Search publications of the members of the Royal Belgian institute of natural Sciences
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Structure and mechanical strength of larval cuticle of sawflies capable of "easy bleeding" a defence strategy against predators evolved in Tenthredinidae (Hymenoptera)
- The cuticle of Tenthredinidae (Hymenoptera) larvae is ruptured after a very mild mechanical stress and hemolymph is released. This phenomenon, called "easy bleeding" is considered as a defence strategy against predators. We investigated the cuticle structure of some selected species capable of "easy bleeding" by light-, transmission- and scanning microscopy and compared it with phylogenetically-related sawfly species without this defense strategy (control). Cuticle thickness has no major influence on mechanical stability, but pronounced local differences in thickness seem to be inversely correlated with mechanical strength. The density of chitin fibrils is considerably reduced in animals capable of easy bleeding and the fibres form a loose 3D network in contrast to the densely packed layers seen in related "non easy bleeding" species like Strongylogaster multifasciata, where a pressure of 2584 kPa is needed to pierce the integument, and which served as a control. Efficient bleeders like Phymatocera aterrima (203 kPa) and Rhadinoceraea nodicornis (219 kPa) are devoid of any defined layers or other structural elements. In Athalia rosae (Allantinae) (539 kPa), and Aglaostigma discolor (Tenthredininae) (1494 kPa)–bleeders with medium efficiency–a reduced number of layers are observed by LM and TEM and a loose network of chitin fibres is visible after treatment with KOH.
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Differing patterns of chemically-mediated defence strategies in Nematinae versus Phymatocerini larvae (Hymenoptera: Tenthredinidae)
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Sequestration of furostanol saponins by Monophadnus sawfly larvae.
- Sawfly larvae of the tribe Phymatocerini (Hymenoptera: Tenthredinidae), which are specialized on toxic plants in the orders Liliales and Ranunculales, exude a droplet of deterrent hemolymph upon attack by a predator. We investigated whether secondary plant metabolites from Ranunculaceae leaves are sequestered by phymatocerine Monophadnus species, i.e., Monophadnus alpicola feeding upon Pulsatilla alpina and Monophadnus monticola feeding upon Ranunculus lanuginosus. Moreover, two undescribed Monophadnus species were studied: species A collected from Helleborusfoetidus and species B collected from Helleborus viridis. Comparative high-performance liquid chromatographicphotodiode array detection-electrospray ionization-mass spectrometric analyses of plant leaf and insect hemolymph extracts revealed the presence of furostanol saponins in all samples. Larvae of species A and B actively sequestered (25R)-26-[(alpha-L-rhamnopyranosyl) oxy]-22alpha-methoxyfurost-5-en-3beta-yl O-beta-D-glucopyranosyl-(1–3)-O-[6-acetyl-beta-D-glucopyranosyl-(1–3)]-O-beta-D-glucopyranoside (compound 1). This compound occurred at a 65- to 200-fold higher concentration in the hemolymph of the two species (1.6 and 17.5 micromol/g FW, respectively) than in their host plant (0.008 and 0.268 micromol/g FW, respectively). In M. monticola, compound 1 was found at a concentration (1.2 micromol/g FW) similar to that in the host plant (1.36 micromol/g FW). The compound could not be detected consistently in M. alpicola larvae where, however, a related saponin may be present. Additional furostanol saponins were found in H. foetidus and H. viridis, but not in the two Monophadnus species feeding on them, indicating that sequestration of compound 1 is a highly specific process. In laboratory bioassays, crude hemolymph of three Monophadnus species showed a significant feeding deterrent activity against a potential predator, Myrmica rubra ant workers. Isolated furostanol saponins were also active against the ants, at a concentration range similar to that found in the hemolymph. Thus, these compounds seem to play a major role for chemical defense of Monophadnus larvae, although other plant secondary metabolites (glycosylated ecdysteroids) were also detected in their hemolymph. Physiological and ecological implications of the sequestered furostanol saponins are discussed.
