■NORTH AMERICAN SPRING-TAILS— FOLSOM. 



455 



Fig. 7.— TOMOCER0S flavescens, vae. amekicanus. 



POSTERIOK SEGMENTS OF ABDOMEN OF MALE, SHOW- 

 ING PROTRUDED GENITAL SEGMENT. /, FOLD. X 30. 



The furcula is an appendage of the fourth abdominal segment in all 

 CoUemboia. This relation, evident in the more generaUzed genera, 

 as Acliorutes, is not obvious in such forms as Tomocerus without 

 the aid derived from a study of certain other genera, particularly 

 Isotoma. Indeed, it has been said frequently that the furcula in 

 Tomocerus is appended to the fifth abdominal segment. The correct 

 view is that maintamed by 

 Willem ('00, pp. 40, 45), to 

 whose evidence I can add a 

 little, as follows: 



In a specimen treated with 

 hot potassium hydroxide a 

 suture becomes evident that 

 separates the sternum of the 

 fourth abdominal segment 

 from the tergum of the fifth, 

 as in figure 6. 



Before such treatment the 

 suture is indistinguishable, 

 and the furcula appears to be attached to the fifth abdominal 

 segment. 



Willem ('00, p. 44) notes that the genital segment (fifth abdominal) 

 is more elongate in the male than in the female. This elongate con- 

 dition is shown in figure 7, in which the genital segment is extended 

 for its entire length. When not extended the posterior part of the 

 segment is telescoped into the anterior part, giving the appearance 

 shown in figure 6, in which / represents the external line of fold- 

 ing, which might easily be mistaken for a 

 suture. In the female the genital segment 

 is telescoped but slightly. 



Willem called attention also to the pres- 

 ence of three ''cerci" on the anal segment, 

 and stated that these are shorter in the 

 male than in the female. The so-called 

 cerci, one dorsal and two ventral, are shown 

 in figures 7 and 8. 



Returning to the furcula — each dens is 

 divided by two transverse sutures into 

 three regions (fig. 9). In brittle alcoholic specimens the dens fre- 

 quently breaks at one or the other of these sutures. The dental 

 spines are limited to the middle and the proximal region or to 

 the middle region. These spines, as regards number, relative size, 

 form, and arrangement, furnish good specific characters. The spines 

 increase in number with the age of the individual and vary a little in 

 number in individuals of the same age. It follows that one should 



Fig. 8.— Tomoceeus flavescens, 

 vae. ameeicanus. Anal seg- 

 ment op male, showing ceeci. 

 X 102. 



