Lars Koerner

Tasty but Protected—First Evidence of Chemical Defense in Oribatid Mites

Oribatid mites (Acari, Oribatida) represent one of the most abundant and speciose groups of microarthropods in the decomposer food webs of soils, but little is known of their top-down regulation by predators. Oribatids are relatively long-lived and have numerous morphological defensive adaptations, and so have been proposed to live in ‘enemy-free space’. Most also possess a pair of large exocrine oil glands that produce species-specific mixtures of hydrocarbons, terpenes, aromatics, and alkaloids with presumably allomonal functions, although their adaptive value has never been tested empirically. We developed a protocol that discharges the oil glands of the model oribatid species, Archegozetes longisetosus. and offered ‘disarmed’ individuals as prey to polyphagous Stenus beetles (Staphylinidae), using untreated mites as controls. Stenus juno fed on disarmed mites with behavioral sequences and success rates similar to those observed when they prey on springtails, a common prey. In contrast, mites from the control group with full glands were almost completely rejected; contact with the gland region elicited a strong reaction and cleaning behavior in the beetle. This is the first evidence of an adaptive value of oribatid mite oil gland secretions for chemical defense. The protocol of discharging oil glands should facilitate future studies on top-down control of oribatid mites that aim to differentiate between morphological and chemical aspects of defensive strategies.

Small but powerful: the oribatid mite Archegozetes longisetosus Aoki (Acari, Oribatida) produces disproportionately high forces.

SUMMARY We investigated the holding and pulling forces generated by claws of the microarthropod Archegozetes longisetosus (Chelicerata, Acari, Oribatida) on three substrates with different roughness (Ra=0.05 μm, 1 μm, 30 μm). Holding forces were measured perpendicular to the substrate using a strain gage force transducer; pulling forces were measured parallel to the substrate using an analytical scale. We found a significant positive correlation of surface roughness and the forces generated. Mites produced holding forces on horizontal rough surfaces (Ra=30 μm) of up to 1180 times their weight; on vertical rough surfaces (Ra=30 μm) they can pull with 530 times their weight, effectively involving only two pairs of legs. The relative forces are five times higher than theoretically expected for organisms of this size (<1 mm, 100 μg) and higher than any relative forces reported for insect claws. Muscles involved in claw action produced stresses up to 1170 kN m–2, a value that is only excelled by decapod crustacean claw closer muscles. Ours is the first study of performance by chelicerate apoteles and claws and also the first to measure forces generated by any microarthropod.

Functional morphology and adhesive performance of the stick-capture apparatus of the rove beetles Stenus spp. (Coleoptera, Staphylinidae)

The adhesive prey-capture apparatus of the representatives of the rove beetle genus Stenus (Coleoptera, Staphylinidae) is an outstanding example of biological adhesive systems. This unique prey-capture device is used for catching elusive prey by combining (i) hierarchically structured adhesive outgrowths, (ii) an adhesive secretion, and (iii) a network of cuticular fibres within the pad. The outgrowths arise from a pad-like cuticle and are completely immersed within the secretion. To date, the forces generated during the predatory strike of these beetles have only been estimated theoretically. In the present study, we used force transducers to measure both the compressive and adhesive forces during the predatory strike of two Stenus species. The experiments revealed that the compressive forces are low, ranging from 0.10 mN (Stenus bimaculatus) to 0.18 mN (Stenus juno), whereas the corresponding adhesive forces attain up to 1.0 mN in S. juno and 1.08 mN in S. bimaculatus. The tenacity or adhesive strength (adhesive force per apparent unit area) amounts to 51.9 kPa (S. bimaculatus) and 69.7 kPa (S. juno). S. juno beetles possess significantly smaller pad surface areas than S. bimaculatus but seem to compensate for this disadvantage by generating higher compressive forces. Consequently, S. juno beetles reach almost identical adhesive properties and an equal prey-capture success in attacks on larger prey. The possible functions of the various parts of the adhesive system during the adhesive prey-capture process are discussed in detail.