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Sawflies (Hymenoptera: Tenthredinidae)
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Easily damaged integument of some sawflies (Hymenoptera) is part of a defence strategy against predators
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Secretion of the ventral glands in Craesus sawfly larvae
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Sawflies (Hymenoptera, Symphyta) from the region of Dinant, with new records for Belgium
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Defense by volatiles in leaf-mining insect larvae
- The defense strategy of an insect toward natural enemies can include a trait that appears at first sight to contradict its defensive function. We explored phylogeny, chemistry, and defense efficiency of a peculiar group of hymenopteran sawfly larvae where this contradiction is obvious. Pseudodineurini larvae live in leaf mines that protect them from some enemies. Disturbed larvae also emit a clearly perceptible lemon-like odor produced by ventral glands, although the mine hampers the evaporation of the secretion. The mine could also lead to autointoxication of a larva by its own emitted volatiles. Citral was the major component in all Pseudodineurini species, and it efficiently repels ants. We conclude that full-grown larvae that leave their mine to pupate in the soil benefit from citral by avoiding attacks from ground-dwelling arthropods such as ants. In some species, we also detected biosynthetically related compounds, two 8-oxocitral diastereomers (i.e., (2E,6E)- and (2E,6Z)-2,6-dimethylocta-2,6-dienedial). Synthetic 8-oxocitral proved to be a potent fungicide, but not an ant repellent. The discrete distribution of 8-oxocitral was unrelated to species grouping in the phylogenetic tree. In contrast, we discovered that its presence was associated with species from humid and cold zones but absent in species favoring warm and dry environments. The former should be protected by 8-oxocitral when faced with a fungal infestation while crawling into the soil. Our work shows the importance of integrating knowledge about behavior, morphology, and life history stages for understanding the complex evolution of insects and especially their defense strategies.
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Hymenoptera Apoidea et Braconidae de quelques forêts sacrées du Sud-Bénin
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Bioassays to elucidate chemical defense strategies in sawfly larvae
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Ecophysiology of dorsal versus ventral cuticle in flattened sawfly larvae.
- Platycampus larvae are highly cryptic leaf feeders characterised by a dorso-ventrally flattened body, the dorsal integument resembling a shield. Dorsal and ventral cuticles from Platycampus luridiventris were compared by histology and gel electrophoresis. By Azan-staining, a red and a blue layer were distinguished in the dorsal cuticle, while the ventral cuticle showed one, almost uniform blue layer, as in both cuticles of control species. The two cuticles from P. luridiventris had similar amounts and sodium dodecyl sulphate-polyacrylamide gel electrophoresis profiles of soluble proteins, but not insoluble proteins. One insoluble protein (MW approximately 41 kDa) was visible as a large band in the ventral cuticle only. It is likely that this protein renders the cuticle elastic, and that the dorsal, red layer is the exocuticle, mainly composed of insoluble proteins. We discuss eco-physiological implications of the exocuticle in insects. Further, data from the literature indicate that the defence strategy in P. luridiventris larvae relies on being visually cryptic towards avian predators and tactically cryptic towards arthropod predators and parasitoids. Crypsis in both senses is favoured by the shield effect, itself based on an abnormally thick dorsal exocuticle. Although the larvae are external feeders, they may be considered as hidden from an ecological perspective.
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How common is ecological speciation in plant-feeding insects? A 'Higher' Nematinae perspective
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Crystalline wax coverage of the cuticle in easy bleeding sawfly larvae
- The larvae of some sawfly species belonging to the family Tenthredinidae (Hymenoptera) are capable of 'easy bleeding', an anti-predator defence strategy based on a specialised cuticle that can readily break, which frees droplets of distasteful haemolymph. Using high-resolution cryo-scanning electron microscopy, we compared the cuticle surface between easy bleeder (Rhadinoceraea micans, Phymatocera aterrima, Aneugmenus padi) and non-easy bleeder (Strongylogaster multifasciata, Nematus pavidus, Arge pagana) sawfly species. We detected crystalline waxes only on the cuticle surface of easy bleeders. Wax crystals varied in shape and dimension depending on species. We assume the reduction of surface wettability by oozed haemolymph to be the primary function of the wax crystal coverage in the easy bleeding defence strategy.
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Flavonoid glycosides and naphthodianthrones in the sawfly Tenthredo zonula and its host-plants, Hypericum perforatum and H. hirsutum.
