INSECTS : WINGS AND THEIR APPENDAGES. 73 



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 

 thickness throughout, but there are enlargements where 

 the branches join the main trunks. These nervures are 

 hollow, and are during life filled with a subtle fluid, which 

 is supplied from the vessels of the body. They contain 

 also ramifications of the exquisite spiral air-vessels, which 

 we shall presently consider, so that both air and blood 

 circulate in them. 



In this wing of the Bee all of these structures may be 

 seen to greater advantage. 'The membrane appears per- 

 fectly uniform by transmitted light, even with so high a 

 magnifying power as 600 diameters, at least on a cursory 

 examination; though, by careful manipulation, we may 

 discern faint traces of angular lines which divide the whole 

 surface into irregular areas. But by using reflected light 

 at an oblique angle, this areolation, which indicates the 

 primary cells of the structure, is much plainer, and each 

 area is perceived to carry a single hair in its centre. 



The hairs themselves here take the character of curved 

 spines, not unlike those of a rose-tree. Along the front 

 edge of the wing they are straight, stout, densely crowded, 

 and overlapping in an inclined position ; but the most 

 interesting modification of these organs is seen at the front 

 edge of the posterior pair. Unlike the Fly, which has but 

 a single pair of wings, the Bee has two pairs, of which the 

 fore pair is the larger and more horny, the hinder pair 



