Histiogaster altilis, KolesnikovK & VorontsovK & PerkovskyK & KlimovK, 2025

KolesnikovK, Vasiliy B., VorontsovK, Dmitry D., PerkovskyK, Evgeny E. & KlimovK, Pavel B., 2025, An exceptionally well-preserved Eocene fossil mite, Histiogaster altilis sp. n. (Acari: Astigmata), from tree sap: Evidence of morphological and ecological niche conservatism, with a review of fossil Astigmata, Acarologia 65 (1), pp. 213-241 : 237-238

Habitat of Histiogaster altilis sp. n.

Fossil mites of H. altilis sp. n. likely lived close to the site where resin was released, as indicated by the presence of several males and females that probably belong to the same population. Additionally, the numerous microinclusions around the mites, which we interpret as phloem sap emulsion, along with abundant filamentous hyphae, suggest proximity to a rich and likely organic substrate. It should be noted that these hyphae are likely to belong to resinicolous fungi related to those found both in Cretaceous ambers and in extant coniferous resins ( Speranza et al., 2015 ; Peñalver et al., 2025), which, in our opinion, supports that the

Eocene resin was enriched with a sugary tree sap. Resinicolous fungi were also reported from

Rovno amber ( Sukhomlyn et al., 2021).

Moreover, H. altilis sp. n. is morphologically similar to H. carpio (Kramer, 1882) and H.

iberica Kadzhaya, 1959, which are modern inhabitants of tree sap flows. However, the fossil mite also shares similarities with H. anops , which inhabits the larval tunnels of two species of pine weevils ( Hylobius pales (Herbst, 1797) and Pachylobius picivorus (Germar, 1824)) in the southern United States ( Davis and Hunter, 1963). Considering all available evidence, we propose that H. altilis sp. n. inhabited fermented tree sap, like many modern Histiogaster species ( Türk and Türk, 1957 ; Bugrov, 1997 ; Klimov et al., 2022). However, we cannot entirely exclude the possibility of an association with beetle galleries given the data at hand.

It is interesting to compare the newly described H. altilis sp. n. with another fossil astigmatan, Glaesacarus rhombeus , which is common in Baltic and Rovno amber ( Perkovsky et al. 2007). The probable microhabitats of G. rhombeus may include insect burrows in wood,

sap flow, or water-filled tree holes ( Sidorchuk and Klimov, 2011), although subcortical spaces are the most likely habitat. One view suggests that this species could forage on hardened resin surfaces and possibly feed on carrion ( Grünemaier, 2017). Surprisingly, despite the abundance of this species in Eocene amber, modern congeners have not yet been found. In contrast, mites of the genus Histiogaster have been able to maintain a conserved phenotype and ecology,

presumably feeding on tree sap flows since the Eocene.

Advanced imaging

Here, we approached the limit of optical resolution in amber by using modern confocal microscopy ( Vorontsov and Voronezhskaya, 2022 ; Vorontsov et al., 2023). This was needed to observe minute taxonomically important features, like the shape of the Grandjean’s organ ( Fig.2E ) or the male’s anal suckers ( Fig.16B–D ). To reach high resolution of imaging, we (1) fine-polished the amber pieces, leaving only a very thin layer of amber over the inclusion and (2) used high-resolution DIC microscopy and CLSM with a super-resolution detector. We found that successful application of CLSM highly depends on the level of fossil autofluorescence, which is usually better in deeper, less deteriorated layers of amber. Deteriorated surface layers of amber become fluorescent and mask the fluorescence from the fossil (Vorontsov et al., 2023). Among the specimens that we studied here, K-15219-C and K-15220-A1 were found closer to the surface of the original pieces, while K-15219-A and -B were extracted from deeper layers (compare background fluorescence in Fig.10C , Fig.11A and Fig.6A to Fig.8A ). However, even partially deteriorated amber did not prevent high-resolution imaging ( Fig. 8 CD). As a result, with our advanced imaging technique, we scored almost all anatomical characters. In contrast, the most complete fossils typically preserve less one third of the anatomical characters used to infer evolutionary relationships among living species ( Edgecombe, 2010).

Figures

Figure 2. Histiogaster altilis sp. n., adult female, holotype, specimen K-15220-B, dorso-lateral view, micrographs: A⚶focus-stacked light micrograph, showing the imprint in amber (outer contour) and, inside it, fossilized remains of a mite; ⚶B maximum intensity projection of confocal stack. Note the alveoli of setae e1 brightly fluorescent; C ⚶gnathosoma, super-resolution (Airyscan) confocal stack, maximum intensity projection; inset shows supracoxal seta (scx) separately stacked in Helicon Focus; D, E ⚶bifurcate Grandjean’s organ was best resolved in transmitted light (100x, DIC) and in the CLSM scan (63x, Airyscan).

Figure 6. Histiogaster altilis sp. n., adult female, specimen K-15219-B, paratype, lateral views, micrographs: A⚶maximum intensity projection of confocal stack. B ⚶DIC light micrograph. C ⚶ chelicera, enlarged.

Figure 8. Histiogaster altilis sp. n., adult female, specimen K-15220-A1, paratype, ventral and frontal views: A ⚶ventral view, maximum intensity projection of confocal stack. Gnathosomal structures and most of the legs are projected ventrally;⚶ Bfrontal view, showing gnathosoma

Figure 10. Histiogaster altilis sp. n., adult male, specimen K-15219-A, dorso-lateral view, micrographs: A, B⚶DIC light micrographs showing the dorsal aspect of the mite and the bifurcate Grandjean’s organ⚶; C maximum intensity projection of the confocal stack. Note the bright fluorescence of setae1 clearly shown against the body; D, E, ⚶ Funidentified microscopic structures attached to (or originating from) the posterior part of the body; D, E⚶dorsal view; F⚶posterior view. One of the possible interpretations could be that these are fractures in amber.

Figure 11. Histiogaster altilis sp. n., adult male, specimen K-15219-A, ventro-lateral view, confocal images: A ⚶whole mite; B ⚶bifurcate Grandjean’s organ; C⚶parts of gnathosoma and palpi; D, E⚶genital structures. All images are maximum intensity projections except E, which is a manually focus-stacked image from the part of the stack shown in D.

Figure 16. Histiogaster altilis sp. n., adult male, specimen K-15219-E, micrographs: A ⚶ventral view, maximum intensity projection of confocal stack; B ⚶anal suckers, confocal Airyscan super-resolution mode, maximum intensity projection; C, D ⚶the same, partial stacking showing the ps3 setae.