Introduction

The mite family Cheyletidae Leach, 1815 (Acariformes: Cheyle toidea) is cosmopolitan and includes more than 440 species in 75 genera (Zhang et al. 2011). Most of these mites are free-living predators, though representatives of several taxonomic groups are permanent ectoparasites of small mammals and birds (Bochkov 2004, 2009). Fossil finds of cheyletids are extremely scarce and described inadequately: one specimen is known from Baltic amber (Koch and Berendt 1854) and another, from Cretaceous Burmese amber (Cockerell 1917).

Recently developed preparation techniques (Sidorchuk 2011, 2013) allow for much closer observation of amber inclusions, especially if the amber is transparent, as was the case for our piece. It was therefore possible to assess the morphological traits normally used for description of extant mites.

In the present paper, we describe a new species of cheyletid from Baltic amber based on a female belonging to the genus Cheletomimus Oudemans, 1904 (Oudemans 1904b) in tribe Cheyletini Leach, 1815. This genus is represented by about 36 extant species of the free-living predators, commonly inhabiting leaves of plants. They are known from all the continents except Antarctica, but mostly from the warmer climatic regions (Fain et al. 2002).

Institutional abbreviations.-CCHH, the private collection of Christel and Hans-Werner Hoffeins, Collection Christel & Hans-Werner Hoffeins, Hamburg, Germany; SMF, Senckenberg Forschungsinstitut und Naturmuseum, Frankfurt, Germany.

Other abbreviations.-1a–c, 2c, 3a–c, 4a–c, coxal setae of coxae 1–4, respectively; a, antelateral leg tarsus seta; acm, antero culminal palpal eupathidia; ag1–3, aggenital setae; b.s., bush-like seta; c2, d2, e2, f2, h1–h3, dorsal setae associated with body segments C, D, E, F, H, respectively; d, dorsal seta; elc, supracoxal seta; emp, empodium; F, femur; ft, fastigial leg tarsus seta; G, genu; g1–2, genital setae; l, lateral seta; m, medial subcapitular seta; oc, ocellus; p, proral leg tarsus seta; per, peritreme; ps1–3, pseudanal setae; se, scapular external prodorsal seta; si, scapular internal prodorsal seta; sul, subultimal palpal eupathidium; tc, tectal leg tarsus seta; Ti, tibia; ti.cl, tibial claw; u, unguinal leg tarsus seta; ul, ultimal palpal eupathidium; v, ventral seta; ve, vertical external prodorsal seta; vi, vertical internal prodorsal seta; vs, ventrosagittal leg tarsus seta; ω, tarsal solenidion; ϕ, tibial solenidion; ′ and ′′ refer to position of seta in pseudosymmetrical pair on appendage: anterior or posterior, respectively.

Material and methods

The piece of Baltic amber (SMF Be 2537, originally CCHH # 448-1), originally ca. 5 × 5 × 10 mm, was affixed with double- side sticky tape to a plastic rod and trimmed with a diamond disk saw using a Proxxon GG 12 tool. Then it was polished on four sides with the help of pit holders and an OpenScience MiniPolly polisher, as described by Sidorchuk (2013), to obtain a rectangular piece ca. 1 × 0.3 × 0.3 mm. The piece is stored in an o-ring 2 ml centrifuge tube in an aqueous solution of Thymol (2 drops of saturated Thymol solution per 1 ml of water).

Individual focal planes were taken using a Nikon Eclipse 800 microscope in brightfield mode with water immersion lenses (40× and 60×). In some cases, a green interference filter was used to reduce chromatic aberration. Clarity-enhanced and noise-reducted stacks of images (focal planes) were taken with an AmScope MU 900, and then processed with Adobe Photoshop Lightroom 5. Layers were combined with Helicon Focus Pro 4 and 5.3 software, always with some manual addition of significant details from the individual focal planes to the combined images. Micropanoramas were created with GIMP software, with correction of the colour levels when necessary.

Drawings were made using a phase contrast Leica microscope with a camera lucida. In the descriptions below, the idiosomal chaetotaxy follows Grandjean (1939) as adapted for Prostigmata by Kethley (1990). The nomenclature for leg setae follows that of Grandjean (1944). All measurements are in mikrometers (μm).

Fig. 1.

