1919] Walker: Structure of Orthopteroid Insects 287 



the ninth, and consists of two narrow lateral plates prolonged 

 forward into a pair of slender, curved bars, which pass between 

 the similar processes of the ninth tergite, under cover of the 

 eighth tergite, but meet in the middle line between the two 

 main parts of the latter and three other small plates which 

 apparently represent the median elongated portion of this 

 tergite in Acrydium. The eleventh tergite is possibly rep- 

 resented by the two small oblique plates at the base of the 

 supra-anal plate, and two others laterad of these at the bases 

 of the cerci (cereal basipodites). These relations are also 

 similar to those of the Acrydium. The cerci are short, 

 cylindrical and unsegmented, although in Tridactylus they are 

 distinctly two-segmented, and in certain other species of 

 Ripipteryx {R. mexicana Sauss.) they are imperfectly divided 

 into a number of secondary segments (de Saussure & Zehntner, 

 loc. cit.). 



The supra-anal plate is similar in form to that of Acrydium 

 but is semi-membranous and undivided. It varies, however, 

 considerably in form in different species of the family, as do also 

 the subdivisions of the abdominal tergites, so that it is ques- 

 tionable to what extent these parts are really homologous to 

 those of the Acrydiidae. 



The paraprocts are of very remarkable form, being greatly 

 elongated, with a terminal segment like a pair of styli or a 

 second pair of cerci. These, however, have nothing to do with 

 true styli, for which they were mistaken by Crampton ('18)^''; 

 for the styli, when present in female insects, are always borne 

 by the dorsal valvulae or their homologues, and thus belong to 

 the ninth segment. These peculiar structures, which are present 

 in both sexes in Tridactylus as well as Ripipteryx, were 

 correctly interpreted by de Saussure and Zehntner as out- 

 growths of the paraprocts. 



Another feature possessed by Ripipteryx in common with 

 Acrydium is the position of the eighth spiracle in the eighth 

 tergite, and the other spiracles in the pleural membranes. 



Turning to the sternal region, we find that, except in the 

 case of the eighth, the sternites overlap the tergites instead of 

 vice-versa. The eighth sternum forms the subgenital plate, 

 but is not specially modified for the purpose and lacks an egg- 



1- Bull. Brooklyn Ent. Soc, Vol. XIII, No. 7, PI. V, Fig. 48, 1918. 



