528 IXSECXA. 



course of the nervures. The nervures, therefore, always start from 

 the root of the wing as two or three principal stems, and distribute 

 their branches more especially to the upper half. The first (fig. 436) 

 of the main trunks which runs beneath the upper margin of the 

 wing is called the costa, and often ends in a horny dilatation. Be- 

 neath the costa there is a second main stem, the radius, and behind 

 this a third, the cubitus, which rarely remains simple, but usually 

 bifurcates before the middle of its course into branches, which are 

 often further divided so that a more or less complicated network is 

 formed in the upper half of the wing. The spaces of this network 

 may be distinguished as marginal spaces or radial cells, and as sub- 

 marginal spaces or cubital cells. Not rarely there may also be 

 present one or more lower nervures (anal, axillar nervures). 



The form and structure of the wings present various modifications. 

 The anterior wings may become coriaceous by the stronger chitinisation 

 of their substance, as for instance in the Orthoptera and Rhync\ota ; 

 or, as in the Coleoptera, they may have a firm horny structure (teg- 

 mina or elytra), and be used less for flight than as a protection 

 of the back, the skin of which is soft. The anterior wings in 

 the Rhynchota group of the Hemiptera are mostly horny and only 

 membranous at the tip, while the posterior wings are membranous. 

 When both pairs of wings are of a membranous structure, their 

 surface is either thickly covered with scales, Lepidoptera and Phry- 

 ganidce (group of Neuroptera), or remains naked and is marked out 

 into a number of very conspicuous spaces, which may not unfrequently 

 have the form of a close net-like mesh-work, as in the Ncuroptera. 

 In general the two pairs of wings differ in size. Those insects which 

 have coriaceous anterior wings and half or whole wing covers, have 

 much larger posterior wings, while in the insects with membranous 

 wings the anterior wings are, as a rule, the largest. In many of the 

 Ifeuroptera, the wings are pi'etty nearly the same size, while in the 

 Diptera the posterior wings are aborted and reduced to small knobs 

 (halter es). Finally we find in all the orders of insects examples of a 

 complete absence of wings either in both sexes, or in the female sex 

 alone. 



The third region of the body, which contains most of the vegeta- 

 tive organs, as well as the organs of reproduction, is the elongated 

 and well-segmented abdomen. In the adult insect this region is 

 destitute of appendages, although very often in larval life, and as an 

 exception in the sexually adult animal (Japyx), short appendages 

 are present. The abdominal segments are very definitely separated 



