NO. 8 INSECT ABDOMEN SNODGRASS I7 



ing to the paired gonopores and the paired ducts have been matters 

 of discussion and diversities of opinion for more than a century. A 

 historical survey of the subject from 1815, when Herold first showed 

 that a part of the female duct system is of ectodermal origin, is given 

 by George (1929). 



In the Dermaptera. as we have seen, the lateral oviducts open into 

 a short median ectodermal ingrowth between the seventh and eighth 

 abdominal sterna. Several writers have asserted that the same condi- 

 tion exists in Mantidae and Blattidae, but, as will later be shown, 

 this idea is based on an erroneous interpretation of the anatomical 

 facts in the adult insects of these families. Many insects, however, 

 in their embryonic or postembryonic development, recapitulate the 

 primitive condition found in Dermaptera, in that the first rudiment 

 of the common oviduct appears as a median ingrowth on the posterior 

 part of the seventh venter, with which the approximated lateral ducts 

 become united. 



The variable position of the female genital opening in different 

 groups of insects has created a perplexing morphological problem, 

 but the recent work of Nel (1930) on Acrididae, and of George 

 (1929) and Metcalfe (1932a) on the hemipteron Philaenns leaves 

 no doubt that the primitive position of the median gonopore in female 

 insects is on the posterior part of the seventh abdominal segment 

 l)ehind the seventh sternum, and that the definitive opening, whether 

 on the eighth or the ninth segment, is a subsequent formation. These 

 writers are not entirely in accord as to how the egg duct is extended 

 from the seventh to the eighth or ninth segment, but their observations 

 lead to the same essential conclusions. 



It is agreed that the first rudiment of the median duct is formed 

 in a late embryonic or an early nymphal stage as an ectodermal in- 

 vagination on the posterior part of the venter of the seventh abdominal 

 segment (fig. 4 A. Ode'). According to Nel and George the primitive 

 gonopore {Gpr') here located in Acrididae and Philaenus runs out 

 into a median groove on the eighth venter. Later, the edges of this 

 groove, beginning anteriorly, grow together and convert the channel 

 into a cuticle-lined tube. In this way the primary opening on the 

 seventh segment (A, Gpr') is lost as the median duct is extended 

 Ihrough the eighth segment (B, Ode) and acquires its definitive open- 

 ing (Gpr) behind the sternal plate of this segment. 



A second invagination is formed in most insects at the posterior end 

 of the eighth abdominal segment, which gives rise to the sperm re- 

 ceptacle, or spermatheea (B. Spt). Hence, with the posterior exten- 

 sion of tlie oviduct, the gonopore and the spermathecal opening come 



