NO. 14 INSECT ABDOMEN — SNODGRASS 73 



to be merely a tubular evagination of the body wall with the opening 

 of the ejaculatory duct on its extremity. It may be differentiated by 

 a circular fold into a proximal part (phallobase) and a distal part 

 (aedeagus). 



According to Heymons (1897) the embryonic gonoducts of 

 Lepisma saccharina extend first to the tenth abdominal segment in 

 the male, and to the seventh in the female. In each case the ducts end 

 with ampullar enlargements. With the later reduction of the tenth 

 segment during embryonic development the ampullae of the male are 

 transposed to the ninth segment and become attached to the ectoderm 

 at the posterior margin of this segment. Here an ectodermal ingrowth 

 takes place between the ampullae, in which later is formed an invagi- 

 nation that, uniting with the ampullae, becomes the definitive ductus 

 ejaculatorius. It is thus evident that male Thysanura must have had 

 primarily paired genital openings on the tenth abdominal segment, 

 and that these primitive gonopores secondarily migrated forward and 

 approximated each other at the posterior edge of the venter of the 

 ninth segment. Here they acquired a common outlet by the ingrowth 

 of a median ectodermal tube. The definitive ejaculatory duct, there- 

 fore, is a ductus communis, and not the product of a union of the 

 ends of the primary ducts. The common genital duct of the female, 

 Heymons says, is similarly formed by a median ectodermal invagina- 

 tion on the eighth abdominal segment, that is, on the first somite 

 behind the one on which the lateral ducts primarily opened. 



EPHEMEROPTERA 



The external male genitalia of the mayflies include a pair of mov- 

 able appendicular clasping organs (fig. 25 A, Sty) carried by the 

 ninth abdominal segment, and a pair of small penes (Pen) arising 

 behind the sternal plate of this segment. The presence of two penes 

 in the male might be supposed to be correlated functionally with the 

 presence of two oviducal openings in the female, but there is no evi- 

 dence that coition takes place during copulation, and spermatozoa have 

 not been found in the oviducts of the female (a spermatheca being 

 absent). It seems very probable, therefore, that the eggs are insemi- 

 nated as they issue from the oviducts, since they are carried by the 

 female until deposited in the water. The eggs are said to be easily 

 fertilized artificially outside the body of the female (Wiebe, 1926). 

 The ovaries and the testes are fully developed in the last larval stage, 

 and it is then that the sex elements are brought to maturity. Prior to 

 mating, the eggs are massed in the greatly distended saclike calyces 

 of the oviducts, and the spermatozoa are stored in the seminal vesicles. 



