614 



CLASS IV. INSECTA. 



Insects are wingless and in most groups there are certain species 

 that have lost their wings. In some cases this apterous con- 

 dition is found only in one sex and in many species there is a 

 correlation between the simple eyes or ocelli and the presence of 

 wings. Both structures tend to disappear simultaneously. 



The attachment of the wing to the thorax lies along the 

 lateral edge of the tergum and is often of a very complex nature. 

 When the wing is expanded it is seen to be roughly triangular in 

 shape, the apex being at the point of attachment. The anterior 

 edge in this position is called the costa (Fig. 360 C), the external 

 is called the outer margin and the posterior is called the inner 



margin. 



The wings are outgrowths of the skin, which consists in i 



simplest condition of 

 a layer of hypodermal 

 cells covered by a 

 thin secretion, the 

 cuticle. The o u t - 

 growths may arise in 

 the embryo or larva 

 in one of two ways : 

 (i) either the skin 

 grows out and " the 

 wings begin and al- 

 ways remain as ex- 

 ternal folds or e vagina tions," or (ii) the wings commence by an in- 

 growth into the body, which eventually, either by turning inside 

 out or by some other means, becomes external. At least 90 per 

 cent, of existing insects belong to the latter category which 

 form the ENDOPTERYGOTA. Those whose wings are external 

 throughout life form the group EXOPTERYGOTA. 



The membranous expansions which form the tissue of the 

 wings are strengthened and supported by the nervures as an 

 umbrella is spread on its ribs (Fig. 367). The arrangement 

 of the nervures is of the greatest systematic importance. The 

 areas they bound are termed cells. The nervure primitively 

 consists of a channel in which blood circulates, and this 

 channel is usually traversed by a trachea and by a rod-like 

 bar known as " Semper's rib." 



The hard, thickened fore-wings of the Coleoptera are termed 



FIG. 367. Wing of Tipula (after Fr. Brauer). H sub- 

 costa; 1 first longitudinal nervure ( costa mediana) ; 

 2 radial rib (radius or sector) ; 3 cubital rib ; 4 dis- 

 coidal rib (or cubitus anticus) ; 5 submedian (or cubitus 

 posticus) ; 6 anal rib (or postcosta) ; 7 axillar rib ; R 

 marginal cell ; U submarginal cell ; D discoidal cell ; 

 7-F'posterior marginal cells; VB anterior basal cell; 

 HB posterior basal cell ; AZ anal cell. 



