DEVELOPMENT 



FIG. 213. Obtect 

 pupa of milkweed but- 

 terfly, Anosia plexippus, 

 natural size. 



bug, Rocky Mountain locust, aphids and sawfly larvae. The effect of 

 moisture may be an indirect one, however; thus moisture may favor the 

 development of bacteria and fungi, or a heavy rain may be disastrous 

 not only by drowning larvae, but also by washing them off their food 

 plants. 



As a result of secondary adaptive modifications, larvae may differ 



far more than their imagines. Thus Platygaster in 



its extraordinary first larval form (Fig. 219) is en- 

 tirely unlike the larvae of other parasitic Hymenop- 

 tera, reminding one, indeed, of the crustacean 

 Cyclops rather than the larva of an insect. As 

 Lubbock has said, the characters of a larva depend 

 (i) upon the group of insects to which the larva 

 belongs and (2) upon the special environment of 

 the larva. 



Pupa. The term pupa is strictly applicable to 

 holometabolous insects only. Most Lepidoptera 

 and many Diptera have an obtect pupa (Fig. 213), 

 or one in which the appendages and body are com- 

 pactly united; as distinguished from the free pupa of Neuroptera, Trichop- 

 tera, Coleoptera and others, in which the appendages are free (Fig. 204). 

 This distinction, however, cannot always be drawn sharply. Diptera 

 present also the coarctate type of pupa (Fig. 205), in which the pupa re- 

 mains enclosed in the old larval skin, or puparium. 



Pupal characters, though doubtless of great adaptive and phylogenetic 

 significance, have received but little attention. Lepi- 

 dopterous pupae present many puzzling characters, for 

 example, an eye-like structure (Fig. 214) suggesting 

 an ancestral active condition, such as still occurs 

 among heterometabolous insects. 



Pupation of a Caterpillar. The process of pupa- 

 tion in a caterpillar has been carefully observed by 

 Riley. The caterpillar of the milkweed butterfly 

 (PL I, A) spins a mass of silk in which it entangles 

 its suranal plate and anal prolegs and then hangs down- 

 ward, bending up the anterior part of the body (B), which gradually 

 becomes swollen. The skin of the caterpillar splits dorsally from the 

 head backward, and is worked back toward the tail (C and D) by the 

 contortions of the larva. 



The way in which the pupa becomes attached to its silken support 



FIG. 214. Head 

 of chrysalis of Pa- 

 pilio polyxenes, to 

 show eye-like struc- 

 ture. Enlarged. 



