﻿12 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 102 



The variation in the number of legless rings is often used as a 

 taxonomic character. This appears to be a useful character in South 

 African forms of Apus (Barnard, 1929), but in the North American 

 forms I have not found it to be of much aid in separating species. 

 In Lepidurus, the number varies within narrow limits (4-5.5 in the 

 great majority of forms), with no specific differences corresponding to 

 the differences in number; a few specimens having 3, 3.5, or 6 legless 

 rings show no other essential differences. There is, of course, the 

 possibility that L. packardi may have a high number and that this 

 character may be of taxonomic importance, the type specimen having 

 6 legless rings. But I do not think this is probable. In two lots (5 

 specimens) that I consider belonging to this species I found 4.5-6 

 legless rings. 



In Apus, the whole range in the American specimens I have seen 

 is 4.5+i-13 in the females and 8-15+i in the males. It seems that 

 this range of variation ought to be great enough to include several 

 species (or subspecies, or forms) with varying ranges, and I will cer- 

 tainly not deny the possibility that lower taxonomic units than species 

 may be separated. However, I have seen a single lot of Apus that 

 covered much of this range (9+i-13 in the females and 12.5-15 +i 

 in the males, as shown in table 6). This is an extreme example, the 

 range of variation being smaller in other samples, especially in those 

 with only a few specimens. 



As a rule, the males have more legless rings than the females of the 

 same species because they usually have fewer leg-bearing rings but 

 a higher total number of rings. 



BODY-RINGS IN PARTHENOGENETIC POPULATIONS 



It seems rather remarkable that an author like Zaddach (1841), 

 extremely careful in so many details, gives a fixed number of 34 

 body-rings in Apus cancriformis. And Kosenberg (1947) definitely 

 states that all specimens, several hundred in number, of his species 

 A. oryzaphagus have 35 body-rings (36 including the telson, according 

 to him). I have seen 6 of them (U.S.N.M. No. 88360) and found 

 5 to have 35 and 1 to have 34 -fi body-rings. Rosenberg describes 

 another form, under the name of A. biggsi, where all specimens have 

 36 rings (37 including the telson), but here it seems uncertain whether 

 he had enough specimens for comparison. I found 3 of them having 

 36 and 1 having 35+i rings (U.S.N.M. No. 88361). In these cases 

 the statements of a fixed number may result from the counting of 

 incomplete rings as ordinary rings. Even so, the range of variation 

 is Very small. A significant observation is that these authors had 

 only females in their samples. 



