PHYSIOLOGY OF INSECTS. 123 



pressing each other in every direction ; these 

 nervures give the wings the appearance of beau- 

 tiful net- work : dragon-flies have wings of this 

 character. 



442. The hind wings (rneialce) possess the 

 characters of being scaly, membranaceous, or 

 reticulated, the same as the fore wings ; but they 

 are never crustaceous, leathery, or half-leathery. 



443. The hind wings rise from the sides of the 

 metathorax, one behind each of the fore-wings ; 

 they occasionally possess characters which the 

 fore wings have not. 



444. Petiolated hind wings (metalcE petiolata') 

 are when they have ceased to bear the appearance 

 of wings, and have become mere knobs, placed at 

 the extremity of a short foot-stalk : in this form 

 .they are usually termed poisers (halteres), and are 

 possessed by all flies and gnats. 



445. Setaceous hind wings (metalce setiformes) 

 are when they have ceased to bear the appearance 

 of wings, and have become simple bristles, totally 

 Useless in flight. 



446. The nervures in membraneous wings divide 

 the membranes into small compartments, called 

 cells (cellulce) ; these cells are very constant in 

 their form and proportion in the same kinds of 

 insects ; consequently their variations distinguish 

 one kind from another. 



447. The strong nervure which runs along the 

 upper edge of each wing, either on the extreme 



