CHAP. VI. THE IMAGO STATE. 41 



of glass in a window; these are called cells. When one of these 

 cells is entirely surrounded by veins and cross-veins, it is said 

 to be closed (Fig. 125, 2, 2> 2); but if the outer or the posterior 

 margin of the wing forms one of its sides, it is then said to be 

 open (Fig. 125, 4, *) 



Naturalists determine the different genera and species of 

 Plant-lice chiefly by the difference in the veining of the wings, 

 these being the most reliable characteristics which these insects 

 possess. 



Fig. 126* represents the venation of the wings of the Rose 

 Aphis and Grain- Aphis; Figure 127, that of the Apple-tree 

 Aphis, and Figure 128, that of the -Woolly Aphis. 



THE ABDOMEN AND ITS APPENDAGES. 



The posterior division of the body of a.i insect is termed 

 the abdomen, and contains the organs of nutrition and' of 



Fig. 126. 



reproduction. The abdomen is sometimes united to the thorax 

 by its entire width, but in some insects such as Wasps, etc, 

 these^ two parts are connected by a slender stem or petiole. 



* EXPLANATION OP FIG. 1261. basal cell; 2, first discoidal cell; 3, second discoidal 

 cell; 4, mfra-marginal cell; 5, first cubital cell; between 4 and 5, second cubital cell; 7, 

 stigma; toward base of wing from stigma, costal cell; between 4 and 7, marginal cell 

 (The latter and No 4 are sometimes called "apical cells.") 



