INSECTS: WINGS AND THEIR APPENDAGES 89 



the stage- forceps, we shall be able to turn it in any direc- 

 tion for observation beneath the microscope. 



At first it seems a very thin transparent membrane, of 

 a shape between triangular and oval, with a few fine black 

 lines running through it, and along one edge. But on 

 bringing a greater magnifying power to bear on it, we see 

 that the clear surface is covered with minute short stiff 

 hairs, each of which has an expanded base. And 'still 

 further, by delicate focusing, we find that there are two 

 sets of these hairs, which come into view alternately, those 

 of one row projecting upward toward our eye, those of 

 the other downward. They are placed on both the upper 

 and under-surface, and are in fact appendages of two dis- 

 tinct membranes, applied to each other. There is some 

 reason to believe that these hairs are delicate organs of 

 touch communicating impressions through the skin to a 

 sensitive layer beneath; at least such seems their function 

 on the body, and we may judge from analogy that it is not 

 different here. 



The black lines are elastic, horny tubes, over which 

 the membranes are spread and stretched, like the silk of 

 an umbrella by its ribs. The upper membrane is firmly 

 attached to the tubes (which are called nervures); the 

 lower has but a slight adhesion, and is easily stripped 

 from them. The nervures originate in the body, and di- 

 verge like a fan to various points of the tip, and to the 

 upper and lower edges; some of them, however, terminate 

 in the substance of the wing without reaching the edge, 

 and some send off cross branches by which two are con- 

 nected together. They generally maintain the same thick- 

 ness throughout, but there are enlargements where the 

 branches join the main trunks. These nervures are hollow, 



