48 THE UNIVERSITY SCIENCE BULLETIN. 



and the oedagus from the membrane of the genital chamber, 

 and thus the styles, the connective, and the oedagus be freed 

 intact. These were then usually drawn in their normal posi- 

 tion, that is a dorsal view of them was obtained. Thus with the 

 previous lateral in situ view, and a dorsal view, a fairly accu- 

 rate idea of these organs can be gained. Both these drawings 

 were later verified and if necessary corrected when the 

 mounted genitalia were studied with the higher magnification 

 of the compound microscope. 



The drawing of these organs was greatly facilitated by us- 

 ing, in one of the oculars, an eye-piece scale ruled into squares. 

 The drawing paper was then ruled into inch squares corre- 

 sponding to these squares. In this way drawings can be made 

 quickly and accurately and with all the various species drawn 

 to the same scale. Our drawings are about 40 times the size 

 of the genitalia. 



After they are dissected out and drawn, the genitalia are 

 transferred to 95 per cent alcohol for a few minutes, then to 

 xylol for a similar period, and finally mounted on slides in 

 Canada balsam. A pin with a small loop in the end and with 

 the other end fastened into a wooden handle is an excellent 

 tool for the transfer of these tiny organs from one liquid to 

 another. 



As in other groups of insects it will be found that these 

 genitalia show distinct and specific differences in some genera, 

 while in others they are, for purposes of classification, of 

 little or no value. In some cases however I believe they are 

 practically the only criterion that will enable us to correctly 

 decide between species and varieties, and also to decide the 

 generic position of some forms, which though specifically dis- 

 tinct, yet give much trouble as to their true generic disposition. 



The value of these internal genitalia may readily be shown in 

 the little that has been done on the Ar/allia group. In their re- 

 view of the members of this genus Osborn & Ball pointed out 

 the existence of three groups within the genus. The differences 

 between these groups, while based partly on adult characters, 

 were more particularly indicated in the nymphs, which vary 

 considerably both as to structure and life history. In 1907 

 Kirkaldy, recognizing the distinctions between these groups, 

 gave to them subgeneric rank, and accordingly divided the 



