THE METAMORPHOSIS OF INSECTS 195 



0. THE DEVELOPMENT OP WINGS 



Two quite distinct methods of development of wings exist in 

 insects; by one method, the wings are developed as outward project- 

 ing appendages of the body; by the other, they reach an advanced 

 stage of development within the body. The former method of 

 development takes place with nymphs and naiads, the latter with 

 larva?.* 



i. The Development of the Wings of Nymphs and Naiads 



In insects with a gradual or with an incomplete metamorphosis the 

 development of the appendages proceeds in a direct manner. The 

 wings of nymphs and naiads are sac-like outgrowths of the body-wall, 

 which appear comparatively early in life and become larger and larger 

 with successive molts, the expanding of the wing-buds taking place 

 immediately after the molt ; an illustration of this has been given in 

 the discussion of gradual metamorphosis, page 175. 



2. Development of the Wings in Insects with a Complete 

 Metamorphosis 



Although there are differences in details in the development of the 

 wings in the different insects undergoing a complete metamorphosis, 

 the essential features are the same in all. The most striking feature 

 is that the rudiments of the wings, the wing-buds, arise within the 

 body and become exposed for the first time when the last larval skin 

 is shed. The development of the wings of the cabbage butterfly 

 (Pontia rapes) will serve as an example of this type of development of 

 wings. The tracing of that part of this development which takes 

 place during the larval life can be observed by making sections of the 

 body-wall of the wing-bearing segments of the successive instars of 

 this insect. 



The first indication of a wing-bud is a thickening of the hypo- 

 dermis; this thickening, known as a histoblast or an imaginal disc, 

 has been observed in the embryos of certain insects, in the first 

 larval instar of the cabbage butterfly it is quite prominent (Fig. 

 216, a). During the second stadium, it becomes more prominent 

 and is invaginated, forming a pocket-like structure (Fig. 216, 6). 

 During the third stadium a part of this imagination becomes 

 thickened and evaginated into the pocket formed by the thinner 



*Only the more general features of the development of wings are discussed 

 here. For a fuller account see "The Wings of Insects" (Comstock 'i 8, a). 



