When SEM becomes a deceptive tool of analysis: the unexpected discovery of epidermal glands with stalked ducts on the ultimate legs of geophilomorph centipedes

Andy Sombke,Carsten H G Müller

doi:10.1186/s12983-021-00402-3

Andy Sombke, Carsten H G Müller

Open Access

https://doi.org/10.1186/s12983-021-00402-3

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Abstract

BackgroundThe jointed appendage is a key novelty in arthropod evolution and arthropod legs are known to vary enormously in relation to function. Among centipedes, the ultimate legs always are distinctly different from locomotory legs, and different centipede taxa evolved different structural and functional modifications. In Geophilomorpha (soil centipedes), ultimate legs do not participate in locomotion and were interpret to serve a sensory function. They can be sexually dimorphic and in some species, male ultimate legs notably appear “hairy”. It can be assumed that the high abundance of sensilla indicates a pronounced sensory function. This study seeks for assessing the sensory diversity, however, documents the surprising and unique case of an extensive glandular epithelium in the ultimate legs of three phylogenetically distant species.ResultsThe tightly aggregated epidermal glands with stalked ducts – mistakenly thought to be sensilla – were scrutinized using a multimodal microscopic approach comprising histology as well as scanning and transmission electron microscopy in Haplophilus subterraneus. Hence, this is the first detailed account on centipede ultimate legs demonstrating an evolutionary transformation into a “secretory leg”. Additionally, we investigated sensory structures as well as anatomical features using microCT analysis. Contrary to its nomination as a tarsus, tarsus 1 possesses intrinsic musculature, which is an indication that this podomere might be a derivate of the tibia.DiscussionThe presence and identity of ultimate leg associated epidermal glands with stalked ducts is a new discovery for myriapods. A pronounced secretory as well as moderate sensory function in Haplophilus subterraneus can be concluded. The set of characters will improve future taxonomic studies, to test the hypotheses whether the presence of these specialized glands is a common feature in Geophilomorpha, and that tarsus 1 may be a derivate of the tibia. As the number of epidermal glands with stalked ducts is sexually dimorphic, their function might be connected to reproduction or a sex-specific defensive role. Our results, in particular the unexpected discovery of ‘glandular hairs’, may account for a striking example for how deceptive morphological descriptions of epidermal organs may be, if based on non-invasive techniques alone.

Highlights

The jointed appendage is a key novelty in arthropod evolution and arthropod legs are known to vary enormously in relation to function
External morphology of the ultimate legs – sexual dimorphisms and specialized epidermal glands Macrophotography and SEM analyses of female and male ultimate legs in Haplophilus subterraneus revealed that their external morphology differs from locomotory legs, and that there is sexual dimorphism in size and proportions (Fig. 1b, c)
Epidermal glands with stalked ducts Based on SEM analyses, the majority of cuticular structures on the male ultimate leg are shaft-like protuberances

Summary

Introduction

The jointed appendage is a key novelty in arthropod evolution and arthropod legs are known to vary enormously in relation to function. In Geophilomorpha (soil centipedes), ultimate legs do not participate in locomotion and were interpret to serve a sensory function They can be sexually dimorphic and in some species, male ultimate legs notably appear “hairy”. As there is a high degree of interspecific or -generic disparity in ultimate leg morphology, a functional differentiation must be assumed Based on their resemblance to antennae, scutigeromorph ultimate legs may be considered as sensory appendages, while ultimate legs in some lithobiomorph species play a major role in predator avoidance by secretion of sticky substances [7, 8]. The claw is reduced and the ultimate legs might possess a higher abundance of cuticular structures, like trichomes or sensilla (they notably appear “hirsute”) Among others, these characteristics are considered relevant for species identification [12, 13, 15]

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