THE METAMORPHOSIS OF INSECTS 197 



parts (a, b, c, and d) of Figure 216; temporary respiratoiy organs, 

 consisting of bundles of tracheoles, are developed (e and/) ; and later, 

 near the close of the larval period, the trachese of the wing are devel- 

 oped, and the bundles of tracheoles disappear. During the later 

 stages in the development of the wing the basement membranes of the 

 hypodermis of the upper and lower sides of the wing come together, 

 except along the lines where the veins are to be developed later, and 

 become united. In this way the wing is transformed from a bag-like 

 organ to a sheet-like one. The hnes along which the two sides of the 

 wing remain separate are the vein cavities ; in these the trunks of the 

 wing-tracheag extend. During the final stages of the development of 

 the wing, the walls of the vein-cavities are thickened, thus the wing- 

 veins are formed; and the spaces between the wing-veins become thin. 



By reference to Figure 216, c and d, it will be seen that the histo- 

 blast consists of two quite distinct parts, a greatly thickened portion 

 which is the wing-bud and a thinner portion which connects the wing- 

 bud with the hypodermis of the body-wall, and which constitutes the 

 neck of the sac-like histoblast, this is termed the peripodal membrane, 

 a term suggested by the similar part of the histoblast of a leg ; and the 

 enclosed cavity is known as the peripodal cavity. 



In the more specialized Diptera, the peripodal membranes are 

 very long and both the wing-buds and the leg-buds are far removed 

 from the body-wall. A condition intermediate between that which 

 exists in the Lepidoptera, as shown in Figure 216, and that of the 

 more speciaHzed Diptera was found by Kellogg (07) in the larva of 

 Holorusia rubiginosa, one of the 

 crane-flies (Fig. 217). 



b. THE DEVELOPMENT OF LEGS 



The development of the legs 

 pi oceeds in widely different ways 



in different insects. In the ^. „,. , , . , , ^ ,, 



,. , . , Fig. 217. — Wing- bud m the larva of the 



more generalized forms, the giant crane-fly, Holorusia rubiginosa; 

 legs of the embryo reach an ^3-, hypodermis; pm peripodal mem- 

 brane; /, trachea; wo, wmg-bud (Alter 

 advanced stage of development Kellogg). 



before the nymph or naiad 



leaves the egg-shell, and are functional when the insect is born; on 

 the other hand, in those specialized insects that have vermiform larvas, 

 the development of the legs is retarded, and these organs do not 

 become functional until the adult stage is reached. Almost every 

 conceivable intergrade between these two extremes exist. 



