Rediscovery and redescription of the type species of Myrmozercon, Myrmozercon brevipes Berlese, 1902 (Acari: Mesostigmata: Laelapidae)

A BSTRACT — The type species of Myrmozercon (Mesostigmata: Laelapidae), M. brevipes Berlese, 1902, previously known only from its type collection from Tapinoma erraticum Latreille (Hymenoptera: Formicidae), is rediscovered after more than 100 years. The species is redescribed from specimens collected from two nests of T. erraticum in western Greece, providing new insights into this unusual species, which represents the most highly modiﬁed member of its genus. This species is unique in Myrmozercon in having its ﬁrst pair of legs much shorter than legs II-IV, highly specialised chelicerae that probably lack a ﬁxed cheliceral digit and have a ﬂange-like moveable digit, and the deutosternal rows each with a single denticle.


INTRODUCTION
The genus Myrmozercon Berlese, 1902 is one of the better known genera of mites from ant nests, being found in most regions of the world (Rosario and Hunter 1988;Walter 2003). Berlese (1902) provided the written description of the type species, Myrmozercon brevipes. The following year, Berlese (1903) described three closely related species in the genus Myrmonyssus, distinguished by lacking the highly hypertrichous dorsal shield found in M. brevipes. Berlese (1904) added a further species of Myrmonyssus and also illustrated M. brevipes and all four species of Myrmonyssus.
Nine species were gradually added to Myrmonyssus (Banks 1916;Berlese 1916;Hull 1923;Vitzthum 1930;Sellnick 1941;Baker and Strandtmann 1948;Hunter and Hunter 1963). However no further species were added until Rosario and Hunter (1988) described another two species, and more importantly synonymised the Myrmonyssus under Myrmozercon because the characters used to define each genus (form of the ventral shields and hypertrichy of the dorsum) were shared by species placed in either genus.
During recent field work in Greece, several specimens of the type species M. brevipes were found. This collection was the first since its description, and the species has not been redescribed until now.

