Putative thermo-/hygroreceptive tarsal sensilla on the sensory legs of an armored harvestman (Arachnida, Opiliones)

Functional fibrillar interfaces: Biological hair as inspiration across scales

Hair, or hair-like fibrillar structures, are ubiquitous in biology, from fur on the bodies of mammals, over trichomes of plants, to the mastigonemes on the flagella of single-celled organisms. While these long and slender protuberances are passive, they are multifunctional and help to mediate interactions with the environment. They provide thermal insulation, sensory information, reversible adhesion, and surface modulation (e.g., superhydrophobicity). This review will present various functions that biological hairs have been discovered to carry out, with the hairs spanning across six orders of magnitude in size, from the millimeter-thick fur of mammals down to the nanometer-thick fibrillar ultrastructures on bateriophages. The hairs are categorized according to their functions, including protection (e.g., thermal regulation and defense), locomotion, feeding, and sensing. By understanding the versatile functions of biological hairs, bio-inspired solutions may be developed across length scales.

Eggs to long-legs: embryonic staging of the harvestman Phalangium opilio (Opiliones), an emerging model arachnid

Background

The comparative embryology of Chelicerata has greatly advanced in recent years with the integration of classical studies and genetics, prominently spearheaded by developmental genetic works in spiders. Nonetheless, the understanding of the evolution of development and polarization of embryological characters in Chelicerata is presently limited, as few non-spider species have been well studied. A promising focal species for chelicerate evo-devo is the daddy-long-legs (harvestman) Phalangium opilio, a member of the order Opiliones. Phalangium opilio, breeds prolifically and is easily accessible in many parts of the world, as well as tractable in a laboratory setting. Resources for this species include developmental transcriptomes, a draft genome, and protocols for RNA interference, but a modern staging system is critically missing for this emerging model system.

Results

We present a staging system of P. opilio embryogenesis that spans the most important morphogenetic events with respect to segment formation, appendage elongation and head development. Using time-lapse imaging, confocal microscopy, colorimetric in situ hybridization, and immunohistochemistry, we tracked the development of synchronous clutches from egg laying to adulthood. We describe key events in segmentation, myogenesis, neurogenesis, and germ cell formation.

Conclusion

Considering the phylogenetic position of Opiliones and the unduplicated condition of its genome (in contrast to groups like spiders and scorpions), this species is poised to serve as a linchpin for comparative studies in arthropod development and genome evolution. The staging system presented herein provides a valuable reference for P. opilio that we anticipate being useful to the arthropod evo-devo community, with the goal of revitalizing research in the comparative development of non-spider arachnids.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-022-00454-z.

Water locomotion and survival under water in a riparian harvestman (Opiliones, Arachnida)

Animals that live by rivers may benefit from being able to cross them, but behavioral adaptations are needed. Additionally, being able to remain submerged is also important if the animal moves under water. Here we asked whether the harvestman Heteromitobates discolor (Opiliones), that lives by rivers, (a) can propel itself across the water surface, (b) moves onto the water if disturbed and (c) can survive for long periods when submerged. Heteromitobates discolor exhibited two gaits on water, whereas a strictly terrestrial species was not able to propel itself. When experimentally submitted to simulated predator attack on a rock on the river, H. discolor walked onto the water, while a strictly terrestrial species did not. Finally, it was able to survive for 6 h under water, presumably due to the conspicuous air film that formed around its body, which was also observed in a strictly terrestrial species. Altogether, these observations suggest that the aquatic environment is not a barrier for regular activity and can be used as an extension of the terrestrial environment for H. discolor.

Evolution of a sensory cluster on the legs of Opiliones (Arachnida) informs multi-level phylogenetic relationships

Phylogenetic relationships in Opiliones (Arachnida) at the suborder level have greatly stabilized in recent years, largely due to advances in molecular systematics. Nonetheless, identifying morphological characters in the context of well-resolved phylogenies is essential for testing new systematic hypotheses and establishing diagnostic markers. Here, we investigate with SEM a promising character system across Opiliones: the sensilla on the distalmost article of legs I and II. We identified four discrete characters and scored species of nearly all families of Laniatores (28 families, 44 species), three Dyspnoi, two Eupnoi and two Cyphophthalmi. Using a phylogenetic backbone compiled from recent and ongoing phylogenomic studies, we trace the evolution of these sensilla using ancestral state reconstruction. We discover a widespread occurrence of three sensilla (a pair of sensilla basiconica and one hooded sensillum) on the anterior legs of all families of Laniatores studied, and that comparable structures occur in the other suborders of Opiliones. Our analysis shows that this sensory field provides diagnostic information at different levels of phylogenetic relationships. We discuss the implications of the widespread occurrence of these sensilla in Opiliones, which have recently been hypothesized as hygro-/thermoreceptors and their putative homology with tarsal organs in Arachnida.

The sensory equipment of a sandokanid: An extreme case of tarsal reduction in harvestmen (Arachnida, Opiliones, Laniatores)

The study of sensory structures has the potential to provide insights into the natural history and evolution of animals. The sensory structures of arachnids are usually concentrated on the pedipalps (the tritocerebral appendages) or on the distal podomere (tarsus) of the anterior walking legs, the latter being the case for armored harvestmen (Opiliones, Laniatores). Therefore, modifications of the tarsus could have direct impacts on the sensory equipment of these animals. Using scanning electron microscopy, we investigated the sensory equipment in an extreme case of reduction in tarsal articles in the harvestman Sandokan truncatus (Sandokanidae), which bears a single tarsomere in all legs, and the potential consequences of this reduction. Additionally, we review the literature on the natural history of the family Sandokanidae. Tarsomeres of all legs are equipped with gustatory sensilla, mechanoreceptors, and a pore organ, but wall-pored olfactory sensilla are restricted to tarsi I and II. Tarsi II present a higher density of olfactory sensilla and also putative campaniform sensilla (strain detectors), which indicates a special sensory function of this pair of legs. Other podomeres are covered with shelled sensilla, a probable chemoreceptor previously unreported in Opiliones. Overall, S. truncatus has types of sensilla largely comparable to harvestmen with longer and subdivided tarsi. However, S. truncatus also exhibits extra-tarsal sensory fields of sensilla basiconica (putative thermo-/hygroreceptors) in previously undescribed sites, and the unique pore organs. Our results establish a basis for further research investigating the natural history, as well as the evolutionary correlations and mechanistic causes of the tarsal reduction in this enigmatic lineage.