DEVELOPMENT 



139 



or may serve merely as a foundation for the cocoon. While silk and 

 often a water-proof gum or cement form the basis of a cocoon, much 

 foreign material, such as bits of soil or wood, is often mixed in ; the cocoons 

 of many common Arctiidae, as Diacrisia mrginica and Isia Isabella, con- 

 sist principally of hairs, stripped from the body of the larva. 



Butterflies have discarded the cocoon, the last traces of which occur 

 in Hesperiidae, which draw together a few leaves with a scanty supply of 

 silk to make a flimsy substitute for a cocoon. Papilionid and pierid pupae 

 are supported by a silken girdle (Fig. 27), and nymphalid chrysalides 

 hang freely suspended by the tail (Fig. 213). 



Cocoon-Spinning. The caterpillar of Telea polyphemus " feels 

 with its head in all directions, to discover any leaves to which to attach 



FIG. 216. Cocoon of Samia cecropia, cut open to show the two silken layers and the enclosed 



pupa. Natural size. 



the fibres that are to give form to the cocoon. If it finds the place suit- 

 able, it begins to wind a layer of silk around a twig, then a fibre is attached 

 to a leaf near by, and by many times doubling this fibre and making it 

 shorter every time, the leaf is made to approach the twig at the distance 

 necessary to build the cocoon; two or three leaves are disposed like this 

 one, and then fibres are spread between them in all directions, and soon 

 the ovoid form of the cocoon distinctly appears. This seems to be the 

 most difficult feat for the worm to accomplish, as after this the work is 

 simply mechanical, the cocoon being made of regular layers of silk united 

 by a gummy substance. The silk is distributed in zigzag lines about 

 one-eighth of an inch long. When the cocoon is made, the worm will 

 have moved his head to and fro, in order to distribute the silk, about two 

 hundred and fifty-four thousand times. After about half a day's work, 

 the cocoon is so far completed that the worm can hardly be distinguished 



