ABSTRACT
As in other arthropods, sensory functions in Crustacea are mainly fulfilled by the setal system. Other sensory functions are performed by visual organs and still poorly known receptors. Setae are the basic units of the setal system. There are macrosetal and a microsetal systems that are distinguished by morphological type, size, and distribution of their elements (microsetae are found on macroscopically smooth surfaces of the exoskeleton). In this paper, only macrosetae are considered. Every seta is a continuation of the cuticle, to which it is joined by a flexible, articular membrane. The setal wall is made of an outer epicuticle lined by cuticular strata. An analysis of setal structure and functional significance requires detailed knowledge of their development during the molt cycle. In Decapoda, the most common setae can be divided into at least fifteen types defined by characters of the shaft and shape of outgrowths. Shape and position of setae reflect their grooming and feeding functions. The dimensions of setae vary according to the body area in which they are found. Within groups, they show dimensional variations following specific gradients. Most setae are arranged in well-defined geometrical groups and can easily be classified. These groups are simple when isolated, but associations of more than one such group form compound arrangements of higher order. Within a species, topographical arrangement of groups (both isolated and compound) is constant.