Adhesive performance of the stick-capture apparatus of rove beetles of the genus Stenus (Coleoptera, Staphylinidae) toward various surfaces

Rove beetles of the genus Stenus possess a unique adhesive prey-capture apparatus that enables them to catch elusive prey such as springtails over a distance of several millimeters. The prey-capture device combines the hierarchically organized morphology of dry adhesive systems with the properties of wet ones, since an adhesive secretion is released into the contact zone. We hypothesize that this combination enables Stenus species successfully to capture prey possessing a wide range of surface structures and chemistries. We have investigated the influence of both surface energy and roughness of the substrate on the adhesive performance of the prey-capture apparatus in two Stenus species. Force transducers have been used to measure both the compressive and adhesive forces generated during the predatory strike of the beetles on (1) epoxy resin surfaces with defined roughness values (smooth versus rough with asperity diameters ranging from 0.3 to 12 μm) and (2) hydrophobic versus hydrophilic glass surfaces. Our experiments show that neither the surface roughness nor the surface energy significantly influences the attachment ability of the prey-capture apparatus. Thus, in contrast to the performance of locomotory adhesive systems in geckos, beetles, and flies, no critical surface roughness exists that might impede adhesion of the prey-capture apparatus of Stenus beetles. The prey-capture apparatus of Stenus beetles is therefore well adapted to adhere to the various unpredictable surfaces with diverse roughness and surface energy occurring in a wide range of potential prey.

An insect’s tongue as the model for two-phase viscous adhesives?

The first stage in developing two-phase biomimetic adhesives is to analyse the key biomechanical data. One possible biological model is the apparatus which rove beetles use to trap the animals that they feed on. The beetles’ extended, rod-shaped lower lips have two sticky pads and shoot out like a catapult to capture potential prey.

Phylogenetic relationships and chemical evolution of the genera Stenus and Dianous (Coleoptera: Staphylinidae)

The subfamily Steninae, composed of the genera Dianous Leach 1819 and Stenus Latreille 1797, belongs to the family of staphylinid beetles (Staphylinidae). Some unique features characterize Stenus beetles, e.g., a distinct prey-capture apparatus (not found in the genus Dianous) and special pygidial gland secretion constituents such as the alkaloids stenusine (1), norstenusine (2), 3-(2-methyl-1-butenyl)pyridine (3), cicindeloine (4) as well as several terpenes like α-pinene (5), 1,8-cineole (6) and 6-methyl-5-heptene-2-one (7); (only 1, 2 and terpenes found in Dianous). As the secretion composition of Stenus beetles is species specific, it can be used for a chemotaxonomic approach to investigate relationships within the Steninae. Based on the alkaloid gland content, Steninae can be grouped into three clusters: the piperidine-, the pyridine- and the epoxypiperideine group. To clarify the phylogenetic relationships within Stenus and between Stenus and Dianous, and to evaluate our chemotaxonomic approach, we analyzed a combined dataset of three gene sequences aligned of mitochondrial cytochrome c oxidase I (COI), 16S rRNA and Histone H3 of 17 Stenus and 4 Dianous species. Our investigations based on two algorithms such as Maximum Likelihood and Bayesian analyses support the chemotaxonomic approach. Furthermore, our results clearly support former analyses concerning the evolutionary origin of Dianous within Stenus, which suggests a secondary loss of the specialized prey-capture apparatus. Finally, phylogenetic aspects based on the morphology of Steninae are discussed.

The Biology of Steninae

Current knowledge of the biology of the megadiverse beetle subfamily Steninae is reviewed here with regard to its systematics, general morphology, life history, behaviour, (chemical) ecology and evolution into various ecomorphs. Comprising >3000 species worldwide, the staphylinid genus Stenus is one of the most speciose animal genera on Earth. Steninae are well characterized by a number of adult and larval autapomorphies. Adult Stenus beetles are diurnal, optically oriented, epigeic predators of springtails and other small arthropods. The most obvious autapomorphic character defining Stenus is its protrusible elongated labium with the paraglossae being modified into adhesive pads. This prey-capture apparatus can be rapidly ejected towards potential prey by increased haemolymph pressure. The paired anal glands of Steninae are described morphologically and with respect to their secretion chemistry. The alkaloid and terpenoid secretions significantly act as defensive compounds against both bacteria and various predators. The unique skimming behaviour of selected species on water surfaces is described in detail, and the chemotaxonomic value of all gland constituents is discussed.

Insektenzunge als Vorbild für biphasisch viskose Klebstoffe

Fur die Entwicklung biphasischer viskoser Klebstoffe bedarf es zunachst der Analyse biomechanischer Eckdaten. Als biologisches Vorbild konnte dabei der Klebfangapparat bestimmter Kurzflugelkafer dienen, bei dem die stabformig verlangerte Unterlippe mit ihren zwei Klebpolstern katapultartig auf Beutetiere ausgeschleudert wird.