ABSTRACT
Setae are ubiquitous structures of crustaceans, but their classification has remained a significant problem. The schemes that do exist utilize a variety of morphological features, most of which have been arbitrarily chosen and emphasized. Further, they were developed for decapods and do not transfer well to other crustacean groups. In this paper, criteria established by Riedl (1978), and modified by Rieger and Tyler (1979) for determining the probability that similar structures are homologous, are applied. Structures considered to indicate homology, and which are therefore referred to as primary structures of setae, are presence or absence of an annulation, presence or absence of setules on the shaft, mode of articulation, and (possibly) the presence of a chemoreceptive tip. Features that are primarily mechanical or structural in nature, and which therefore have no value for indicating homology, are presence or absence of denticulations, strength of denticulations, degree of cuticularization of the shaft, diameter of setal lumen, features of the basal septum, and ringing of the distal part of the shaft. On the basis of the homologous structures, four fundamental types of setae were determined to exist: I, annulate, with setules; II, annulate, without setules; III, non-annulate, robust; IV, non-annulate, small, non-robust. Examples of all types and an expanded classification are given.
