﻿NORTH AMERICAN NOTOSTRACA — LINDER 



19 



too, that these measurements are not easily determined with reason- 

 able accuracy. They increase, of course, with increasing age, but 

 to a degree that certainly varies. 



The shape of the spines near the caudal border of the rings is rather 

 uniform in each of the North Am(»rican species. Accessory small 

 spines sometimes occur on the ventral side of the legless rings, but 

 this does not seem to be a specific character. In several species of 

 Lepidurus, the ventral spines on the hind margin of the legless rings 

 are smaller than the lateral and dorsal ones. The number of these 



4/si 

 body -ring 



Figure 3. — a. Dorsolateral view of distal portion of abdomen of a male of Afus longicaudatus 

 LeConte (U.S.N.M. No. 11602, type of A. lucasanus Packard) showing a right-handed 

 spiral of a little more than two rounds, beginning in the middorsal line and evenly tapering 

 at distal end, X 29. {Sp, spiral; T, telson; S, dorsal sensory setae; C/., caudal filaments.) 

 b, Schematic drawing of the spiral. 



spines, and of the other ones, too, is subject to considerable individual 

 variation. Even in Apus the number of spines on each ring will not 

 do as a specific character, though it is more constant in some lots than 

 in others. Caution certainly is needed in dealing with this character, 

 frequently used and much overrated in old descriptions. 



INCOMPLETE BODY-RINGS 



Barnard (1929, p. 232) seems to be the only author who has paid 

 any attention to the interesting subject of incomplete body-rings. 

 He states: "The interpolation of an incomplete segment immediately 