MATERIALS AND METHODS
Specimens were collected during an expedition to Greece by the Hungarian Natural History Museum and the Systematic Zoology Research Group of the Hungarian Academy of Sciences. Some specimens were cleared in lactic acid and mounted in glycerol for examination; these specimens are stored in 75% ethanol. The remaining specimens were cleared in Nesbitt's solution and slide-mounted in Hoyer's medium. Mite specimens were examined with a light microscope (ODS: Nikon Eclipse 80i with DIC); drawings were made with the aid of a drawing tube. Scanning micrographs were taken in the Hungarian Natural History Museum with a HI-TACHI SN 2600 scanning electron microscope; the specimens investigated were sputter-coated with gold-palladium. The specimens are deposited in the Soil Zoology Collections of the Hungarian Natural History Museum (HNHM) and Queensland Museum, South Brisbane, Australia (QM). Leg and idiosomal chaetotaxy follows Evans (1963) and Evans and Till (1965) as adapted from Lindquist and Evans (1965). Width of the idiosoma was taken at the level of coxae IV; its length along the midline. Measurements in the description and scale bars are in micrometres.
Legs -Chaetotaxy as in female, legs subequal in length compared with female. Tarsus I as in female; all other leg setae unspecialised, as in female.
Remarks -The only previous record for M. brevipes is from Italy with the ant T. erraticum (Berlese 1904). Our new specimens are from Greece, but from the same host species, and we expect M. brevipes to be present throughout the intervening terrain in the Balkan Peninsula and central Europe where T. erraticum is known to occur.
Species of Myrmozercon are highly host specific, with no species known from more than one species of ant, although in three known cases two species of Myrmozercon are known to utilize the same species of ant (Shawand Seeman 2009). In these cases, the species pairs conform to a Hutchinsonian size ratio (Shawand Seeman 2009). Therefore, the host species and size of the mite is informative. Our specimens match Berlese's (1904) illustrations in all regards, including the bizarre chelicerae of the female and male. Their size is also similar. Berlese (1902) measured the female as 800 µm × 609 µm and the male at 700 µm × 560 µm. Dr Roberto Nannelli (Berlese Collection, Florence) measured all available intact females, including the holotype (n = 4, on slides 5/39 and 5/40, holotype measurement first followed by range of values based on three other females at the Berlese Collection), and recorded the length as 792 µm (720-768) and width as 600 µm (552 -576). Berlese's male was remeasured at 720 µm × 552 µm. Our females (640 -760 µm × 550 -640 µm) and males (650 -660 µm × 550 -560 µm) tended to be slightly shorter, and Berlese (1904) also illustrated the species with a slightly tapering idiosoma, while ours are ovate. We regard our specimens as the same species, attributing the small differences in size and shape to intraspecific variation.
The chelicerae of M. brevipes are unlike any other member of the genus (or perhaps the Acari) and are difficult to interpret. Two hypotheses are given. In the first, the lateral cheliceral lyrifissure has enlarged to such a size that it separates the second cheliceral segment into two pseudosegments (Fig.  5B). The dorsal lyrifissure and vestigial cheliceral seta remain on the proximal pseudosegment, while the distal pseudosegment terminates in a membranous movable digit that includes small rod-like sclerotised elements. The dorsal, membranous flange at the margin of the pseudosegmental division is a novel structure. The lyrifissures and setae are absent or not visible in males. The terminology in the description is based on this hypothesis.
An alternative interpretation is that the movable digit is equivalent to the distal pseudosegment of the first hypothesis, and this bears a novel structure distally (Fig. 5C). In support of this is Berlese's (1904) drawing of cheliceral musculature for M. brevipes, showing the adductor muscle of the movable digit attaching to the first separation in the second cheliceral digit. In this interpretation, the small membranous flange is a much reduced fixed digit. Cheliceral musculature was poorly visible in our specimens, but what was visible supported Berlese's drawing. Ghafarian et al. (2013) suspected that the species Myrmozercon ovatum Karawajew (1909) was a synonym of M. brevipes, an opinion we concur with, but we also could not locate the type specimens of M. ovatum in Kiev, where the myrmecologist Karawajew (also spelled Karavaiev or Karawaiew) was based in 1909. Myrmozercon ovatum matches the few comparable characters in the original description, most notably the much reduced legs I, and comes from the same host species and is the same size as M. brevipes. Joharchi et al. (2011) discussed character variation in Myrmozercon, noting instability in character states within the genus, such as hypertrichy of the dorsal shield, hypo-or hypertrichy of leg coxae, hypotrichy of palp coxae, hypotrichy of other leg segments, and reduction of sternal shields and their setation. Myrmozercon brevipes is a remarkably specialised member of the genus, presumably explaining why Berlese erected the genus Myrmonyssus for all subsequent species he described. Some of its unusual or reductionist characters do occur in other species, such as the highly hypertrichous dorsum, legs with scarcely more setae than those found in a laelapid larva, and a highly reduced horseshoeshaped sternal shield. However, the form of the chelicerae, tiny legs I and the deutosternum reduced to single denticles are unique.

DISCUSSION
Myrmozercon brevipes and its likely synonym, M. ovatum, are also probably unique in the extreme reduction of legs I. In other Myrmozercon species where species are described in sufficient detail, legs I are the longest or subequal in length compared to legs II-IV. However, M. brevipes has extremely short (ca. 2/3 length of leg II) and slender legs I, unlike any other Myrmozercon (note: leg size of Myrmozercon titan Berlese (1916) was not reported, but its first pair of legs are very long [R. Nannelli, pers. comm.]).
In summary, Myrmozercon brevipes -the type species of Myrmozercon -is a highly specialised representative of its genus, so much so that it begs the question whether Myrmonyssus should be reinstated. However, the closest relatives of M. brevipes seem to be other Myrmozercon species, so it is likely that M. brevipes has arisen within the genus Myrmozercon, and that further investigations should uncover similarly specialised species.