Eur. J. Entomol. 105: 889–907, 2008 http://www.eje.cz/scripts/viewabstract.php?abstract=1411 ISSN 1210-5759 (print), 1802-8829 (online) Predatory behaviour of some Central European pselaphine beetles (Coleoptera: Staphylinidae: Pselaphinae) with descriptions of relevant morphological features of their heads ANDREA SCHOMANN1, KERSTIN AFFLERBACH2 and OLIVER BETZ3 1Natural History Museum of Denmark/University of Copenhagen, Zoological Museum, Universitetsparken 15, DK-2100 Copenhagen, Denmark; e-mail: [email protected]; [email protected] 2Eichenstraße 8, 56305 Puderbach, Germany; e-mail: [email protected] 3Universität Tübingen, Zoologisches Institut, Abteilung für Evolutionsbiologie der Invertebraten, Auf der Morgenstelle 28E, D-72076 Tübingen, Germany; e-mail: [email protected] Key words. Staphylinidae, Pselaphinae, Brachygluta, Bryaxis, Bythinus, Pselaphus, Rybaxis, Tyrus, predatory behaviour, prey-capture, morphology, head, antenna, mouthparts, maxillary palp, sensillum, sensory organ Abstract. The Pselaphinae is a large subfamily of staphylinid beetles with a characteristic habitus and small body size. Detailed morphological and behavioural studies on these beetles are scarce. In this study, specimens of Bryaxis puncticollis (Denny, 1825), Bryaxis bulbifer (Reichenbach, 1816), Bythinus burrelli (Denny, 1825), Brachygluta fossulata (Reichenbach, 1816), Rybaxis longi- cornis (Leach, 1817), Pselaphus heisei (Herbst, 1792) and Tyrus mucronatus (Panzer, 1803), all collected in Northern Germany, have been examined with regard to their sensory organs (eyes and antennae), mouthparts and method of capturing prey. Scanning electron microscope studies revealed sex-specific differences in the numbers of ommatidia in Bryaxis puncticollis. A multitude of different sensilla on the antennae and great differences in the shape of the mouthparts were observed and peculiarities of the antennae and maxillary palps (e.g., the segment-like appendage) were examined using scanning and transmission electron micros- copy. The prey-capture behaviour of these species is described in detail for the first time based on laboratory experiments using Het- eromurus nitidus (Templeton, 1835) (Collembola) as prey. This behaviour seems to be tribe specific, ranging from simple seizure with the mandibles (e.g., Rybaxis longicornis, tribe Brachyglutini) to the employment of raptorial legs (Tyrus mucronatus, tribe Tyr- ini). The two Bryaxis species (tribe Bythinini) even employ their apparently sticky maxillary palps to capture prey. The assumption that a viscous secretion is used by these species is supported by the finding of glandular structures in the interior of their maxillary palps. Prey-capture is preceded by a complicated preparation phase in most of the species and followed by a sequence of prey- handling movements that seem to be adapted to restrain prey such as Collembola. In simple prey-choice experiments the beetles of several species preferred small prey, irrespective of their own body size. In these experiments, Bryaxis bulbifer and Brachygluta fos- sulata were more successful in capturing prey than Bryaxis puncticollis and Pselaphus heisei. This might be related to their different sensory equipment and different methods of capturing prey. INTRODUCTION Research on pselaphines has mostly focused on their The Pselaphinae is a globally distributed and very systematics and geographical distribution, whereas diverse subfamily of the beetle family Staphylinidae, studies on their behaviour (De Marzo, 1985, 1986, 1988; comprising 9267 described species (A.F. Newton, pers. Engelmann, 1956; Poggi, 1990) and ecology (Reichle, comm.). They are small predators (0.5–5.5 mm in body 1967) are scarce. Furthermore, there are only a few gen- length) with a characteristic appearance. They possess eral morphological studies on this group (e.g., Chandler, compound eyes with only a few ommatidia. The mouth- 2001; Jeannel, 1950; Sabella et al., 1998; Thayer, 2005). parts are prognathous, often with prominent maxillary Most of the morphological data on Pselaphinae are gen- palps and strong mandibles, indicative of a predatory life eral taxonomic morphological descriptions, often with style. Saprophagy is recorded from some species of the notes on the conspicuous structures used for genera Batrisodes, Claviger and Adranes, but not myco- identification. or phytophagy (Thayer, 2005). The aim of this publication is to extend the knowledge Pselaphinae are most species-rich and diverse in the on the behaviour and morphology of Pselaphinae by tropics, but also abundant in temperate regions. They usu- examining several Central European species. Detailed ally occur in moist forest leaf litter or debris, or moist information on the way that adult pselaphines