Redescription of the enigmatic neotropical inquiline Paramyrmetes foveipennis Bruch, 1929 with notes on myrmecophily (Coleoptera, Histeridae) in the Saprininae subfamily

Abstract The poorly-known and highly autapomorphic myrmecophilous Neotropical taxon Paramyrmetes foveipennis Bruch, 1929 is redescribed, figured and its lectotype designated. Notes on the evolution of the inquilinous lifestyle (myrmecophily) in the subfamily Saprininae are given.


Introduction
Several years ago the first higher-level phylogeny of the Saprininae was published, where all but three genera and subgenera of this subfamily were included . The outstanding taxa were Auchmosaprinus Wenzel, 1962, a subgenus of Xerosaprinus Wenzel, 1962Satrapister Bickhardt, 1912;and Paramyrmetes Bruch, 1929. Satrapister was the subject of a separate paper (Lackner 2016), whereas Paramyrmetes is the subject of the present study. The monotypic genus Paramyrmetes was described by Carlos Bruch based on five specimens collected by himself and Prof. J. Hubrich in 1923 and 1927, respectively, inside refuse chambers of nests of the ant Pogonomyrmex cunicularis var. carnivora Santschi, 1925 (= currently a junior synonym of Pogonomyrmex serpens Santchi, 1922) in the Santa Fé province of Argentina (Bruch 1929). Bruch had difficulties with the systematic placement of this species, and, referring to the key to histerid genera available to him at the time (Bickhardt 1916), placed the taxon into the Saprininae subfamily between ant-inquilines Myrmetes Marseul, 1862 and Platysaprinus Bickhardt, 1916. The same author noted the peculiarities of Paramyrmetes: the rounded, tongue-like labrum and rectangular head, with a broad protruding clypeus that is fused with the frons, and the absent frontal stria. The position of the antennae placed pressed against and parallel to the prosternal process when the head is retracted as well as the prosternum itself resembles, according to Bruch, the taxon Platysaprinus (currently a subgenus of Euspilotus Lewis, 1907). Bruch also noted several autapomorphies of this genus: the presence of elytral depressions as well as reduced dorsal elytral striae. He pointed out the structure of the antennae and the dilated tibiae as putative morphological adaptations to myrmecophily and observed that Paramyrmetes foveipennis was the first recorded (beetle) ant-guest of Pogonomyrmex cunicularis var. carnivora (= Pogonomyrmex serpens Santchi, 1922). In his catalogues (1984,1997,2011) Mazur consistently placed Paramyrmetes between the Australian Tomogenius Marseul, 1862 and Palaearctic Myrmetes Marseul, 1862.
In this paper, Paramyrmetes is re-described based on the type material. Habitus images as well as drawings of male genitalia are provided. This work represents another contribution to the systematics and higher taxonomy of the Saprininae (see e.g. Lackner 2014 or 2016 and the references therein).

Material and methods
A dry-mounted syntype of Paramyrmetes foveipennis was relaxed in warm water for several hours. After removal from the original card, it was side-mounted on a triangular point and examined under a Nikon 102 binocular microscope and viewed with diffuse light. Male genitalia were first macerated in 10% KOH solution for about 3 hours, cleared in 80% alcohol and macerated in lactic acid with fuchsine, incubated at 60 ºC for another 30 minutes, and subsequently cleared in 80% alcohol and then observed in α-terpineol in a small dish. Digital photographs of male genitalia were taken by a Nikon 4500 Coolpix camera and edited in Adobe Photoshop CS5. Genitalia drawings based on the photographs, or direct observations were produced with the aid of Hakuba klv-7000 light box. Habitus photographs were taken by F. Slamka (Bratislava, Slovakia). The specimen was measured with an ocular micrometer. Beetle terminology follows that of Ôhara (1994) and Lackner (2010).
The specimen examined for this study is deposited in the following collection: MNNC Museo Nacional de Historia Natural (Santiago de Chile, Chile).

