Cheliceral chelal design in free-living astigmatid mites

Abstract

Cheliceral chelal design in free-living astigmatid mites (Arthropoda: Acari) is reviewed within a mechanical model. Trophic access (body size and cheliceral reach) and food morsel handling (chelal gape and estimated static adductive crushing force) are morphologically investigated. Forty-seven commonly occurring astigmatid mite species from 20 genera (covering the Acaridae, Aeroglyphidae, Carpoglyphidae, Chortoglyphidae, Glycyphagidae, Lardoglyphidae, Pyroglyphidae, Suidasiidae, and Winterschmidtiidae) are categorised into functional groups using heuristics. Conclusions are confirmed with statistical tests and multivariate morphometrics. Despite these saprophagous acarines in general being simple ‘shrunken/swollen’ versions of each other, clear statistical correlations in the specifics of their mechanical design (cheliceral and chelal scale and general shape) with the type of habitat and food consumed (their ‘biome’) are found. Using multivariate analyses, macro- and microsaprophagous subtypes are delineated. Relative ratios of sizes on their own are not highly informative of adaptive syndromes. Sympatric resource competition is examined. Evidence for a maximum doubling of approximate body volume within nominal taxa is detected but larger mites are not more ‘generalist’ feeding types. Two contrasting types of basic ‘Bauplan’ are found differing in general scale: (i) a large, chunk-crunching, ‘demolition’-feeding omnivore design (comprising 10 macrosaprophagous astigmatid species), and (ii) a small selective picking, squashing/slicing or fragmentary/‘plankton’ feeding design (which may indicate obligate fungivory/microbivory) comprising 20 microsaprophagous acarid-shaped species. Seventeen other species appear to be specialists. Eleven of these are either: small (interstitial/burrowing) omnivores—or a derived form designed for processing large hard food morsels (debris durophagy, typified by the pyroglyphid Dermatophagoides farinae), or a specialist sub-type of particular surface gleaning/scraping fragmentary feeding. Six possible other minor specialist gleaning/scraping fragmentary feeders types each comprising one to two species are described. Details of these astigmatid trophic-processing functional groups need field validation and more corroborative comparative enzymology. Chelal velocity ratio in itself is not highly predictive of habitat but with cheliceral aspect ratio (or chelal adductive force) is indicative of life-style. Herbivores and pest species are typified by a predicted large chelal adductive force. Pest species may be ‘shredders’ derived from protein-seeking necrophages. Carpoglyphus lactis typifies a mite with tweezer-like chelae of very feeble adductive force. It is suggested that possible zoophagy (hypocarnivory) is associated with low chelal adductive force together with a small or large gape depending upon the size of the nematode being consumed. Kuzinia laevis typifies an oophagous durophage. Functional form is correlated with taxonomic position within the Astigmata—pyroglyphids and glycyphagids being distinct from acarids. A synthesis with mesostigmatid and oribatid feeding types is offered together with clarification of terminologies. The chelal lyrifissure in the daintiest chelicerae of these astigmatids is located similar to where the action of the chelal moveable digit folds the cheliceral shaft in uropodoids, suggesting mechanical similarities of function. Acarid astigmatids are trophically structured like microphytophagous/fragmentary feeding oribatids. Some larger astigmatids (Aleuroglyphus ovatus, Kuzinia laevis, Tyroborus lini) approximate, and Neosuidasia sp. matches, the design of macrophytophagous oribatids. Most astigmatid species reviewed appear to be positioned with other oribatid secondary decomposers. Only Dermatophagoides microceras might be a primary decomposer approximating a lichenivorous oribatid (Austrachipteria sp.) in trophic form. Astigmatid differences are consilient with the morphological trend from micro- to macrophytophagy in oribatids. The key competency in these actinotrichid mites is a type of ‘gnathosomisation’ through increased chelal and cheliceral height (i.e., a shape change that adjusts the chelal input effort arm and input adductive force) unrestricted by the dorsal constraint of a mesostigmatid-like gnathotectum. A predictive nomogram for ecologists to use on field samples is included. Future work is proposed in detail.

Transitional chelal digit patterns in saprophagous astigmatan mites

“…Digit nomenclature and division into rami follows Fig. 5 in Bowman (2024). Mite chelae were orientated by reflection and rotation such that their adductive output lever moment arm directions (i.e., L2M see Bowman 2021b) were aligned.…”

“…This is true if you map the vertebrate head and jaws to a mite's gnathosoma and chelicerae. Many useful conclusions about differential trophic adaptations are available from hominid studies (e.g., Laird et al (2016)), however looking at just the sizes of chelal ornamentations (Bowman 2024) in mites is not the whole story in order to help explain any co-existence of apparently competitive species. Another issue arises.…”

“…As Bowman (2024) illustrates, each digit feature interacts with the others in an arrangement (or model type) to make up the particular operating characteristics of the chela. One is interested here in the overall pattern, not just the relative sizes of each moveable digit feature (as estimated by say division or regression of specific dental features with some sort of body size measure) on this composite masticatory tool.…”

“…2D geometric morphometrics is inappropriate. Bowman (2024) introduces the concept of two different areas of possible moveable digit evolution. In one case, digit tip elongation is the main modality (together with dental ornamentation).…”

Growth, development, and host preference of Histiostoma feroniarum (Acaridida: Histiostomatidae): effects of temperature and types of mushroom cultivar

“…Astigmatid mites are phylogenetically closely related to Oribatids. Many species are fungivorous and commonly occur as detritivorous saprophytes in decaying organic matter in the soil (Duek et al ., 2001; Bowman, 2021). The fungivorous astigmatid mite Histiostoma feroniarum Dufour is an economically important pest of vegetables, Chinese herbs, mushrooms, and stored products.…”