Danger detection and escape behaviour in wood crickets

International audienceThe wind-sensitive cercal system of Orthopteroid insects that mediates the detection of the approach of a predator is a very sensitive sensory system. It has been intensively analysed from a behavioural and neurobiological point of view, and constitutes a classical model system in neuroethology. The escape behaviour is triggered in orthopteroids by the detection of air-currents produced by approaching objects, allowing these insects to keep away from potential dangers. Nevertheless, escape behaviour has not been studied in terms of success. Moreover, an attacking predator is more than ''air movement'', it is also a visible moving entity. The sensory basis of predator detection is thus probably more complex than the perception of air movement by the cerci. We have used a piston mimicking an attacking running predator for a quantitative evaluation of the escape behaviour of wood crickets Nemobius sylvestris. The movement of the piston not only generates air movement, but it can be seen by the insect and can touch it as a natural predator. This procedure allowed us to study the escape behaviour in terms of detection and also in terms of success. Our results showed that 5-52% of crickets that detected the piston thrust were indeed touched. Crickets escaped to stimulation from behind better than to a stimulation from the front, even though they detected the approaching object similarly in both cases. After cerci ablation, 48% crickets were still able to detect a piston approaching from behind (compared with 79% of detection in intact insects) and 24% crickets escaped successfully (compared with 62% in the case of intact insects). So, cerci play a major role in the detection of an approaching object but other mechanoreceptors or sensory modalities are implicated in this detection. It is not possible to assure that other sensory modalities participate (in the case of intact animals) in the behaviour; rather, than in the absence of cerci other sensory modalities can partially mediate the behaviour. Nevertheless, neither antennae nor eyes seem to be used for detecting approaching objects, as their inactivation did not reduce their detection and escape abilities in the presence of cerci

Insect-induced effects on plants and possible effectors used by galling and leaf-mining insects to manipulate their host-plant

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Leaf-Miners Co-opt Microorganisms to Enhance their Nutritional Environment

International audienceOrganisms make the best of their mother's oviposition choices and utilize specific feeding options that meet energetic requirements and cope with environmental constraints. This is particularly true for leaf-miner insects that develop enclosed in the two epidermis layers of a single leaf for their entire larval life. Cytokinins (CKs) play a central role in plant physiology - including regulation of senescence and nutrient translocation - and, as such, can be the specific target of plant exploiters that manipulate plant primary metabolism. 'Green-islands' are striking examples of a CKinduced phenotype where green areas are induced by plant pathogens/insects in otherwise yellow senescent leaves. Here, we document how the leaf-miner caterpillar Phyllonorycter blancardella, working through an endosymbiotic bacteria, modifies phytohormonal profiles, not only on senescing (photosynthetically inactive) but also on normal (photosynthetically active) leaf tissues of its host plant (Malus domestica). This leaf physiological manipulation allows the insect to maintain sugar-rich green tissues and to create an enhanced nutritional microenvironment in an otherwise degenerating context. It also allows them to maintain a nutritional homeostasis even under distinct leaf environments. Our study also highlights that only larvae harboring bacterial symbionts contain significant amounts of CKs that are most likely not plant-derived. This suggests that insects are able to provide CKs to the plant through their symbiotic association, thus extending further the role of insect bacterial symbionts in plant-insect interactions

Amino acid composition of the bushcricket spermatophore and the function of courtship feeding: variable composition suggests a dynamic role of the nuptial gift

National audienceNuptial gifts are packages of non-gametic material transferred by males to females at mating. These gifts are common in bushcrickets, where males produce a complex spermatophore consisting in a sperm-containing ampulla and an edible sperm-free spermatophylax. Two non-mutually exclusive hypotheses have been suggested to explain the function of the spermatophylax: the paternal investment hypothesis proposes that it represents a male nutritional investment in offspring; the mating effort hypothesis proposes that the spermatophylax maximizes the male's sperm transfer. Because gift production may represent significant energy expenditure, males are expected to adjust their investment relative to the perceived quality of the female. In this study, we first examined the free amino acid composition and protein-bound amino acid composition of the nuptial gift in the bushcricket, Ephippiger diurnus (Orthoptera: Tettigoniidae). Second, we investigated whether this composition was altered according to female age and body weight. Our study represents the first investigation of both free and protein-bound amino acid fractions of a bushcricket spermatophylax. We found that composition of the nuptial gift varied both qualitatively and quantitatively with respect to traits of the receiving female: older females received larger amounts of protein-bound amino acids (both essential and non-essential), less water and less free glycine. This result suggests that gift composition is highly labile in E. diurnus, and we propose that gift allocation might represent a form of cryptic male mate choice, allowing males to maximize their chances of paternity according to the risk of sperm competition that is associated with mate quality

Seasonal selection and resource dynamics in a seasonally polyphenic butterfly

International audienceSeasonal polyphenisms are widespread in nature, yet the selective pressures responsible for their evolution remain poorly understood. Previous work has largely focussed either on the developmental regulation of seasonal polyphenisms or putative 'top-down' selective pressures such as predation that may have acted to drive phenotypic divergence. Much less is known about the influence of seasonal variation in resource availability or seasonal selection on optimal resource allocation. We studied seasonal variation in resource availability, uptake and allocation in Araschnia levana L., a butterfly species that exhibits a striking seasonal colour polyphenism consisting of predominantly orange 'spring form' adults and black-and-white 'summer form' adults. 'Spring form' individuals develop as larvae in the late summer, enter a pupal diapause in the fall and emerge in the spring, whereas 'summer form' individuals develop directly during the summer months. We find evidence for seasonal declines in host plant quality, and we identify similar reductions in resource uptake in late summer, 'spring form' larvae. Further, we report shifts in the body composition of diapausing 'spring form' pupae consistent with a physiological cost to overwintering. However, these differences do not translate into detectable differences in adult body composition. Instead, we find minor seasonal differences in adult body composition consistent with augmented flight capacity in 'summer form' adults. In comparison, we find much stronger signatures of sex-specific selection on patterns of resource uptake and allocation. Our results indicate that resource dynamics in A. levana are shaped by seasonal fluctuations in host plant nutrition, climatic conditions and intraspecific interactions

Converging strategies in plant-manipulating insects: insect-induced effects on plants and possible mechanisms used by leaf-miners to manipulate their host-plant

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