupae 

 till the June following f . I am unable, therefore, to as- 

 sign any plausible cause for these extraordinary varia- 



a De Geer i. 339—. t Reaum. ii. 423, and iii. 497. 



c Ibid. i. 605. 'i De Geer ii. 941. 



e Brahm Insek. 72. f Reaum. ubi supra. 