catch and mosses at the margin of water bodies. Some pselaphines handle their prey, and comparative descriptions of the display unusual biological adaptations (e.g., myrme- morphology of their sensory organs and mouthparts are cophily), which sometimes also involve interesting provided. It was examined whether different species- behavioural features (especially remarkable in Claviger specific strategies or structures are used in this process. testaceus: Cammaerts, 1991). Although the assembled information is limited, we attempt to establish correlations between certain aspects 889 TABLE 1. Pselaphine species investigated in the present study (classification into tribes according to Löbl & Besuchet, 2004) (+ = studied, – = not studied, (+) = partly studied). Tribe Genus Species Prey-capture Prey-capture Morphology General Morphology (Supertribe) behaviour success and prey of eyes morphology of mouthparts size preference of antennae Bythinini Bryaxis puncticollis (Goniaceritae) Kugelann, 1794 (Denny, 1825) + + + + + bulbifer + + + + + (Reichenbach, 1816) Bythinus burrelli – – – (+) structural – Leach, 1817 Denny, 1825 peculiarities Brachyglutini Brachygluta fossulata + + + + + (Goniaceritae) Thomson, 1859 (Reichenbach, 1816) Rybaxis longicornis + – + + + Saulcy, 1876 (Leach, 1817) Pselaphini Pselaphus heisei + + + + + (Pselaphitae) Herbst, 1792 Herbst, 1792 Tyrini Tyrus mucronatus + – (+) number – (+) maxillary (Pselaphitae) Aubé, 1833 (Panzer, 1803) of ommatidia palps of behaviour and particular morphological traits. The TRV345E, and Sony Handycam, DCR-HC17E) fitted with observations and conclusions are discussed in the context close-up lenses (hama Nah (Close-Up) +1, +2, and +4). Light of earlier observations made by Engelmann (1956). was provided by cold-light illuminators (Schott KL105B). Actual data were obtained by single frame analysis of the video MATERIAL AND METHODS recording. Evaluation of the preferred prey size and the prey-capture Adult male and female beetles were collected in wet habitats success: Beetles that had been starved for two days were placed mostly around the city of Kiel (Schleswig-Holstein, Northern in small 5 by 5 cm containers with a base of moist plaster of Germany) by searching the ground litter at the margins of water Paris under natural light conditions, along with defined numbers bodies using a Reitter’s sieve or “sifter”. and size classes of Heteromurus nitidus for two hours [presented The animals were transferred in small containers (4.5 cm in prey-size classes: (I) 0.6 mm, (II) 0.6–0.9 mm, (III) 0.9–1.2 diameter) with a floor of moist plaster of Paris, and kept at d mm, and (IV) >1.2 mm]. To determine prey-capture success, the 14–20°C in an outdoor shelter. The insects were maintained on absolute number of Collembola caught during these two hours a diet of small collembolans [Heteromurus nitidus (Templeton, was recorded, and the proportion of the size classes in this catch 1835)]. The photoperiodic conditions were that of natural day was used to determine the preferred prey size. In order to avoid length. any external disturbances, this was conducted as a “blackbox” The specimens examined (in all 144) belong to 7 species in 6 experiment, i.e., captured Collembola were not replaced. How- genera, representing four tribes in two supertribes (Table 1). ever, since only in a small percentage of these experiments all The smallest were Bryaxis puncticollis, Bryaxis bulbifer and the Collembola of one size-class were captured, the preference Bythinus burrelli, all around 1.3 mm in length (from base of for a specific prey type could be clearly established. labrum to abdominal tip). These have maxillary palps with a Statistical analysis: If multiple data sets per specimen were voluminous fourth segment. Brachygluta fossulata (ca. 1.8 mm) available, species-specific grand means were calculated, sum- and Rybaxis longicornis* (ca. 2.0 mm) have less well developed marizing the mean values for several individuals. All means and maxillary palps compared to the previously mentioned species. standard deviations refer to these grand means. There was only Tyrus mucronatus, a large pselaphine beetle of about 2.3 mm, a single specimen of Tm available for behavioural observations which is found beneath bark of dead trees, has comparatively (marked as “one individual” in the tables), so that in this case small maxillary palps. Finally, Pselaphus heisei (1.8 mm long) only the means for this specimen together with their standard has extremely long maxillary palps with a slender and termi- deviations are given. nally clubbed fourth segment. These species will be referred to All the statistical analyses were performed with SPSS 11.0 by abbreviations in the following text: Bp = Bryaxis (SPSS