﻿NORTH AMERICAN NOTOSTRACA — LESIDER 11 



and large specimens in the same sample, and second, by comparing 

 the old and the new integument of specimens in molting. All my ob- 

 servations agree that there is no general rule that larger specimens have 

 a higher total number of rings than smaller ones. On the contrary, I 

 have not infrequently found relatively small specimens with a high 

 number (see fig. 14). Also, I have thoroughly examined about a 

 hundred molting specimens of various species, and in none of them 

 have I found a new ring appearing from the telson in the underl3dng, 

 new integument. Judging from this, I think I am entitled to assume 

 a fixed total number of rings in an adult specimen. 



As for the variation in the number of leg-bearmg rings, I have 

 found T^ clear rule when comparmg larger and smaller specimens 

 from the same lot. Wlien examining moltmg specimens, however, I 

 have found two cases where the number of pairs of legs is increased 

 by one in the underlying, new integument. Thus wo must reckon 

 with the possibility of an increase in the number of legs, and, conse- 

 quently, even a shglit increase in the number of abdominal leg-bearing 

 rings, with an increase in the size of the animal. However, I think 

 we can be sure that this increase, if present, will not be great. 



The usual range m the number of leg-bearing abdominal rmgs in 

 the genus Lepiclurus is 9.5-13. This applies to all extra-American 

 species I have examined {L. apus, L. kirkii, L. viridis), and to most 

 American species {L. arcticus, L. packardi, L. couesii). This condi- 

 tion, together with the similarly small number of legless rmgs, at least 

 in some cases helps to explain the short abdomen commonly men- 

 tioned as a character separatmg Lepidurus from Apus. Packard 

 (1883), however, mentions that L. hilohatus has an unusually long 

 abdomen, with many body-rings exposed behind the carapace; and I 

 found, when examining a specimen of this species marked "type" in 

 the U. S. National Ivluseum., that it lias 16.5 leg-bearing abdominal 

 rings. Further, in material consistmg of more than a hundred speci- 

 mens of a new species from the State of Washington, I found this 

 number to be 14.5-18, while 12 specunens of a variety of this species had 

 16-17.5. Corresponding to the high number of leg-bearing abdominal 

 rings we have a higher total number of body-rings here than in other 

 species of the genus; there are more legs, and, generally, more rings 

 exposed behind the carapace. In my opinion, these figures indicate 

 that there are two groups of species within the genus Lepidurus, 

 whatever the taxonomic category we choose for them. 



In the American forms of Apus, the number of leg-bearing abdom- 

 inal rings ranges between 16 and 21 (males 16-18.5, females 17.5-21). 

 In one large lot I have seen every number in this range represented (see 

 fig. 31). Usually, the females have more, sometimes many more, 

 leg-bearing abdominal rings than the males of the same species. 



