Costs of walking: differences in egg size and starvation resistance of females between strains of the red flour beetle (Tribolium castaneum) artificially selected for walking ability

Experimental arena settings might lead to misinterpretation of movement properties

Movement is an important animal behavior contributing to reproduction and survival. Animal movement is often examined in arenas or enclosures under laboratory conditions. We used the red flour beetle (Tribolium castaneum) to examine here the effect of the arena size, shape, number of barriers, access to the arena's center, and illumination on six movement properties. We demonstrate great differences among arenas. For example, the beetles moved over longer distances in clear arenas than in obstructed ones. Movement along the arena's perimeter was greater in smaller arenas than in larger ones. Movement was more directional in round arenas than in rectangular ones. In general, the beetles stopped moving closer to the perimeter and closer to corners (in the square and rectangular arenas) than expected by chance. In some cases, the arena properties interacted with the beetle sex to affect several movement properties. All these suggest that arena properties might also interact with experimental manipulations to affect the outcome of studies and lead to results specific to the arena used. In other words, instead of examining animal movement, we in fact examine the animal interaction with the arena structure. Caution is therefore advised in interpreting the results of studies on movement in arenas under laboratory conditions and we recommend paying attention also to barriers or obstacles in field experiments. For instance, movement along the arena's perimeter is often interpreted as centrophobism or thigmotaxis but the results here show that such movement is arena dependent.

Insect egg morphology: evolution, development, and ecology

The insect egg can be viewed through many lenses: it is the single-celled developmental stage, a resource investment in the next generation, an unusually large and complex cell type, and the protective vessel for embryonic development. In this review, I describe the morphological diversity of insect eggs and then identify recent advances in understanding the patterns of egg evolution, the cellular mechanisms underlying egg development, and notable aspects of egg ecology. I also suggest areas for particularly promising future research on insect egg morphology; these topics touch upon diverse areas such as tissue morphogenesis, life history evolution, organismal scaling, cellular secretion, and oviposition ecology.

Dispersal syndrome and landscape fragmentation in the salt-marsh specialist spider Erigone longipalpis

Dispersal and its evolution play a key role for population persistence in fragmented landscapes where habitat loss and fragmentation increase the cost of between-habitat movements. In such contexts, it is important to know how variation in dispersal and other traits is structured, and whether responses to landscape fragmentation are aligned with underlying dispersal-trait correlations, or dispersal syndromes. We therefore studied trait variation in Erigone longipalpis, a European spider species specialist of (often patchy) salt marshes. We collected spiders in two salt-marsh landscapes differing in habitat availability. We then reared lab-born spiders for two generations in controlled conditions, and measured dispersal and its association with various key traits. E. longipalpis population densities were lower in the more fragmented landscape. Despite this, we found no evidence of differences in dispersal, or any other trait we studied, between the two landscapes. While a dispersal syndrome was present at the among-individual level (dispersers were more fecund and faster growing, among others), there was no indication it was genetically driven: among-family differences in dispersal were not correlated with differences in other traits. Instead, we showed that the observed phenotypic covariations were mostly due to within-family correlations. We hypothesize that the dispersal syndrome is the result of asymmetric food access among siblings, leading to variation in development rates and carrying over to adult traits. Our results show we need to better understand the sources of dispersal variation and syndromes, especially when dispersal may evolve rapidly in response to environmental change.

IDH knockdown alters foraging behavior in the termite Odontotermes formosanus in different social contexts

Foraging, as an energy-consuming behavior, is very important for colony survival in termites. How energy metabolism related to glucose decomposition and adenosine triphosphate (ATP) production influences foraging behavior in termites is still unclear. Here, we analyzed the change in energy metabolism in the whole organism and brain after silencing the key metabolic gene isocitrate dehydrogenase (IDH) and then investigated its impact on foraging behavior in the subterranean termite Odontotermes formosanus in different social contexts. The IDH gene exhibited higher expression in the abdomen and head of O. formosanus. The knockdown of IDH resulted in metabolic disorders in the whole organism. The dsIDH-injected workers showed significantly reduced walking activity but increased foraging success. Interestingly, IDH knockdown altered brain energy metabolism, resulting in a decline in ATP levels and an increase in IDH activity. Additionally, the social context affected brain energy metabolism and, thus, altered foraging behavior in O. formosanus. We found that the presence of predator ants increased the negative influence on the foraging behavior of dsIDH-injected workers, including a decrease in foraging success. However, an increase in the number of nestmate soldiers could provide social buffering to relieve the adverse effect of predator ants on worker foraging behavior. Our orthogonal experiments further verified that the role of the IDH gene as an inherent factor was dominant in manipulating termite foraging behavior compared with external social contexts, suggesting that energy metabolism, especially brain energy metabolism, plays a crucial role in regulating termite foraging behavior.

