66 THE STUDY OF INSECTS. 



the five branches of the typical radius, or vein III, are 

 designated either thus, R b R 2 , R 3 , R 4 , R 5 ; or thus, IIIi, 

 III 2 , III,, III 4 , III 5 . 



When two or more branches of a branched vein coalesce, 

 the compound vein is designated by an expression indicating 

 this coalescence, as III 2+3 or R 2+3 . In this way it is possible to 

 indicate some of the changes that have taken place in the de- 

 velopment of the species; and to make use of them in working 

 out the classification of the group to which the species belongs. 



The cells of the wing are designated by applying to each 

 the number or the abbreviation of the name of the vein that 

 forms its cephalic (front) margin. In Figure 78 the veins are 

 designated by numbers at the margin of the figure ; the cells 

 by numbers within the figure. When a cell is divided by a 

 cross-vein the parts are numbered, as in the case of cell V 2 in 

 Figure 497, page 422. 



The Abdomen and its Appendages. 



The abdomen is the third or caudal region of the body. 

 Its segments are more simple, distinct, and ring-like than 

 those of the other regions. The number of segments of 

 which it appears to be composed varies greatly. In the 

 Cuckoo-flies (Chrysididce) there are usually only three or four 

 visible, while in many other insects nine appear. Except in 

 the lowest order of insects (Thysanurct) the abdomen of the 

 adult bears no locomotive appendages. But many larvae 

 have fleshy appendages which aid in locomotion : these are 

 termed prolcgs. In the adult the end of the body in many 

 families is furnished with jointed filaments the cerci, and 

 caudal setce. Frequently also the body is furnished in the 

 male with organs for clasping the claspers ; and in the female 

 with saws, pierces, or borers the ovipositor. In the female of 

 certain insects there is a sting, a modified ovipositor, which 

 is used as an organ of defence ; and the abdomen of plant- 

 lice and certain other insects bears a pair of tubes or tuber- 



