CHAPTER III 

 THE MORE GENERAL FEATURES OF THE WINGS OF INSECTS 



The series of investigations briefly reviewed in the preceding chapters 

 has led to the adoption of a uniform terminology of the wing- veins of insects. 

 The application of this terminology to the wings of representatives of the 

 several orders of insects is discussed in later chapters ; but, in order to avoid 

 repetition, the more general features of the wings of insects are discussed 

 here, and the terms applied to the difterent types of wings and to the parts 

 of wings are defined. 



The presence or absence of wings in insects.^ — Excepting in that divi- 

 sion of the Hexapoda, or insects, that is known as the Apterygogenea, and 

 which includes the Collembola, the Campodeoidea, and the Thysanura, 

 wings are usually present in adult insects. Regarding the absence of wings 

 in the Apterygogenea two very different theories are held. The first was 

 proposed by Friedrich Brauer, in his Systematisch-Zoologische Studien 

 (1885). According to this theory, the Collembola, Campodeoidea, and 

 Thysanura represent a branch of the insect series that separated from the 

 main stem before wings were evolved, and which consequently is phylo- 

 genetically distinct from the series of orders of insects that decended froms 

 the primitive winged insects, and in which the absence of wings, when such 

 is the case, is due to their having been lost. 



Brauer, therefore, separated the Hexapoda into two primary divisions: 

 the Apterygogenea, or originally wingless insects; and Pterygogenea, or 

 originally winged insects. The former division includes only the Collem- 

 bola, Campodeoidea, and the Thysanura; the latter includes all other 

 insects. 



The second theory is that maintained by Handlirsch in his Die Fossilen 

 Insekten (i 906-1 908).. According to this theory all living insects have 

 descended from the winged Palseodictyoptera of Paleozoic times ; and the 

 absence of wings in the so-called Apterygogenea is due to specialization by 

 reduction. It does not fall within the scope of this essay to discuss these 

 rival theories. 



Even in the Pterygogenea many wingless forms are found; there being 

 wingless representatives of nearly all of the orders of winged insects. But 

 here the wingless condition is unquestionably an acquired one. 



The loss of wings by members of the Pterygogenea is often confined to a 

 single sex of a species ; thus with the canker-worm moths, for example, the 

 females are wingless, while the males have well-developed wings; on the 

 other hand, with the fig-insect, Blastophaga, the female is winged and the 

 male wingless. 



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