Sensing from both ends - transformation of formerly locomotory into sensory arthropodia in terminal trunk segments of centipedes
Final Report Abstract
In this project, we investigated the ultimate legs of various centipede taxa using scanning and transmission electron microscopy as well as immunohistochemical and electrophysiological experiments to detect functional transformations. Main goals have been (1) analysis of external morphology of centipede ultimate legs including cuticular sensilla, (2) TEM-analyses of sensilla and ultrastructure-related deduction of sensory modalities, (3) functional verification via electrophysiological experiments, and (4) the analysis of the sensory pathway to primary processing areas in the central nervous system. Main hypotheses to be evaluated in the course of this project have been that (1) enlarged ultimate legs in Lithobiomorpha and Scolopendromorpha serve a defensive function, and (2) sexual dimorphisms correlate with an enhancement of sensory functions, and (3) scutigeromorph ultimate legs primarily have a sensory function (establishment of a posterior antenna). Analyses of the external morphology were conducted for all selected species, including ultrastructural analyses of trichoid sensilla and epidermal exocrine glands. As some of the proposed sensory structures turned out to be glandular (e.g., ultimate legs of geophilomorphs), we consequently shifted our focus to investigate the ultrastructure of “glandular legs” which seem to be much more widespread in centipedes than previously expected. Good examples for glandular ultimate legs are the defensive legs of lithobiid Lithobiomorpha which include numerous, tightly aggregated telopodal glands present on almost every podomere (defensive action provided also by high-speed-camera footages) as well as the “hairy” ultimate legs of many geophilomorphs. As for the latter example, we could show for the first time that the distinctly sexually dimorphic ultimate legs of himantariid geophilomorphs, such as Haplophilus subterraneus, though looking densely equipped with trichoid sensilla, are actually primarily glandular because the epidermis is rich in numerous, aggregated exocrine glands with stalked ducts resembling sensillum shafts. As SEM-observations indicate the presence of large, tightly adjoined gland pores on proximal podomeres of the ultimate legs of various scolopendromorphs and craterostigmomorphs, it becomes more likely that the ultimate leg of the last common ancestor of Pleurostigmophora (including all centipedes except Scutigeromorpha) had ultimate legs with a secretory specialization. However, this hypothesis needs to be evaluated in future studies. We were able to confirm our hypothesis that the extremely elongated ultimate legs of scutigeromorph centipedes primarily have a sensory function and may be considered second (posterior) antennae. Many characters, such as the multi-annulated, flagellar appearance, the number and distribution of beak-like tip-pore sensilla, the absence of “glandular” sensilla, and the specific splitting of sensillar afferents into two distinct domains (striated lamellated and convoluted allantoidal neuropils) indicate (functional) morphological similarity of antenna and ultimate leg. However, the evolutionary origin of the scutigeromorph ultimate leg from a former locomotory appendage is suggested by some morphological traits such as the sequence and proportions of proximal podomeres and the specific ultrastructure of beak-like tip-pore sensilla (5-7 receptor cells). Our assumption that beak-like tip-pore sensilla do not only contain taste receptors but also have the ability to smell volatile odors was confirmed by preliminary electrophysiological recordings (antennoand arthropodiograms).
Publications
- (2017) Serotonergic neurons in the ventral nerve cord of Chilopoda – a mandibulate pattern of individually identifiable neurons. Zoological Letters 3: 9
Sombke A, Stemme T
(See online at https://doi.org/10.1186/s40851-017-0070-y) - (2017) The ultimate legs of Chilopoda (Myriapoda): a review on their morphological disparity and functional variability. Peerj 5: e4023
Kenning M, Müller CHG, Sombke A
(See online at https://doi.org/10.7717/peerj.4023) - (2018) A comparative analysis of the ventral nerve cord of Lithobius forficatus (Chilopoda: Lithobiomorpha): morphology, neuroanatomy, and individually identifiable neurons. Arthropod Systematics & Phylogeny 76(3): 377-394
Schendel V, Kenning M, Sombke A
- A new species of Newportia (Scolopendromorpha: Scolopocryptopidae) from Cuba with a standardized terminology for cuticular structures and on the transformation of ultimate legs. 111th Annual meeting of the German Zoological Society. 10.-15.09.2018 Greifswald
Martinez-Muñoz CA, Sombke A
- The ultimate legs of Lithobius forficatus – a morphological analysis of transformed arthropodia. 111th Annual meeting of the German Zoological Society. 10.-15.09.2018 Greifswald
Kenning M, Müller CHG, Sombke A
- (2019) Comparative morphology of ultimate and walking legs in the centipede Lithobius forficatus (Myriapoda) with functional implications. Zoological Letters 5:3
Kenning M, Schendel V, Müller CHG, Sombke A
(See online at https://doi.org/10.1186/s40851-018-0115-x) - Sensing from both ends? Functional morphology of scutigeromorph ultimate legs. 18th International Congress of Myriapodology. 25.- 31.08.2019 Budapest, Hungary
Kenning M, Müller CHG, Sombke A
- (2021) When SEM becomes a deceptive tool of analysis: the unexpected discovery of epidermal glands with stalked ducts on the ultimate legs of geophilomorph centipedes. Frontiers in Zoology 18: 17
Sombke A, Müller CHG
(See online at https://doi.org/10.1186/s12983-021-00402-3)
