﻿NORTH AMERICAN NOTOSTRACA — LINDER 17 



GROUPING SPECIES BY NUMBER OF BODY-RINGS 



To sum up some of the facts I have observed about the number of 

 rings in species from all over the world, we get the distribution shown 

 in table 3, p. 42, in w^hich I have included the number of pahs of legs. 

 The range of variation given in this table applies to the group of 

 species as a whole, each species usually covering only a part of the 

 whole range. Several species of both genera are missing because I 

 have not yet been able to get specimens, but there is strong reason to 

 believe that the missing species are merely synonyms of those recorded. 

 Even some of the listed species are probably synonyms. 



I have not seen any specimens of Lepidurus patagonicus Berg, but 

 judging from the description (Berg, 1900) and redescription (Biraben, 

 1945) we may assume that it belongs, with L. arcticus, to the first group 

 in table 3. The author says that it has 29 segments, 5 of which are 

 apodous. Notliing is mentioned about variation or about number of 

 legs. Only two specimens seem to be known, the later report of 

 Bruch (1916) about a new locality being very uncertain because the 

 illustration of a specimen (op. cit., fig. 1) clearly shows a form of the 

 genus Apus. Thus L. patagonicus seems to be closely related to 

 L. packardi and is perhaps identical with this species. It should be 

 added that I regard all North American forms of Apus, and at least 

 some of the South American ones, too, as belonging to the species 

 A. longicaudatus, and L. macrurus as a synonym of L. couesii. 



There are strong reasons to consider the groups of species in Lepi- 

 durus (see table 3) to be just as homogeneous and well demarcated 

 from each other as genera usually are. Further, A. cancriformis is 

 in important respects well distinguished from the other species of 

 Apus. However, I think it best to postpone the taxonomic alterations 

 thus implied until more is known about the number of body rings and 

 legs, an extensive field of investigation that has only recently been 

 brought to our attention. 



EXPOSED BODY-RINGS 



In most descriptions the number of exposed rings is mentioned 

 and is often stressed as an important character. Very small differ- 

 ences in number have been given the value of a specific character, 

 as w^hen Lilljeborg (1877) says that L. macrurus has sLx to eight 

 exposed rings and L. couesii only five, but Gurney (1923) says he does 

 not think that too much importance should be attached to this. Sars 

 (1901) and Barnard (1929) doubt the importance of the differences, 

 because the varying influences of preservation cause an artificial vari- 

 ation in this respect. I agree with them that this is obviously often 

 the case (see p. 5). 



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