1918] Morphology of Genitalia of Insects 133 



generalized condition is almost as clearly seen in Vespa as in 

 the saw-fly. 



3. ■ Somite Ten. — The typical form of this somite holds 

 good, for the tergum in the male and female Lepisma, the 

 sternum in these forms having as its main characteristic a deep 

 indentation in the middle of the caudal margin. In the grass- 

 hoppers this tergum is narrow and only partly divided from the 

 ninth by an incomplete suture, in both male and female, but 

 the sternum is nearly typical. In the female cockroach it is so 

 deeply indented in the middle of the caudal margin as to seem 

 almost forked, and in the male this sclerite is very short with 

 an entire, caudal margin, while the sternum is very small and 

 retracted in both cases. In the male and female crickets the 

 tergum is a sclerite nearly as long as broad, with a well-rounded 

 caudal margin, but the sternum is smaller, much shorter than 

 broad, and indented on the caudal margin, as is also the tergum 

 of the female Xiphidium. The sternum again in this genus, as 

 in all of the Orthoptera studied, shows speciaHzation to a slight 

 degree in its being retracted under the preceding sterna and 

 terga with a consequent reduction in size. In the Hemiptera 

 this small size of somite ten, with its accompanying retraction, 

 is carried to a greater extent, and we have small sclerites, still 

 chitinized, and, for all those studied, very well illustrated by the 

 terminal tergum of Benacus which is typical of such sclerites, 

 well developed, considering that they are retracted. The latter 

 point is clear when such a tergum is compared with the tenth 

 tergum or sternum of Euschistus. In all the Hemiptera studied 

 there was quite perceptible chitinization of the cuticula forming 

 the tenth somite, but this is lacking in the Neuroptera where 

 membrane only is present. The somite in this case is not only 

 short, but also small in circumference, entirely covered by other 

 somites and appendages, and enclosing the anus without very 

 well-marked sutures between the tergum and sternum. Such a 

 description as this would answer for the tenth somite of almost 

 any form taken through the remaining insect orders considered. 

 Exceptions may be made in the case of Panorpa where the whole 

 somite is long in proportion to its width, and firmly chitinized; 

 in forms as reduced and retracted as Hepialus and Tabanus, 

 where the length is very short in proportion to the width; in 

 the female Pryonoxystus, where it is retracted and membranous, 

 yet long and cylindrical. 