Abbreviations.
Abbreviations of morphological measurements follow Ôhara (1994) and are used throughout the text as follows:

APW
width between anterior angles of pronotum EL length of elytron along elytral suture EW maximum width between outer margins of elytra PEL length between anterior angles of pronotum and apices of elytra PPW width between posterior angles of pronotum.
Diagnosis. Medium-sized reddish-brown shining asetose Saprininae beetle with completely punctate and shagreened dorsal cuticle, broadly rectangular head; frontal and supraorbital striae absent, labrum tongue-shaped. Dorsal elytral striae strongly reduced; apical third of elytra deeply depressed; metaventrite and first visible abdominal ventrite with (striolate) depression. Pygidium with prominent round ornamentation; tibiae dilated. Differential diagnosis. Based on the autapomorphies outlined above, Paramyrmetes cannot be confused with any currently known South American Saprininae genus. The overall body coloration, in combination with the depressed apical third of the elytra, metaventral and abdominal depressions and, especially the peculiarly-shaped labrum, will readily set this taxon apart from the other members of the subfamily. Moreover, Paramyrmetes possesses pygidial ornamentation in the male sex. Pygidial ornamentation occurs in the Saprininae subfamily rather seldom, and was observed so far only with female specimens of several taxa (e.g. Euspilotus (Neosaprinus) perrisi Marseul, 1872)). According to my knowledge, male pygidial ornamentation has not been reported in the Saprininae subfamily hitherto.
Biology. The type series was found inside the refuse chambers of the ant Pogonomyrmex serpens Santchi, 1922. This species is apparently a specialised ant inquiline.
Distribution. Known only from the type series collected in the province of Santa Fé, Argentina (Fig. 17).
Remarks. Although Bruch (1929) mused about the metaventral-and abdominal depressions as a possible sexual character of Paramyrmetes, he did not specify the sex of the specimens he examined. As I was only able to examine a single male, I am not sure whether Paramyrmetes foveipennis is a sexually dimorphic species or not.

Discussion
In his catalogues Mazur (1984Mazur ( , 1997Mazur ( , 2011, without any prior analysis, consistently placed Paramyrmetes among the more 'basal' Saprininae, between the cave-dwelling and inquilinous Australopacific genus Tomogenius Marseul, 1862 and Palaearctic antinquiline genus Myrmetes Marseul, 1862. In my analysis, both Tomogenius and Myrmetes were found to be near the root of the tree, and, in fact an inquilinous lifestyle of saprinines is inferred to be plesiomorphic for the subfamily . For the present study, I coded the morphological characters of Paramyrmetes and included them in the matrix used for my Satrapister paper (Lackner 2016). The position of Paramyrmetes on the tree was unstable, varying among the taxa near the root (Myrmetes, Erebidus, Gnathoncus, Tomogenius, Microsaprinus). This instability was mainly due to the impossibility of coding antennal club characters, as the disarticulation of the unique syntype available was not allowed. Furthermore, Paramyrmetes exhibits numerous autapomorphies, which are not informative for the phylogenetic reconstruction. Consequently, I decided not to include the new tree here, opting instead to await the results of the molecular analysis (on-going). Despite the unstable position on the tree, Paramyrmetes, consistently came out near the root of the tree, in fact corroborating Mazur's taxon placement. Inclusion of freshly collected specimens into the ongoing molecular analysis would be highly desirable.
Myrmecophily, or an ant-inquilinous lifestyle is rather rare among saprinine histerids and has hitherto been confirmed in six higher taxa with at least three independent evolutionary events leading to it P. Kovarik, pers. communication). It is interesting to note that four out of the six ant-inquiline higher taxa occur in the New World, and three of those are attaphilic (see the table below for details). Several, but not all inquilinous saprinines exhibit peculiar morphological autapomorphies linked to their specialized lifestyles, and these autapomorphies are not similar between the taxa. Among the shared characters could be listed their wholly punctate bodies (except for Myrmetes), interrupted to absent frontal stria (all taxa), and occasional lack of sutural elytral stria (present in Priscosaprinus and Paramyrmetes). The dilated tibiae, which are present in most taxa (exceptions are: Myrmetes, Priscosaprinus and Phoxonotus) probably function to protect larger portion of venter when in repose, since they are able to cover more space. Autapomorphies of the undescribed Australian genus, which include imbricately setose elytra and abdomen (it is hypothesized that these micro-setae function as trichomes secreting appeasing liquid), metepisternal groove for receiving the mesotarsus, and enlarged antennal club are further results of the selection pressures for inquilinous lifestyle. Many of these are also found in other ant-inquiline histerids (e.g. Haeteriinae). The functions of several autapomorphies, e.g. elytral and metepisternal-abdominal depressions, dorsal tubercles, tongue-shaped labrum or pygidial ornamentation is completely unknown. Larvae of ant-inquiline saprinine beetles are unknown and biology generally poorly understood. The table below is presented to outline several putative morphological adaptations presumably linked with ant-inquilinism in the Saprininae subfamily; a dorsal habitus image of a representative of each taxon is included.