- Larvae of the sawfly Tenthredo zonula are specialized on Hypericum. Whether the sawfly is able to sequester plant metabolites was unknown. Aerial materials of Hypericum perforatum and H. hirsutum, as well as dissected larvae and prepupae of T. zonula, were analyzed by HPLC to determine the presence and content of flavonoid glycosides (rutin, hyperoside, isoquercitrin, and quercitrin) and naphthodianthrones (pseudohypericin and hypericin). All flavonoid glycosides were detected in both Hypericum species, with hyperoside as major compound in H. perforatum (ca. 1.7 μmol/g fresh weight, FW) and isoquercitrin in H. hirsutum (0.7 μmol/g FW). Naphthodianthrones were present at low concentrations (0.02 μmol/g FW) in the former, and almost undetected in the latter species. In the body parts (i.e., hemolymph, digestive tract, salivary glands, or miscellaneous organs) of T. zonula, the surveyed compounds were detected more frequently in prepupae than in larvae. The compounds were not present in every sample, and flavonoid glycosides especially occurred in highly variable amounts, with maximal concentrations of 41 μg rutin/prepupa in salivary glands, 8 μg hyperoside/prepupa in hemolymph (= 0.36 μmol/g FW), 32 μg isoquercitrin/prepupa in salivary glands, and 63 μg quercitrin/larva in miscellaneous organs (mainly composed of the integument). We conclude that flavonoid glycosides are sequestered since they were detected in organs other than the digestive tract of larvae, and because prepupae are a non-feeding stage. The naphthodianthrone pseudohypericin, but not hypericin, occurred generally in the digestive tract (up to 0.25 μg/larva). Both naphthodianthrones and related unidentified compounds, but not flavonoid glycosides, were found in the larval excrement. The highly variable distributions of flavonoid glycosides and naphthodianthrones in T. zonula larvae and prepupae make it difficult to determine the ecological significance of these metabolites.
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Superhydrophobic cuticle with a "pinning effect" in the larvae of the iris sawfly, Rhadinoceraea micans (Hymenoptera, Tenthredinidae)
- The integument of some sawfly larvae can be easily damaged causing haemolymph bleeding (easy bleeding phenomenon). In the present study, we investigated intact cuticle, cuticle without wax coverage and cuticle replicas of sawfly larvae by using cryo-scanning electron microscopy and contact angle (CA) measurements. The easy bleeder Rhadinoceraea micans was compared to the non-easy bleeder Nematus pavidus (both Tenthredinidae), and the examination showed that the surface structures of R. micans were hierarchically arranged, whereas the cuticle surface of N. pavidus appeared much smoother. The intact cuticle proved to be superhydrophobic in R. micans (CAs: 156° dorsally, 120° ventrally), but not in N. pavidus (CAs: 67° dorsally, 47° ventrally). The wettability of R. micans increased significantly after the waxes had been peeled off. Replicas of the intact cuticle surface were hardly wettable in both species. It is suggested that wax crystals in combination with cuticle microsculptures are the most important features for rendering the integument of the easy bleeder species superhydrophobic. The wax-free tips of the cuticle microsculptures in R. micans are sites where haemolymph droplets can adhere. The integument surface of easy bleeders is assumed to be adapted to their particular defence strategy, simultaneously contributing to the maintenance and "reuse" of released haemolymph droplets on the superhydrophobic surface due to this "pinning effect".
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On Scottish sawflies , with results of the 14 th International Sawfly Workshop , in the southern Highlands , 2010
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Die Oberflächeneigenschaften des Integuments der Pflanzenwespenlarven Rhadinoceraea micans und Nematus pavidus (Hymenoptera , Tenthredinidae)
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Integument and defence in larva and prepupa of a sawfly living on a semi-aquatic plant
- The larvae of the sawfly Rhadinoceraea micans live and feed on a semi-aquatic plant, Iris pseudacorus, and their integument is strongly hydrophobic. The hydrophobicity is part of a chemical defence strategy, easy bleeding, also known from congeners. The prepupae burrow into the soil where they form a cocoon in which they pupate, thus implying different micro-environmental conditions. The cuticle structure and wetting defensive effectiveness of R. micans were compared between larvae and prepupae. The two stages were similarly well defended against attacking ants by the bleeding of a deterrent hemolymph, whereas they were dissimilar in the cuticle surface that presented sculptures and wax crystals at the larval stage only. The integument of prepupae was less structured, and hydrophilic. Larvae of R. micans exhibit, among sawflies, an exceptional cuticle structuring and we assume that they occupy this particular niche of a semi-aquatic environment to avoid encounters with ground-dwelling predators whereas prepupae may benefit from the chemical defence acquired at larval stage.
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Palaeogenetics for ostracods
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Structuration de l'espace au Paléolithique moyen : étude tracéologique et répartition spatiale sur le site de Remicourt (province de Liège, Belgique)
