Sombke Andy, Müller Carsten H G
Department of Evolutionary Biology, Integrative Zoology, University of Vienna, Althanstrasse 14, 1090, Vienna, Austria.
University of Greifswald, Zoological Institute and Museum, General and Systematic Zoology, Loitzer Straße 26, 17489, Greifswald, Germany.
Front Zool. 2021 Apr 20;18(1):17. doi: 10.1186/s12983-021-00402-3.
The 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.
The 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.
The 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.
分节附肢是节肢动物进化中的一个关键新特征，已知节肢动物的腿在功能方面差异极大。在蜈蚣中，末对足总是与运动用的腿明显不同，不同的蜈蚣类群进化出了不同的结构和功能变化。在土蜈蚣目（土壤蜈蚣）中，末对足不参与运动，被认为具有感觉功能。它们可能存在两性异形，在某些物种中，雄性末对足明显看起来“多毛”。可以推测，大量的感觉器表明其具有显著的感觉功能。然而，本研究旨在评估感觉多样性，却记录了三个系统发育关系较远的物种的末对足中存在广泛腺上皮这一令人惊讶且独特的情况。
在地下单毛蜈蚣中，使用包括组织学以及扫描和透射电子显微镜在内的多模态显微镜方法，仔细研究了带有柄状导管的紧密聚集的表皮腺——之前误认其为感觉器。因此，这是关于蜈蚣末对足的首个详细描述，证明其进化转变为“分泌性的腿”。此外，我们使用显微CT分析研究了感觉结构以及解剖特征。与它被命名为跗节不同，第一跗节具有内在肌肉组织，这表明这个体节可能是胫节的衍生物。
带有柄状导管的末对足相关表皮腺的存在和特征对于多足类动物来说是一项新发现。可以得出结论，地下单毛蜈蚣的末对足具有显著的分泌功能以及适度的感觉功能。这些特征将有助于未来的分类学研究，以检验这些特殊腺体的存在是否是土蜈蚣目的一个共同特征，以及第一跗节是否可能是胫节的衍生物这两个假设。由于带有柄状导管的表皮腺数量存在两性异形，其功能可能与繁殖或特定性别的防御作用有关。我们的结果，特别是“腺毛”这一意外发现，可能说明了如果仅基于非侵入性技术，表皮器官的形态描述可能会有多么具有误导性。
