248 



Arthropoda. 



of the adult {e.g., in Lepidopteraj Fig. 210).* In many the salivary 

 glands are modified to form a pair of silk glands, i^-hose secre- 

 tion forms a protective covering for the larva, either alone, or with 

 the assistance of foreign particles which it binds together. This 

 covering or cocoon is usually developed for protection during 

 pupation."' The tracheal system also exhibits striking deviations, for 



Fig. 210. Larva, pupa, and imago of a Lepidopteran (Sphinx) "with various 

 organs msifu; somewhat diagrammatic. All three figures of the same magnification. 

 6 legs, c brain, ch mesenteron, e proctodeum, h heart, i sub-oesophageal ganglion, k head, 

 03 oesophagus, s proboscis, t testis. 1 — 3 three thoracic ganglia. /, II, III, IV the four 

 first abdominal ganglia. — Adapted from Ne^vport. 



some of the stigmata which are open in the imago are closed in the 

 larva, or conversely ; the structure of the stigmata may also vary. 

 In the body-cavity of the larva there are large masses of fat, the fat 

 bodies, which are to a great extent used up during metamorphosis, 

 if they are not entirely absent from the adult. The genital organs 

 are only incipient. 



During larval life, the Insect moults repeatedly, and gradually 

 increases in size, usually without an actual change of form. When 



* In certain insectan larTse {e.g., the larvae of Bees and Ant-lions) a peculiar condition 

 obtains, in that the proctodseum into which the Malpighian tubules open, is not in 

 connection with the mesenteron ; but both the posterior end of the latter and the 

 anterior end of the former are closed. Only after metamorphosis do the two- 

 communicate. 



t In the larvse of the Ant-lions (and presumably in their allies) the secretion from 

 which the silk of the pupa-case is constructed, is probably formed in some of the 

 Malpighian tubules (of., the analogous conditions of some Fish where the Mdneys 

 secrete mticous threads). 



