INSECTS : WINGS AND TniuLR APPENDAGES. 81 



black lines running tlirougli it, and along one edge. 

 But on bringing a greater magnifying power to hear on 

 it, we see that the clear surface is covered with n.inute 

 short stiff hairs, each of which has an expanded base. 

 And still further, by delicate focussing, we find that 

 there are two sets of these hairs, which come into view 

 alternately, those of one row projecting upward to- 

 wards our eye, those of the other downward. They arc 

 placed on both the upper and under-surface, and are in 

 fact appendages of two distinct membranes, applied to 

 each other. There is some reason to believe that these 

 hairs are delicate orgaus of touch communicating im- 

 pressions 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 ner- 

 vures) ; the lower has but a slight adhesion, and is 

 easily stripped from them. The nervures originate in 

 the body, and diverge 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 connected together. Tliey 

 generally maintain the same thickness throughout, but 

 there are enlargements where the branches join the 

 main trunks. Tliese nervures are hollow, and are, dur- 

 ing life, filled with a subtile fluid, which is supplied 

 from the vessels of the body. They contain also rami- 

 fications of the exquisite spiral air-vessels, which wo 

 4* 



