NO. I INSECT THORAX — SNOUGRASS 59 



appendage through the membranization of its 1)asal part, and which 

 has become movable in a definite manner through the close association 

 of certain chitinous points in its base with special points on the edges 

 of the tergum and pleuron of its segment. The wing is a secondary 

 structure, acquired long after the legs were fully developed and the 

 subcoxfe transformed into chitinous pluera. The first movements of 

 the wings were made probably through successive alterations in the 

 shape of the thorax produced by the contractions of l)ody muscles 

 already present ; and insects in general have developed few muscles 

 particularly for the movement of the wings. The Odonata stand alone 

 among modern insects in having acquired special sets of wing muscles 

 attached directly to the bases of the wings, which have completely 

 replaced the older thoracic muscles. Since the dragonflies represent 

 an ancient group of insects, it must be supposed that they developed 

 their peculiar thoracic musculature during an early period of their 

 history. According to Poletaiew (1881) the wing muscles of modern 

 Odonata are formed in the individual during the postembryonic stages 

 of growth and attain their definitive form only in the last nymphal 

 instar. 



GENERAL STRUCTURE OF THE WINGS 



The insect wing is a flattened, double-layered expansion of the body 

 wall, and, therefore, its own walls consist of the same elements as 

 the body wall — cuticula, hypodermis, basement membrane — and its 

 lumen contains tracheae, nerves, and liquid of the body cavity. 



Development of the zvings. — Though the adult wing preserves the 

 basic structure of the hollow immature wing pad, the progressive 

 changes that take place within it during its growth so alter its tissues 

 that the fully formed wing becomes practically a lifeless appendage 

 serving as a propeller in the mechanism of flight. The histological 

 details of the development of the wings, the formation of the wing 

 tracheae, and the relations of the tracheae to the veins have been de- 

 scribed by Weismann (1864), Gonin (1894), Mayer (1896), Com- 

 stock and Needham (1899), Mercer (1900), Tower (1903), Powell 

 (1903), Marshall, (1915). The principal studies of the origin of 

 the wing tracheae from the trachea of the wing base are those of 

 Chapman (1918), and Beck (1920). 



The wings of insects with incomplete metamorphosis appear in the 

 second or third nymphal instar as hollow, flattened outgrowths of the 

 lateral parts of the dorsum in the mesothorax and metathorax, and 

 they "grow externally in the same manner as do the legs, mouth parts, 

 or other appendicular organs of the body. In insects with complete 



