THE GENITALIA 43 



sclerites being the remains of the sternum of the eleventh segment 

 (Fig. 26). In cases when the cerci apparently arise from the 

 tenth abdominal segment, the eleventh segment having atrophied, 

 examples are afforded of Lankester's sixth law of metamerism. 

 The atrophy of the eleventh segment has resulted in its appendages 

 becoming transferred to the tenth segment. The terminal pair of 

 abdominal appendages found on the ninth or tenth abdominal 

 segments of various larvae are of a problematical nature, especially 

 those which are devoid of articulation and arise from the dorsum 

 of their segment. They are probably best referred to under the 

 general term of cercoids in reference to their frequent resemblance 

 to cerci. 



4. The Genitalia 



During recent years much attention has been given to the 

 genitalia of insects, and more especially to the male organs, in view 

 of their importance in taxonomy. The whole subject has become 

 greatly involved owing to the lack of any convenient, uniform 

 terminology, and to uncertainties which exist with respect to the 

 homologies of the parts concerned in different orders. The 

 greater part of the literature deals with the completed organs and 

 especially those of the male. Ontogenetic studies have not been 

 pursued with the same enthusiasm. They are dependent upon 

 obtaining supplies of material in the proper successive stages 

 which are often difficult to provide. This fact, coupled with the 

 obscurity which so often attends the initial growth phases of the 

 genitalia and the difficulties of interpretation that are involved, 

 has resulted in a large field, so far, being inadequately explored. 

 Nevertheless, the writings of Singh Pruthi (1924, 1925), Mehta 

 (1933), Metcalfe (1932) and others have advanced our knowledge 

 of the developmental aspects of the subject. 



The Female. It is well known that a typical, completely 

 formed insect ovipositor is composed of three pairs of valves 

 which are located in relation with the eighth and ninth abdominal 

 segments (Fig. 26). These valves arise in connection with the 

 basal plates or coxites of their respective segments, so that the 

 coxites of the eighth segment bear one pair of such valves, while 



