466 PROCEEDINGS OF THE NATIONAL MUSEUM vol.88 



CHARACTERS USED IN IDENTIFICATION 



In a previous paper (Meehean, 1937) it was mentioned that many 

 of the specific characters used at present for identification are variable 

 in nature. Consequently positive identification is not possible unless 

 mature male specimens are available. For the same reason it is dif- 

 ficult for persons unfamiliar with the actual specimens to know posi- 

 tively whether the species has been described if only the females are 

 at hand. Therefore, it is felt that it would be of value to have some 

 method for identification based upon characters that are more or less 

 constant and that can be used for both sexes. A study of the speci- 

 mens in the National Museum has shown that certain characters or 

 combinations of characters can be used for this purpose and has in- 

 dicated that some species are synonymous with others already de- 

 scribed when the characters are fitted to them. So far there has been 

 only one exception to the reliability of the characters selected, and it 

 concerns a foreign species of which a more thorough study will be 

 made later when other data are available. 



The characters utilized are suitable for all the American fresh- water 

 and salt-water forms. The most important of these are the respira- 

 tory areas. Wilson has said many times that they are specific, yet 

 many descriptions have failed not only to figure them but even to 

 mention them. In a few instances they are very similar in closely 

 related species but are always slightly different and when combined 

 with other characters serve to separate the species. Such examples 

 are maculosii^ and versicolor. As the characteristic shapes are dif- 

 ficult to describe, they are figured for each species. 



The segments making up the ribs on the rim of the suction cups 

 are also specific. Although there is slight variation in the number 

 of segments, the formation is very characteristic. In only two species 

 examined was there any great difference in this respect. The male 

 of pugettensis has a smaller number of segments than the female, 

 with a basal segment of a different type. However, one vial con- 

 taining immature specimens had females with typical segments like 

 the male and stages between the two. Since these chitinous thick- 

 enings are known to develop at points of great stress, the difference 

 in the two sexes might be attributed to the fact that the male is 

 much smaller than the female and would probably develop fewer 

 segments. In other instances there is some similarity in the seg- 

 mentation, but when combined with other characters it loses its 

 significance. 



The antennae have characteristic appearances for some species 

 and in many instances help in the identification. The number of 

 spines, the character of their arrangement, and whether a spine or 

 hook is found on the anterior surface of the first antennae are all of 

 assistance. 



