SUBTERRANEAN AMPHIPOD STYGONECTES 13 



parts of animals in this genus have at least some diagnostic value, 

 and at one time or another almost every structiu-e was utilized to 

 some extent taxonomically. 



Probably one of the most reliable and accurate "tools" used in 

 separating morphologically closely related species was the "yard- 

 stick of difference" derived from knowledge of morphological differ- 

 ences between closely related sympatric (or syntopic) species. Appli- 

 cation of observable differences between sympatric species was further 

 useful because it was one of the few means available which could be 

 relied upon to distinguish between phenotypically similar but geneti- 

 cally distinct populations. 



Two major problems inherent in amphipod taxonomy are: (1) 

 continuous growth and development thi'ough successive instars, 

 and (2) variation in size and proportion of a number of taxonomically 

 important characters in older animals. Although sexes can generally 

 be recognized quite early in the hfe cycle of many ampliipods, molting 

 and its attendant structural and size changes continue throughout 

 most of the animal's normal life span. With the above factors in 

 mind, it is of singular importance to base, whenever possible, tax- 

 onomic determinations and descriptions on animals that have reached 

 sexual maturity and to have sufficient knowledge of the amount of 

 variation which can be expected in diagnostically significant body 

 structures of older animals. 



Recognition of sexually mature females was far easier in most 

 species than recognition of se?aially mature males. In the Gam- 

 maridae in general, sexual maturity is typically reached in the female 

 when brood plates become enlarged and fringed with long setae 

 (Hynes, 1955). Hynes has further pointed out that in some gam- 

 marids the brood plates acquire serrated edges at the last molt before 

 maturity. Although his observations were based on epigean species, 

 they apparently have direct apphcation to the material at hand. 

 In the present study sexually mature females of different sizes were 

 often noted in samples, but appreciable differences in meristic char- 

 acters between smaller and larger specimens were not usually ap- 

 parent, despite the fact that growth undoubtedly continued after 

 sexual maturity was initially attained. At most, larger, sexually 

 mature females had only a few more spines and setae than smaller 

 ones. In view of these observations it was generally assumed through- 

 out this investigation that the presence of marginally setose brood 

 plates was indicative of sexual maturity in females, and animals that 

 had reached this stage of development were readily utihzed for tax- 

 onomic discrimination. The most appreciable distinction between 

 larger and smaller sexually mature females were allometric size 

 differences in a number of the appendages, especially in length of 



