Abstract
Coxal setal organs (CSO) have been described by Weyda and Stys (1974). They occur on the apices of coxae of the first to the seventh abdominal segment. Topographically and functionally the CSO are associated with eversible vesicles (organs absorbing water from a wet substrate; this type of water uptake is essential for water economy of Archaeognatha and some Zygentoma). CSO differ slightly in structure in various species of Archaeognatha. There are significant differences in the number of mechanosensitive sensillae between some species of individual genera. But this taxonomic character is of low practical value because of the difficulties in counting the total number of sensillae (small size, several hundred of sensillae per one insect). CSO are missing only in Kuschelochilis, which also lacks eversible vesicles (adaptation to the specific very humid environment of the island Juan Fernandez). Eversible vesicles as well as CSO are present only in primitive groups of Zygentoma. For example, Tricholepidion gertschi (“living fossil” from California) has CSO fully developed. Nicoletia phytophila (Nicoletiidae) has CSO in a reduced form. In other more advanced species of Zygentoma, CSO are either greatly reduced or mostly completely absent (Figs. 1–3).
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