142 INSECT TRANSFORMATION 



that throughout pupal life these organs should be kept in con- 

 stant motion. The caddis-pupa (Fig. 82) has, like the larva, 

 strong biting mandibles, though these appendages are wanting 

 or vestigial in the imago. They are required in order that the 

 pupa may bite its way out of the case, so as to rise through 

 the water into the atmosphere before the emergence of the 

 fly. This upward passage through the water is in some cases 

 performed by crawling, and in others by swimming, as the 

 legs of various caddis-pupae are provided either with claws for 

 clinging to the stems and leaves of aquatic plants, or with 

 rows of delicate swimming hairs, which facilitate free loco- 

 motion in the pond or stream (Fig. 82 c, d.) 



Reference has been frequently made to the cocoon or protec- 

 tive case which surrounds the pupa. In the life-history of 

 the caddis-flies, just now under consideration, the " house " 

 of the larva, shortened and closed, becomes the shelter of the 

 pupa, and this is built of surrounding objects or fragments 

 spun together by the silk that the larva secretes from the 

 long tubular glands which open into its mouth (see p. 122 

 above). There are many other insect life-histories in which 

 the pupal cocoon may be regarded as the modification of a 

 larval shelter, and always the cocoon is the result of the larva's 

 work before the final moult, its spinning activity being a 

 preparation for the pupal life. A moth-caterpillar, for example, 

 that has completed its growth from hatching, feeding uncovered 

 on the leaves of plants, surrounds itself with a cocoon (Fig. 83) 

 that has relation not to the needs of its own stage in the 

 transformation but to those of the next. 



Many pupae are found buried in the soil ; such may be pro- 

 tected by a cocoon of fragments of earth fastened together by 

 silk, or the earthen chamber may have a silken lining. The 

 " Goat " caterpillar (Cossus) after its long period of feeding 

 in the wood of trees constructs a cocoon of chips and splinters, 

 while many cocoons are strengthened by the shed hairs or 

 spines of the caterpillar or by some hardened solidified secre- 

 tion. On the other hand the cocoon may be formed entirely 

 of silk as in the well-known work of the " Silkworm " 

 (Bombyx). Such a silken cocoon may be, as in this case, 

 dense and extensive, composed of a comparatively enormous 

 length of thread, or it may be loose and scanty, a mere network 



