Elementary entomology elementaryentomo00sand Year: [c1912] ELEMENTARY ENTOMOLOGY mouth-parts, form the most important basis for classification. Thus most of the orders are distinguished by differences in the wings, as indicated by their names, which usually end in -ptera (from pteron, a wing), and many insects may be classified to the genus or even to the species by the wings alone, this being particularly true of fossil forms, in which the wings are often the only parts well preserved. Most adult insects possess two pairs of wings, borne by the meso- thorax and metathorax, but in some paras
Elementary entomology elementaryentomo00sand Year: [c1912] ELEMENTARY ENTOMOLOGY mouth-parts, form the most important basis for classification. Thus most of the orders are distinguished by differences in the wings, as indicated by their names, which usually end in -ptera (from pteron, a wing), and many insects may be classified to the genus or even to the species by the wings alone, this being particularly true of fossil forms, in which the wings are often the only parts well preserved. Most adult insects possess two pairs of wings, borne by the meso- thorax and metathorax, but in some parasitic orders the wings have been lost, and one order, the Thysanura, represents the primitive insect without wings. In the flies (Diptera) only the mesothoracic wings are developed, and the metathoracic wings are represented by clublike appendages, called Jialtercs, or balancers. The relative shape and size of the two pairs vary greatly, and frequently the two wings of each side over- lap or are held together by various structures, so that they act as a single organ. The wings are strengthened FIG. 27. Wing of house-fly (Mnsca domesti- by numerous thickenings, to), showing specialization of wing venation called veins, whose number through reduction of veins and position form the basig of ,, costa; r, radius; m media; en cubitus; a, ^Q classification of families, anal. (After Comstock) genera, and species. It has been shown by Professors Comstock and Needham that the prin- cipal veins are homologous in all the orders of insects, and that they have been derived from one original type, either by the disap- pearance of certain veins, by their growing together, or by the addition of supplementary veins. The typical longitudinal veins, as shown in Fig. 26, are the costa, subcosta, radius, media, cubitus, and anals. The costa (c) is unbranched and strengthens the anterior margin of the wing. The subcosta (sc) is typically two- branched, though often single, and, where the costa is smal
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