THE MICROSCOPE. 



wasps, dragon-flies, two-winged flies, are made up of 

 a double layer of membrane, with a number of veins 

 or " nervures," within which there are generally found 

 air-vessels, or tracheae. These nervures, by their 



Common Fly. 



subdivision and reunion, form in some cases an ex- 

 ceedingly beautiful network ; this is especially ob- 

 servable in some of the smaller Neuroptera. Besides 

 spiral vessels, or tracheae, the nervures contain a fluid 

 supplied from the body, so that both air and blood 

 circulate in them ; the membrane of the wing often pre> 

 sents an appearance of cellular areolation, as you will 

 see in the above figure of the common fly. Although 

 to the unassisted eye the membrane appears to be 

 clear, transparent, and homogeneous, under the micro- 

 scope you will see it is covered with short stiff hairs ; 

 in the fly there is a single hair in each areola, of the 

 form of a curved spine. In the wings of the Hy- 



