INSTINCT OF INSECTS. 479 



When these larvas cut out their case from the edge of 

 a leaf, they seem aware that, if they were to detach it 

 entirely from the inner side before the process of sew- 

 ing, lining, &c,, is completed, having no support on the 

 exterior edge, it would be liable to fall down ; at the 

 same time they could not sew together the membranes 

 composing it at the inner side, without cutting them in 

 part from the leaf. While, therefore, they divide the 

 major part of their inner side from the leaf, they artfully 

 leave them attached to it by one of the large nerves at 

 each end ; and these supports they do not cut asunder 

 until the intermediate space has been sewed up, and 

 they are ready to step, with their house on tlieir back, 

 upon the terra firma of the disk of the leaf. In this in- 

 stance, therefore, the larvse do not wholly separate 

 their case from the leaf, until it is sewed. But when 

 the same larvae cut out their materials fi'om the middle 

 of the leaf, where, though completely cut round, they 

 are retained in tJaeir situation secure from all danger of 

 falling by the serratures of the incisions made by the 

 jaws of the larvse, these little tailors vary their mode, 

 and entirely detach the pieces from the surrounding- 

 leaf, beft,re they proceed to set a stitch into them". 



In the preceding instances the variation of instinct 

 takes place in the same individual, but Bonnet men- 

 tions a very curious fact in which it occurs in different 

 generations of the same species. There are annually, 

 he informs us, tv/o generations of the Angoumois moth, 

 an insect which has been before mentioned**, as destruc- 

 tive to wheat; the first appear in May and June, and 

 lay their eggs upon the ears of wheat in the fields ; the 

 * lieaimi. iii. Wl-Wi. "^ Vol. I. '^d I'.d. ITX 