A Review of the Phenotypic Traits Associated with Insect Dispersal Polymorphism, and Experimental Designs for Sorting out Resident and Disperser Phenotypes

Dispersal represents a key life-history trait with several implications for the fitness of organisms, population dynamics and resilience, local adaptation, meta-population dynamics, range shifting, and biological invasions. Plastic and evolutionary changes of dispersal traits have been intensively studied over the past decades in entomology, in particular in wing-dimorphic insects for which literature reviews are available. Importantly, dispersal polymorphism also exists in wing-monomorphic and wingless insects, and except for butterflies, fewer syntheses are available. In this perspective, by integrating the very latest research in the fast moving field of insect dispersal ecology, this review article provides an overview of our current knowledge of dispersal polymorphism in insects. In a first part, some of the most often used experimental methodologies for the separation of dispersers and residents in wing-monomorphic and wingless insects are presented. Then, the existing knowledge on the morphological and life-history trait differences between resident and disperser phenotypes is synthetized. In a last part, the effects of range expansion on dispersal traits and performance is examined, in particular for insects from range edges and invasion fronts. Finally, some research perspectives are proposed in the last part of the review.

Artificial selection on walking distance suggests a mobility-sperm competitiveness trade-off

Securing matings is a key determinant of fitness, and in many species, males are the sex that engages in mate searching. Searching for mates is often associated with increased mobility. This elevated investment in movement is predicted to trade-off with sperm competitiveness, but few studies have directly tested whether this trade-off occurs. Here, we assessed whether artificial selection on mobility affected sperm competitiveness and mating behavior, and if increased mobility was due to increased leg length in red flour beetles (Tribolium castaneum). We found that, in general, males selected for decreased mobility copulated for longer, stimulated females more during mating, and tended to be better sperm competitors. Surprisingly, they also had longer legs. However, how well males performed in sperm competition depended on females. Males with reduced mobility always copulated for longer than males with high mobility, but this only translated into greater fertilization success in females from control populations and not the selection populations (i.e. treatment females). These results are consistent with a mate-searching/mating-duration trade-off and broadly support a trade-off between mobility and sperm competitiveness.

The distinct phenotypic signatures of dispersal and stress in an arthropod model: from physiology to life history

Dispersing individuals are expected to encounter costs during transfer and in the novel environment, and may also have experienced stress in their natal patch. Given this, a non-random subset of the population should engage in dispersal and show divergent stress-related responses. This includes physiological shifts as expressed in the metabolome, which form a major part of responses to stress. We analyzed how metabolic profiles and life-history traits varied between dispersers and residents of the model two-spotted spider mite Tetranychus urticae, and whether and how these syndromes varied with exposure to a stressful new host plant (tomato). Regardless of the effect of host plant, we found a physiological dispersal syndrome where, relative to residents, dispersers were characterized by lower leaf consumption and a lower concentration of several amino acids, indicating a potential dispersal-foraging trade-off. As a possible consequence of this lower food intake, dispersers also laid smaller eggs. Responses to tomato were consistent with this plant being a stressor for Tetranychus urticae, including reduced fecundity and reduced feeding. Tomato-exposed mites laid larger eggs, which we interpret as a plastic response to food stress, increasing survival to maturity. Contrary to what was expected from the costs of dispersal and from previous meta-population level studies, there was no interaction between dispersal status and host plant for any of the examined traits, meaning stress impacts were equally incurred by residents and dispersers. We thus provide novel insights in the processes shaping dispersal and the feedbacks on ecological dynamics in spatially structured populations.