Redescription of the female of bumblebee-associated gamasid mite Proctolaelaps sibiriensis (Davydova, 1988) (Acari: Mesostigmata: Melicharidae) from North Asia

The female of Proctolaelaps sibiriensis (Davydova) (Acari: Mesostigmata: Melicharidae) is redescribed on the basis of type series and new materials collected from bumblebees (Hymenoptera: Apidae) in Tyumen Province, close to the type locality in Novosibirsk Province. A key to world bumblebee-associated species of the genus Proctolaelaps is presented.


Introduction
Like other nest-making animals (e.g. birds, rodents, scarab beetles, wasps, ants and termites), bees, including bumblebees, are hosts to a wide diversity of gamasid mites (Eickwort, 1994). Members of the genera Parasitellus Willmann, 1939 (Parasitidae), Pneumolaelaps Berlese, 1920 (Laelapidae) and some species of Proctolaelaps Berlese, 1923 (Melicharidae) have a more intimate association with bumblebees (Westerboer, 1963;Bregetova, 1977b;Hyatt, 1980;Joharchi et al., 2019). The fauna of bumblebee-associated gamasid mites in North Asia has been well studied before (Davydova, 1976;Davydova and Bogdanov, 1976;Davydova and Nikolsky, 1986;Davydova, 1988;Davydova and Bogatyrev, 1990;Klimov, 1998;Joharchi et al., 2019). During a survey of bumblebee-associated gamasid mites in Western Siberia (Tyumen Region) Proctolaelaps sibiriensis (Davydova, 1988) was collected, a species known only from the type series. The aim of this article is to redescribe the female of P. sibiriensis based on type series and new material because the original description of this species is brief and both the description and illustrations lack many important details. Moreover, we provide a world key to bumblebee-associated species of the genus Proctolaelaps.

Materials and methods
Bumblebees were collected with a sweep net from flowers and placed individually in vials of 70% ethanol. Thereafter, alcohol sediments from the vials were inspected for phoretic mites with the aid of the stereomicroscope Discovery V8 (Carl Zeiss). Specimens were cleared in lactic acid solution and mounted in Hoyer's medium (Walter and Krantz, 2009). The morphology of mites was studied with the aid of an Axioskop 40 microscope (Carl Zeiss), and Axio Imager A2 microscope (Carl Zeiss) with DIC and phase-contrast objectives. Photomicrographs were taken with a M3CMOS 10000 camera (Sigeta).
The morphological terminology generally follows Evans and Till (1979). Dorsal and ventral setae were labelled according to the systems of Lindquist andEvans (1965), andLindquist (1994). Palpal and leg chaetotaxy follows Evans (1963a, b). The notation for idiosomal pore-like structures follows Johnston and Moraza (1991). Lengths of shields were measured from the anterior to posterior shield margins along the midline. The length of the second cheliceral segment was measured from their base to the apex of the fixed digit. The length of legs was taken from the base of the coxa to the apex of the tarsus, excluding the ambulacrum. The measurements are given in micrometers (μm).
Proctolaelaps sibiriensis is redescribed based on type series (Novosibirsk Province) and specimens collected in Tyumen Province. The studied material is deposited in Zoological Museum of Institute of Systematics and Ecology of Animals (Novosibirsk, Russia) (type series of P. sibiriensis), the Zoological Museum of Tyumen State University (Tyumen, Russia), and the Department of Zoology of Odessa I. I. Mechnikov National University (Odessa, Ukraine).
Diagnosis -the diagnosis of Proctolaelaps used here is based on that of Moraes et al. (2016).
Proctolaelaps sibiriensis (Davydova, 1988)  Diagnosis -Dorsal shield with complex volumetric shape, harshly sclerotized with distinct reticulate ornamentation over whole surface except more poorly sclerotized regions in anterior extension and posterior; with 43 pairs of setae; dorsal setae simple, needle-like, except spindle-like setae j1 and serrated setae Z5. Pre-sternal area transversely lineate, without platelets. Sternal shield completely reticulated except posteriorly where overlapped by hyaline flap of epigynal shield, with group of small rounded cells in centre of shield. Poroids iv3 absent. Anterior membranous margin of epigynal shield subtriangular, elongate; posterior margin of shield slightly rounded; shield reticulated. Three pairs of postgenital platelets present. Anal shield subquadrate; reticulated; anus small, located in centre of shield. Soft opisthogastric cuticle with 11 pairs of setae. Epistome subtriangular with small blunt process distally and undulated laterally. Deutosternum with seven rows of denticles, rows 1-6 connected, 7 th row free; rows with two lateral and 1-2 medial denticles. Corniculi with paraxial process. Fixed cheliceral digit with apical hook, two teeth on cutting surface and one small tooth on median line; subapical tooth with well-expressed membranous lobe instead of pilus dentilis. Movable digit with apical hook and one small tooth on cutting surface. Leg chaetotaxy formulae normal for genus, including tibia III with 9 setae; most leg setae thickened and born on small tubercles.
Their distribution is limited to the Holarctic (e.g. P. sibiriensis is known only from Western Siberia). Although the genus Bombus Latreille, 1802 is more widely distributed, and data on Proctolaelaps mites associated with bumblebees in the Neotropical and Indomalayan regions are not recorded at all (Williams, 1998;OConnor, Klimov, 2012).
Nothing is known about the feeding behavior of these mites or any other aspects of their biology. Females of these species of mites disperse and overwinter on adult queen bees (Klimov et al., 2016). OConnor and Klimov (2012) are of the opinion that Proctolaelaps mites could be harmful to their bee hosts by feeding on its eggs, larvae, and pupae. However, we agree with Halliday (2019) that the mites and the bees have a symbiotic relationship though these suggestions are still mere speculation and not yet confirmed.
The following key is based on published descriptions and illustrations, except for P. ornatus and P. sibiriensis.