Ultrastructure, functional morphology and evolution of recto-canal epidermal glands in Myriapoda

•We newly define the type of recto-canal epidermal glands (rceg) in Chilopoda.•The rceg are characterized by a broad set of ultrastructural characters.•The rceg are homologous among Chilopoda and probably also across Euarthropoda.•The rceg may become glandular units in aggregated or compound glandular organs.•The rceg and flexo-canal epidermal glands (fceg) are of phylogenetic importance.

In Chilopoda, solitary epidermal glands are composed of a couple of cells only. These glands are highly abundant on the entire body surface and are distributed throughout the single-layered epidermis. Some authors provided more or less comprehensive observations on the structure of epidermal glands of specific chilopod taxa. However, no information is hitherto available on the ultrastructural diversity of these glands. Furthermore, potential homologies of these chilopod epidermal glands and of their characteristic cellular components remain unknown. Based on our results, we are now able to distinguish two types of epidermal glands in Chilopoda that can be clearly distinguished by their structure and the course of their conducting canal: recto-canal epidermal glands (rceg) and flexo-canal epidermal glands (fceg). In the present paper, we focus on the rceg. We examined the ultrastructural organization of these glands in the head region and on the anterior trunk segments of various representatives of the five extant chilopod orders by light- and electron-microscopy. According to our terminology, rceg consist of up to five different cell types including: a) distal canal cells, b) proximal canal cells, c) intermediary cells, and d) two different types of secretory cells. Intermediary and canal cells form a common conducting canal. The rceg may taxon-specifically differ in relative size and subcellular architecture, but all have the following features in common: 1) a wide distribution on various body regions among all five chilopod subtaxa, 2) the straight, broad and locally dilated conducting canal surrounded by closely packed microvilli or microvilliform infoldings around the apex of the canal cell(s), and 3) the tendency to aggregate to form compound glandular organs of massive size and complexity. Tricellular glandular units established by three different cell types are observed in Scutigeromorpha and Geophilomorpha, whereas four cell types constitute rceg in Lithobiomorpha and Craterostigmomorpha. Five different cell types per glandular unit are found only in Scolopendromorpha. The partial cuticularization of the lower part of the conducting canal formed by the intermediary cell, as found in Chilopoda, differs from the pattern described for equivalent euarthropod epidermal glands, as for instance in Hexapoda. Their wide distribution in Chilopoda and Progoneata makes it likely that tricellular rceg were at least present in the last common ancestor of the Myriapoda. Concerning Chilopoda, the evolution of highly diverse rceg is well explained on the basis of the Pleurostigmophora concept. Glands of the recto-canal type are also found in other arthropods. The paper discusses cases where homology of rceg and also fceg may be assumed beyond Myriapoda and briefly evaluates the potentials and the still-to-be-solved issues prior to use them as an additional character system to reconstruct the phylogeny of the Euarthropoda.