NO. 14 INSECT ABDOMEN — SNODGRASS 65 



Phallus (Pill). — The usual median penis or single intromittent 

 organ of male insects, variously formed and probably developed inde- 

 pendently in several insect groups. 



Preputial sac. — See endophallus. 



Siibgenital plate (IXS). — See hypandrium. 



Titillators (Ttl). — Distal external processes of the aedeagus. 



Virga (Vir). — A terminal phallic spine or slender rod usually aris- 

 ing from the wall of the endophallus and therefore protractile with 

 the aversion of the latter. 



COLLEMBOLA 



The Collembola are hexapods with a differentiated thorax and 

 abdomen, but having at most only six abdominal segments. Their 

 embryology gives no evidence of a greater number of somites ever 

 having beeri present, which fact is surprising considering that the 

 immediate ancestors of all the other hexapods undoubtedly had a 

 standardized abdomen of 12 segments. The single genital opening 

 in each sex of the Collembola is on the extreme posterior part of the 

 venter of the fifth abdominal segment ; the gonopore thus has the 

 same position relative to the terminal segment as has the genital open- 

 ing in Qiilopoda and Protura, but the disparity in the number of 

 somites preceding the genital segment shows that the subterminal 

 position of the gonopore in these three arthropod groups is not a 

 matter of anatomical homology. The genital aperture of the Col- 

 lembola is situated sometimes on a slight elevation of the integument, 

 but there is no development of the external genital region sufficient 

 to constitute an intromittent organ or an ovipositor. 



The internal reproductive organs of Collembola have been studied 

 by Sommer (1885), Fernald (1890), Willem (1900), Lecaillon 

 (1901, 1902, 1902a, 1902&), Imms (1906), and De Winter (1913). 

 The gonads lie ventro-laterally in the abdomen and the posterior part 

 of the thorax (fig. 23 A), and in the mature condition they approxi- 

 mate each other beneath the alimentary canal (C, G). The testes 

 are continuous posteriorly with short lateral ducts that join a common 

 exit duct (A, B, Dej), the anterior end of which may be enlarged 

 as a seminal vesicle (B, Vsm). 



In the younger stages of all Collembola, according to Willem 

 (1900), the gonads are simple fusiform sacs, each tapering anteriorly 

 to a suspensory ligament ; as development progresses, however, the 

 organs in most cases lose their primitive form, become greatly en- 

 larged, and variously modified in shape (fig. 23 F, G). The ger- 

 marial area of each organ in some forms is situated in the dorsal or 



