70 INSECT TRANSFORMATION 



As after the final moult, the wings thus expand, the cuticle 

 extends, thickens and hardens ; the skin-cells become flattened 

 and, over the extensive areas of the wings where the two 

 layers are in contact, they may degenerate and die, leaving 

 the upper and lower sheets of cuticle coalesced into a 

 stiff membrane supported by the thickened nervures which 

 form what might be termed the skeleton of the wing 

 (Fig. ?] j, 43 a). 



A subject of much interest in connexion with wing-growth 

 is afforded by the varying degree of development attained by 

 the wings in insects of the same or of nearly allied kinds. In 

 the great majority of grasshoppers, locusts and crickets which 

 all resemble the insect taken as an example in the previous 

 chapter (pp. 5-28) in the elongate and strong hindlegs wings 

 capable of supporting the creature in flight are developed as 

 already described. But not a few of these jumping insects 

 attain the adult condition provided only with short wing- 

 rudiments quite useless for flight, and in some there is no 

 trace of wings at any stage. To the same order (Orthoptera) 

 as grasshoppers and locusts, belong the familiar kitchen and 

 shipboard cockroaches among which wings are developed in 

 essentially the same way. In the common kitchen cockroach 

 (Blatta orientalis] the insect often erroneously termed a 

 " black beetle " the male (Fig. 38 6) has rather short but 

 perfectly formed wings, while in the female (Fig. 38 a c) these 

 organs never get beyond the condition of early rudiments. 

 A similar sexual differentiation is to be noticed in many 

 cockroaches inhabiting tropical countries, and some members 

 of the family are without any wings at all. On the other hand 

 the American cockroach (Periplaneta americana) of ships, 

 quays, and hothouses, has very well developed wings in both 

 sexes, and so has the small but prolific German cockroach 

 (Phyllodromia germanica) of bakehouses. Such facts as these 

 will be found very suggestive when, in a later chapter, the 

 general question of winglessness among insects will come up 

 for discussion. 



There is another group of insects whose life-histories serve 

 to elucidate this subject ; these are the Aphids too well- 

 known to gardeners as plant-lice or " greenfly ". They belong 

 to an order (the Hemiptera) very distinct from that of the 



