Attachment of Galerucella Nymphaeae (Coleoptera, Chrysomelidae)
Total Page：16
File Type：pdf, Size：1020Kb
© 2014. Published by The Company of Biologists Ltd | The Journal of Experimental Biology (2014) 217, 4213-4220 doi:10.1242/jeb.108902 RESEARCH ARTICLE Attachment of Galerucella nymphaeae (Coleoptera, Chrysomelidae) to surfaces with different surface energy Constanze Grohmann*, Andreas Blankenstein, Sven Koops and Stanislav N. Gorb ABSTRACT (wind, waves, rain, etc.), which can cause leaves to be covered by Numerous studies deal with insect attachment onto surfaces with water. According to Hippa and Koponen (Hippa and Koponen, different roughness; however, little is known about insect attachment 1986), the larvae survive underwater temporarily, having an air film onto surfaces that have different chemistry. In the present study, we visible on their cuticle, but they sink immediately when dropped describe the attachment structures of the water-lily leaf beetle below the water surface (Kouki, 1991b). The body of adult beetles Galerucella nymphaeae and test the hypothesis that the larval and is almost completely covered with long setae. Between these setae, adult stages generate the strongest attachment on surfaces with an air film renders the beetle water repellent for some time and contact angles that are similar to those of leaves of their host plants. persists for 2 days when submerging a freshly killed specimen The larvae bear a smooth attachment system with arolium-like (Balmert et al., 2011). Larvae, as well as adults, are able to move structures at their legs and a pygopodium at the abdomen tip. Adults underwater (A.B. and S.K., personal observations). have pointed setae on the ventral side of the two proximal A variety of attachment organs occur in insects (Beutel and tarsomeres and densely arranged spatula-shaped ones on their third Gorb, 2001; Gorb, 2001; Scherge and Gorb, 2001), which evolved tarsomere. In a centrifugal force tester, larvae and adults attained the many times independently based on two main principles – hairy highest friction forces and safety factors on surfaces with a water and smooth. Hairy attachment organs can be found, for example, contact angle of 83 deg compared to those of 6, 26 and 109 deg. This in adult beetles (Bullock and Federle, 2011; Gorb and Gorb, 2002) comes close to the contact angle of their host plant Nuphar lutea and flies (Gorb et al., 2001; Niederegger et al., 2002), whereas (86 deg). The similarity in larval and adult performances might be a smooth adhesive pads are found – amongst other taxa – in result of the similar chemical composition of their attachment fluid. representatives of Orthoptera (Gorb et al., 2000) and Hymenoptera We compare our findings with previous studies on the forces that (Federle et al., 2001). In both types of attachment systems, a fluid insects generate on surfaces with different surface energies. that is secreted into the contact zone with the substrate contributes to the attachment performance. In beetles, this fluid is a mixture KEY WORDS: Adhesion, Friction, Contact angle, Insect, of hydrocarbons, fatty acids and alcohols, similar to the Biomechanics, Locomotion composition of their cuticular lipids (Geiselhardt et al., 2009; Geiselhardt et al., 2010; Ishii, 1987). In this so-called wet INTRODUCTION adhesion, capillary forces account largely for the attachment ability The attachment of insects to surfaces is crucial for their survival. For (Langer et al., 2004). The capillary forces, in turn, strongly depend example, they need to adhere to the plants they live and feed on, on the contact angle between the fluid and the surface of particular attach to their hosts or hold onto their copulation partners. The substrate (e.g. De Souza et al., 2008a). necessity to strongly attach to their host plant holds true especially There are a number of papers concerning the attachment of insects for terrestrial beetles that are associated with water-plants. The to surfaces of different roughness (e.g. Beutel and Gorb, 2001; terrestrial leaf beetle Galerucella nymphaeae (Linnaeus 1758), for Bullock and Federle, 2011; Dai et al., 2002; Gorb and Gorb, 2009; example, lives and forages mainly on the upper surface of floating Voigt et al., 2008). However, the impact of the surface energy on the leaves of the water-lilies Nuphar and Nymphaea (Smirnov, 1960). insects’ attachment ability has been analysed in a few studies only. Adult beetles overwinter in the riparian areas and colonise leaves in In most of them, instead of the free surface energy, the contact angle spring. After mating, females lay clutches of eggs on the upper of water on the surface was measured. In general, one can say that surface of leaves. The larvae undergo three larval stages and pupate. the smaller the contact angle, the higher the free energy of a