Cheyletid mite Cheletomimus crinitus sp. nov., female holotype (SMF Be 2537, ex. CCHH 448-1) from Baltic amber (Baltic Sea coast, most likely Sambian Peninsula, middle to upper Eocene, 37–54.5 Mya), reconstruction (leg positions altered for clarity). Dorsal (B) and ventral (C) views, right palp (A) and left tibia and tarsus I (D) in dorsal views, left tarsus I in ventral view (E). Abbreviations: 1a–c, 2c, 3a–c, 4a–c, coxal setae of coxae 1–4, respectively; c2, d2, e2, f2, h1–h3, dorsal setae, associated with body segments C, D, E, F, H, respectively; ′ and ′′ refer to position of seta in pseudosymmetrical pair on appendage: anterior or posterior, respectively.

Fig. 2.

Cheyletid mite Cheletomimus crinitus sp. nov., female holotype (SMF Be 2537, ex. CCHH 448-1) from Baltic amber (Baltic Sea coast, most likely Sambian Peninsula, middle to upper Eocene, 37–54.5 Mya), brightfield layered micropanaramas, each combined from ca. 170 focal planes. Dorsal (A) and ventral (B) views. Abbreviations: 1a–c, 2c, 3a–c, 4a–c, coxal setae of coxae 1–4, respectively; ag1, anterior aggenital seta; c2, d2, e2, f2, h1–h3, dorsal setae, associated with body segments C, D, E, F, H, respectively; oc, ocellus; se, scapular external prodorsal seta; si, scapular internal prodorsal seta; ve, vertical external prodorsal seta; vi, vertical internal prodorsal seta.

Fig. 3.

Cheyletid mite Cheletomimus crinitus sp. nov., female holotype (SMF Be 2537, ex. CCHH 448-1) from Baltic amber (Baltic Sea coast, most likely Sambian Peninsula, middle to upper Eocene, 37–54.5 Mya). A. Dorsal view of gnathosoma and anterior opisthosoma (micropanorama assembled from ca 80 images). B. Detail of left palp in ventral view (assembled from 18 images). C. Posterior of ano-genital area in lateral antiaxial view in lateral antiaxial view (assembled from ca. 40 images). D. Left leg I tarsus in lateral antiaxial view (from 22 focal planes). Note extremely poor visibility of seta p′′ against the background of the leg. Abbreviations: a, antelateral seta; acm, anteroculminal palpal eupathidia; ag1–3, aggenital setae, b.s., bush-like seta; d, dorsal seta; elc, supracoxal seta; emp, empodium; F, femur; ft, fastigial seta; G, genu; g1–2, genital setae; h3, seta of segment H; l, lateral seta; oc, ocellus; p, proral seta; per, peritreme; ps1–3, pseudanal setae; tc, tectal seta; Ti, tibia; ti.cl, tibial claw; u, unguinal seta; ul, ultimal palpal eupathidium; ve, vertical external prodorsal s4eta; vi, vertical internal prodorsal seta; vs, ventrosagittal seta; ω, tarsal solenidion; ′ and ′′ refer to position of seta in pseudosymmetrical pair on appendage: anterior or posterior, respectively.

Discussion

According to rough estimates made with a molecular clock (18S rDNA), the family Cheyletidae diverged with the Syringophilidae from a common ancestor approximately 180 Mya at the beginning of the Jurassic (Dabert et al. 2010). Despite this long age estimate, only two records of fossil cheyletids are known: Cheyletus portentosus Koch and Berendt, 1854 from Baltic amber (middle to upper Eocene of the Baltic coast, 37– 54.5 Mya; Poinar 1992; Weitschat and Wichard 2010) and C. burmiticus Cockerell, 1917 from the Cretaceous Burmese amber (Hukawng Valley in North Myanmar, Cenomanian-Albian boundary, 100 Mya; Shi et al. 2012). The specimen of C. portentosus is a female, which certainly does not belong to the genus Cheyletus Latreille, 1796 because its anterior legs are too short. It could belong to the genus Cheletomimus or closely related genera. Unfortunately the fine morphological details of this specimen are not described and the type was not available for study, so its generic assignment cannot be determined with confidence and comparison at the species level is impossible. We propose, therefore, to consider this species as nomen dubium. From the description of C. burmiticus, we can only conclude that this is a heteromorphic male of some free-living cheyletid.

We are the first, for Cheletomimus crinitus sp. nov., to study the external morphology of an extinct cheyletid species in detail. Its specialized structures (aberrant dorso-median setae of the idiosoma, granulated rostrum, leg setation, etc.) are comparable to those of extant representatives of the morphologically derived C. (H.) wellsi group, and this species has no plesiomorphic character states that would allow us to put it to the base of the wellsi group phylogeny. We speculate, therefore, that the genus Cheletomimus probably appeared earlier than 50 Mya, and that many modern genera of free-living cheyletids existed in the Eocene.