NO, 2 EXCYCLOPEDIA OF INSECT ANATOMY — SXODGRASS 37 



merit is incorporated into the thorax as the propodium, and the sec- 

 ond segment forms a petiole. The petiolate part of the abdomen is 

 sometimes called the "gaster," but this is an inappropriate name for 

 it, since the gaster is properly the stomach. 



Male genitalia: The external genital equipment of male insects 

 includes primarily organs for the insemination of the female and 

 secondarily copulatory organs for holding her. To the taxonomist 

 the male genitalia offer the best characters he has for the separation 

 of species because of their highly diversified structure. This very 

 fact, however, makes the study of the genital homologies difficult 

 and has given rise to a great deal of confusion in current termi- 

 nology. Recent studies on the development of the organs have given 

 a better understanding of their basic structure and the homologies 

 of the parts. Thus there has been made possible the adoption of a 

 more uniform nomenclature. That confusion still persists is due 

 largely to the fact that specialists in each order of insects insist on 

 retaining their traditional ordinal nomenclature. 



Since the inner organs of reproduction are duplicated on opposite 

 sides of the body, and each has its own outlet duct, it is probable 

 that primitively the ducts opened through paired external apertures, 

 as they still do in some arthropods other than the insects. For effi- 

 cient insemination of the female it became more practical to have the 

 openings of the male ducts carried out to the ends of simple tubular 

 outgrowths. Thus we find paired penes present in Crustacea and 

 Diplopoda and, among the insects, in Ephemerida and Dermaptera. 

 In most of the crustaceans and diplopods the penes arise on the bases 

 of a pair of legs and are not themselves intromittent in function. 

 The sperm is transferred from the male to the receptacle of the fe- 

 male by one or two pairs of modified legs (gonopods) of a following 

 body segment. The insects have not adopted this indirect method 

 of insemination, but they have developed a great variety of struc- 

 tures in the copulatory organs, associated with the organ of insemina- 

 tion, for grasping and holding the female. 



The penes of Ephemeroptera arise from a small ventral plate or 

 a pair of plates above the stylus-bearing plates of the ninth segment ; 

 these evidently belong to the much reduced tenth segment of the 

 abdomen. The penes vary in form in different species, in some that 

 are armed with subterminal prongs, and rarely they are united basally 

 in a single organ, but the ducts are always separate. 



In the Dermaptera the two penes are united basally on a long 

 apodemal plate and are variously developed in their distal parts, so 