surface. All stages live on the upper surface of the leaves and larvae, as well Although in some studies (Gorb and Gorb, 2009; Gorb et al., 2010; as adults, feed on the leaves. At least three generations occur per Hosoda and Gorb, 2012; Lüken et al., 2009; Prüm et al., 2013) the year (Wesenberg-Lund, 1943). Along with other ecological impacts, focus was laid on hairy attachment systems of different beetle G. nymphaeae strongly reduces Nuphar stands and thus increases species, others were done on smooth attachment organs of moths (Al light transmission into the water (Kouki, 1991a; Kouki, 1993; Bitar et al., 2009) and sawfly larvae (Voigt and Gorb, 2012). The Stenberg and Stenberg, 2012; Wahl, 2008; Wallace and O’Hop, results of these papers are not consistent. For example, in two 1985). studies (Al Bitar et al., 2009; Prüm et al., 2013), no differences in For larvae, as well as for adults, it is essential to stay attached to the ability of beetles to attach onto surfaces with different contact the surface of host leaves during various environmental disturbances angles were detected, whereas in different studies (Lüken et al., 2009; Gorb et al., 2010), the insects performed, by far, better on Department of Functional Morphology and Biomechanics, Zoological Institute, surfaces with higher surface energies. Thus, the role of surface Kiel University, Am Botanischen Garten 9, 24118 Kiel, Germany. energy on the adhesion ability of the insects remains unclear. Because different insect species do not only have different *Author for correspondence ([email protected]) attachment pad structures, but also have different degrees of Received 9 July 2014; Accepted 9 October 2014 specialisation to various natural substrates, it might be possible that The Journal of Experimental Biology 4213 RESEARCH ARTICLE The Journal of Experimental Biology (2014) doi:10.1242/jeb.108902 adaptation to the substrate chemistry is species specific and might Adults be exhibited to a greater extent in ‘specialist’ species compared to Adult beetles were ~6 mm long and 3 mm wide (N=26). Each leg ‘generalist’ ones. bears five tarsal segments and a claw. The third tarsal segment In this paper, we study the morphology of the smooth and hairy consists of two lobes and the reduced forth one is hardly visible attachment structures of larval and adult G. nymphaeae beetles. In (Fig. 2A–C,E,F). A claw is situated at the distal end of each tarsus. an experiment, we tested the hypothesis that in G. nymphaeae, an It is composed of two halves similar in shape, each with a longer insect species that has specialised to live on and to attach mainly to strong tip at the outer side and a half as long at the inner side. All Nuphar lutea leaves, the highest attachment forces are reached on but the forth tarsal segment are covered with long and pointed setae; surfaces with surface energies similar to that of their host plant. however, only the third tarsal segment has more densely packed spatula-shaped setae at its the ventral side (Fig. 2D). These spatula- RESULTS shaped setae are cylindrical at their base with a diameter of 1 to Morphology 3 μm and a length of 40 to 62 μm. All but the most proximal setae Larvae widen towards their tips, reaching up to 6 μm in width at their end. The third instar larvae were roughly 7 mm long (N=20). All thoracic The upper side of these spatula-shaped setae is covered with nubs legs bear an arolium-like structure and a claw at their distal end that are up to 2 μm in length. There is a tendency that the width of (Fig. 1A,B,G). The terminal abdominal segment is characterised by the setae’s ends and the number of nubs increase from the proximal a fleshy pygopodium (Fig. 1C–F), which strongly contributes to the to the distal end of the third tarsal segment. total adhesion force of the larvae (C.G., A.B. and S.K., personal Females outweighed males [12.3±1.7 (mean ± s.d., N=4) and observations). While the larvae attach to a surface, the thoracic legs, 10.4±1.1 (N=5), respectively]. We did not find distinct differences ventral parts of the abdominal segments and the pygopodium make between the shape of female and male adhesive setae (Fig. 2E,F). contact with the surface (Fig. 1E–J). However, female beetles possessed more setae compared to males Fig. 1. Habitus and attachment organs of Galerucella nymphaeae at the 3rd larval stage. (A) Lateral view. (B–F) Cryo-scanning electron microscopy images. (B) Thoracic leg, right forefoot. (C) Ventral view. (D) Detail of the abdominal appendage (pygopodium). (E,F) Ventral view of a specimen frozen while it attached to a smooth surface. (E) Whole specimen. (F) Detail of the pygopodium. (G) Arolium-like structure of a thoracic leg attaching to a surface. (H,I) Contact area of abdominal segments (H) and the pygopodium (I) of a larva attaching upside down to a smooth surface.