THORAX, LEGS, AND WINGS, 
39 
The wings themselves are transparent membranes 
covered with very short hairs, intersected by threads 
darker than their substance, called nervures, veins, or 
ribs. These are hollow and thicker towards the root 
of the wing, allowing blood to circulate in them, 
and tracheae also extend into them, the distribu- 
tion of which corresponds to the course of the 
nervures. 
The structure of the wings is well shown in Fig. i8. 
Fig 1 8 . — Wings of Bee. 
The main nervures beginning at the front edge, or 
costa ( a , c), are called the costal, sub-costal, median, 
sub-median, and internal. The cells are also named 
according to their position in the wing, the first row 
