8o SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



the base of the penis from a ductus conjunctus (Dejcn) that pro- 

 ceeds from a seminal reservoir (rs), into which the vasa deferentia 

 discharge by a common duct (Vdcn) after first enlarging as a pair 

 of seminal vesicles (see Jordan, 1909, Heymons, 1912). The two 

 exit ducts of Hemimerus open separately into the end-sac of the penis, 

 which is shown everted in the diagram (B, cc). In Forficula (C), 

 as described by Meinert (1864) and by Walker (1922), the vasa 

 deferentia discharge separately into the seminal reservoir, and a single 

 outlet duct (Dejcn) proceeds posteriorly from the latter but soon 

 divides into two branches ; one branch (dej), however, is rudimentary 

 and has no distal connection, the other (Dej) traverses the penis as 

 the functional duct and opens into the eversible end-sac (cc). 



The presence of a rudimentary ejaculatory duct in the Forficuloidea 

 leaves little doubt that the structure of the male organs in Labi- 

 duroidea (fig. 27 A) represents the more generalized condition in the 

 Dermaptera, and that the imperfectly unified organs of the other 

 groups (B, C) have been derived from completely paired structures. 

 In Hemimerus the terminal parts of both exit ducts remain func- 

 tional in association with a penis conjunctus; in Forficuloidea one 

 duct is functionally suppressed. This manner of transition within the 

 Dermaptera from paired penes with individual exit ducts to a single 

 intromittent organ with one functional duct might be supposed to show 

 the method by which the median penis (phallus) and single exit 

 apparatus of other male insects has been evolved. There is, however, 

 no specific evidence of the production of a median intromittent organ 

 by the union of paired penes in any insects other than Dermaptera 

 and Ephemeroptera, and in these two orders a union of the penes has 

 taken place quite independently. Moreover, the ontogenetic develop- 

 ment of the common ductus ejaculatorius of other insects shows that 

 this duct, from its inception, is an independent median ingrowth of 

 the ectoderm, and suggests that it probably originated phylogeneti- 

 cally as an invagination of the body wall at the approximated mouths 

 of the lateral ducts, as it does in the ontogeny of Thysanura. 



PLECOPTERA 



The Plecoptera in some respects may present a fairly generalized 

 condition of the specialized organization of the wing-flexing group of 

 pterygote insects ; but the male reproductive organs are in no way 

 generalized, and they furnish no lead toward the evolution of the 

 genitalia in higher insects. In fact, the structures that form the basis 

 for most of the elaboration in the external genital mechanism of 



