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TEXT-BOOK OF ENTOMOLOGY 



first to be quite creased together. These wing-like wrinkles in the skin are not 

 empty pouches, but contain tissues and organs within, which are connected 

 with the skin, as the fat of the body, the net-work of tracheae, muscles, etc. 

 Alongside the tracheae, running through the former wing-pouches and accom- 

 panied by the nerves, there are canals through which the blood flows in and 

 out. 



A B 



FIG. 154. 



FIG. 156. 



FIG. 154. Stages in the growth of the wings of the nymph of Terme* .tf<i rijttx : A, young: <t, 

 a wing enlarged. S, older nymph : b, fore wing ; n, a vein. C, wings more advanced ; />, mature. 

 FIG. 155. Wings of nymph of Psocus. 



" After the last moult, however, when the supply of moisture is very much re- 

 duced in the wing-pouches, which are contracted at the bottom, their two layers 

 become closely united, and afterward grow into one single, solid wing-membrane. 



"These thick-walled blood-tubes arising above and beneath the upper and 

 lower membrane of the wing are the veins of the wings ; the development of 



