DEVELOPMENT . 151 



nervous system, uninterrupted in their functions, undergo comparatively 

 little alteration. 



The imaginal hypodermis of the thorax arises from thickenings of 

 the peripodal membrane which spread over the larval hypodermis, while 

 the latter is gradually being broken down by the leucocytes ; in the head 

 and abdomen the process is essentially the same as in the thorax, the 

 new hypodermis arising from imaginal buds. 



Most of the larval muscles, excepting the three pairs of respiratory 

 muscles, undergo dissolution. The imaginal muscles have been traced 

 back to mesodermal cells such as are always associated with imaginal 

 buds. 



Hymenoptera and Lepidoptera. The internal transformation in 

 Hymenoptera, according to Bugnion, is less profound than in Muscidae 

 and more extensive than in Coleoptera and Lepidoptera. The internal 

 metamorphosis in Lepidoptera resembles in many respects that of Core- 

 thra. In both these orders the dorsal pair of prothoracic buds is absent. 

 In a full-grown caterpillar the fundaments of the imaginal legs and wings 

 (Fig. 222) may be seen, the wings in a frontal section of the larva ap- 

 pearing as in Fig. 223. Many of the details of the internal metamorphosis 

 in Lepidoptera have been described by Newport and Gonin. Figure 224, 

 after Newport, shows some of the more evident internal differences in 

 the larva, pupa and imago of a lepidopterous insect. 



Significance of Pupal Stage. To repeat among holometabolous 

 insects the function of nutrition becomes relegated to the larval stage 

 and that of reproduction to the imaginal stage. Larva and imago be- 

 come adapted to widely different environments. So dissimilar are the 

 two environments that a gradual change from the one to the other is no 

 longer possible; the revolutionary changes in structure necessitate a 

 temporary cessation of external activity. 



