194 THE BIOLOGY OF INSECTS 



is revealed, is followed by another moult ushering in what 

 is regarded as the true pupal stage. R. E. Snodgrass (1924) 

 has described in the small muscoid dipteron Rhagoletis, a 

 ** prepupal " cuticle which is formed within the puparium 

 and envelops the pupa (Fig. 54). This instar, however, 

 resembles the contracted maggot in form and has no wing- 

 rudiments. 



Further, in connection with the biology of the pupa, it 

 is noteworthy that among the metamorphic insects there is a 

 great range of variation in the creature's power of movement 

 during this stage of its life. The house-fly pupa lies 

 quiescent within its hard protective puparium — the shrunken 

 and condensed larval cuticle — out of which the fly has to 

 make its way after emerging from the cast pupal coat. The 

 pupa of a butterfly or of a moth belonging to one of the 

 highly organised families can move only a few of its 

 abdominal segments. But among the more primitive 

 Lepidoptera the pupa, provided with rows of locomotor 

 spines on its abdominal segments, works its way partially out 

 of its cocoon or from the earth in which it lay buried ; the 

 empty pupa coat of the Goat Moth (Cossus) may be seen 

 partly protruding from a tree wherein the caterpillar fed, 

 that of a Swift Moth (Hepialus) from the surface of the soil 

 in which the larva devoured roots. Among the more 

 primitive Diptera the same tendency to pupal activity may 

 be noticed in cases where the life-conditions render it 

 appropriate ; the pupa of a Crane-fly (Tipula) raises the 

 front half of its body out of the ground, and gnat pupae 

 swim actively through the water making use of the surface 

 film to obtain atmospheric air for breathing by means 

 of paired '* trumpets *' on the thoracic region of their 

 bodies. 



From the foregoing examples it may be realised that 

 while insects practising the open method of wing-growth 

 are as a rule active, and those practising the hidden method 

 passive in the penultimate stage, there is no absolute devia- 

 tion in this respect between the two great types of insect 

 life-history. The pupa or its corresponding instar seems 



