The Freshwater Isopods of the Genus Lirceus (Asellota, Asellidae)

Abstract

In the genus Asellus we have, in the first and second pleopoda of the male, characters which can be relied upon to distinguish the species, but in Lirceus these appendages are so similar in different species that they are, with one exception, useless as a means of separation. This is not because the species of Lirceus are less distinct than Asellus, but requires that the animal as a whole be studied rather than a single appendage. To an experienced observer the various species are easily distinguished in the field by those subtile differences in form and color which are almost impossible to describe. Difficulties begin when one brings specimens into the laboratory and attempts to define the differences. The commonest cause of difficulty is depauperate specimens. Under adverse conditions the animals are stunted, the gnathopoda do not develop their processes, and the uropoda remain in juvenile form. When this depauperization is extreme the collection cannot be determined, but must be assumed to belong to the species in the range of which it was found.