THE INSECT: ITS EXTERNAL STRUCTURE 



13 



membranes in contact with each other except along certain Hnes (Fig. 

 19). Along these lines each membrane thickens and also rises above the 

 general surface, so that if the two membranes could be separated and 

 examined from the inner surface, they would appear uniform except for 

 grooves with thickened sides and bottoms, running here and there. 



- /^ O—r^ 



— u — o — w v/~ 



Fig. 19. — Diagram of cross-.seotioii of an insect wing showing the two membranes 

 somewhat separated and the ways in which the veins are formed. {Modified from Wood- 

 worth.) 



When the membranes are brought together again, these grooves com- 

 bining form hollow rods which, being stronger than the rest of the mem- 

 brane, serve as its support and hold it stiff. These hollow rods are 

 usually called veins or nerves, though they are nothing of the sort. 

 The main and largest veins arise at the base of the wing and extend 

 outward, diverging as they go, and some branch several times before they 



/A 



'om. 



OITL 



a.an 



Fig. 20. — Diagram of the margins and veins in the wings of moths. A, apex; a. an, 

 anal angle; c, costa; c.c, closed cell; /, frenulum; i.n, inner margin; o.m, outer margin; v, 

 vein. {Original.) 



reach the wing margin (Fig. 20). Cross veins also occur, connecting the 

 radiating main veins or their branches. Areas of membrane between 

 veins are termed cells and where entirely surrounded by veins are called 

 closed cells. These may be relatively few or many, according to the 

 number of veins and their branches present. The arrangement and 

 number of the chief veins and their branches are of importance in 

 identifying insects. 



