Fine structure and phylogenetic significance of ‚flexo-canal epidermal glands‘ in Chilopoda

Abstract

In a comparative light and electron microscopic study, we examined isolated (scattered) epidermal glands, located on the head flanks of various scutigeromorph, lithobiomorph, craterostigmomorph, scolopendromorph and geophilomorph Chilopoda. We describe a distinct type of epidermal glands, named ‘flexo-canal epidermal glands’. This type of epidermal gland is commonly found in all chilopod subtaxa, excluding Scutigeromorpha. The ‘flexo-canal epidermal glands’ exhibit a constant arrangement of one secretory cell, one intermediary secretory cell as well as one canal cell that release the secretion to the outside. Further characteristic features of ‘flexo-canal epidermal glands’ are: 1) their tendency to occur in small aggregations in the direct vicinity of sense organs, 2) a thin, elongated and strongly convoluted/meandering conducting canal running through the canal cell, 3) the presence of a more or less expanded central cavity surrounded by the canal cell, 4) the absence of widening areas in the reservoir (secretory cell) and basal part of the conducting canal (intermediary cell) and 5) the presence of apical loops interconnecting the intermediary cell and the secretory/canal cell. Epidermal glands of the ‘flexocanal’ type are observed in many other euarthropod taxa, including Crustacea, Diplopoda and Hexapoda. It is assumed that ‘flexo-canal epidermal glands’ may have evolved once in the stem lineage of the Mandibulata. Their absence in Scutigeromorpha has to be considered a secondary loss and thus an apomorphy of this subtaxon.