Text-book of comparative anatomy . gs, and originally indeed with 2 pairs,and although Avithin the different orders of the Pterygota the Avings 456 COM PA RA TIVE A NA TOM Y CHAP. may be reduced to 1 pair, or may be entirely wanting (in both sexesor only in the female), we here have to do with a derived con-dition and with animals which have lost the wings once possessed bytheir ancestors. In such insects the rudiments of wings or of organsbelonging to wings can often still be pointed out. The wings are thin lamellate unjointed folds of the body wall, speci-ally of the integument. The 2 lamell
Text-book of comparative anatomy . gs, and originally indeed with 2 pairs,and although Avithin the different orders of the Pterygota the Avings 456 COM PA RA TIVE A NA TOM Y CHAP. may be reduced to 1 pair, or may be entirely wanting (in both sexesor only in the female), we here have to do with a derived con-dition and with animals which have lost the wings once possessed bytheir ancestors. In such insects the rudiments of wings or of organsbelonging to wings can often still be pointed out. The wings are thin lamellate unjointed folds of the body wall, speci-ally of the integument. The 2 lamellae of a wing fold lie close to eachother. The wings are veined like the leaf of a plant. The veinsfor the most part are thickenings of the chitinous cuticle. Withinthe narrow interior space of the wing, nerves and especially tracheaeenter, branching like the veins. Blood-vessels also accompany theThe arrangement of the veins is very important for classifica- courses of the veins and their veins,tion. The exact investigation of the. Fto. 317.—A, Larva. B, female imago of Capnia nigra (Periid) (after Pictet). development, and especially the observation of rudimentary veins orveins in the act of disappearing, have led to the result that the wings ofthe various Hexapodan orders must be traced back not from one to theother, but to a common form of wing. Thus the examination of wingsconfirms the assumption that all orders of winged insects are derivedfrom a common winged racial group. The 2 pairs of wings are appendages of the meso- and meta-thoraxof the Insects. There are never more than 2 pairs. Their narrowedbasal portions are articulated with the dorso-lateral parts of the meso-and meta-thorax. Strongly developed wing muscles serve to movethem (see section on musculature). The problem of the phylogenetic origin of the wings of insects is extremelydifficult, and as yet by no means solved. The rise of such organs is not explainedby saying that they are iiitegumental folds, which
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