Predation Cues in Solitary bee Nests.

Predation at the nesting site can significantly affect solitary bees' reproductive success. We tested female red mason bees' (Osmia bicornis L.) acceptance of potential nesting sites, some of which were marked with cues coming from predated conspecifics (crushed bees) or from a predator itself (rodent excreta). In our experiment, females did not avoid nests marked with either of the two predator cues. We suggest that bee females do not recognize these two cues as risky. Alternatively, costs of abandoning natal aggregation might be too high compared with any perceived predation risk of staying. Moreover, the presence of crushed bees can provide positive information about the presence of conspecifics and, possibly, information about a nesting aggregation that may be preferred by bees when choosing a nesting site.

The thermal environment of the nest affects body and cell size in the solitary red mason bee (Osmia bicornis L.).

Many ectotherms grow larger at lower temperatures than at higher temperatures. This pattern, known as the temperature-size rule, is often accompanied by plastic changes in cell size, which can mechanistically explain the thermal dependence of body size. However, the theory predicts that thermal plasticity in cell size has adaptive value for ectotherms because there are different optimal cell-membrane-to-cell-volume ratios at different temperatures. At high temperatures, the demand for oxygen is high; therefore, a large membrane surface of small cells is beneficial because it allows high rates of oxygen transport into the cell. The metabolic costs of maintaining membranes become more important at low temperatures than at high temperatures, which favours large cells. In a field experiment, we manipulated the thermal conditions inside nests of the red mason bee, a solitary bee that does not regulate the temperature in its nests and whose larvae develop under ambient conditions. We assessed the effect of temperature on body mass and ommatidia size (our proxy of cell size). The body and cell sizes decreased in response to a higher mean temperature and greater temperature fluctuations. This finding is in accordance with predictions of the temperature-size rule and optimal cell size theory and suggests that both the mean temperature and the magnitude of temperature fluctuations are important for determining body and cell sizes. Additionally, we observed that males of the red mason bee tend to have larger ommatidia in relation to their body mass than females, which might play an important role during mating flight.

Orientation Inside Linear Nests by Male and Female Osmia bicornis (Megachilidae).

Numerous species of solitary bees and wasps build linear nests with only one entrance. Developing insects must orient themselves inside their nest to choose the correct direction in which to emerge. Misorientation and chewing towards the dead end of the nest can result in significant mortality. Most insects position themselves towards the nest entrance during cocoon construction; however, some individuals are misoriented. We tested whether imagines can examine and possibly correct their orientation after emerging from their cocoons. Males were usually able to correct their misoriented position based on the shape of the cell wall and emerged through the correct entrance, whereas most females pursued the direction that they faced in their cocoons. We suggest that there can be more than one time point during development when bees can control their position in relation to the nest entrance and that the importance of these time points varies between sexes.

Flies developed small bodies and small cells in warm and in thermally fluctuating environments.

Although plasma membranes benefit cells by regulating the flux of materials to and from the environment, these membranes cost energy to maintain. Because smaller cells provide relatively more membrane area for transport, ectotherms that develop in warm environments should consist of small cells despite the energetic cost. Effects of constant temperatures on cell size qualitatively match this prediction, but effects of thermal fluctuations on cell size are unknown. Thermal fluctuations could favour either small or large cells; small cells facilitate transport during peaks in metabolic demand whereas large cells minimize the resources needed for homeoviscous adaptation. To explore this problem, we examined effects of thermal fluctuations during development on the size of epidermal cells in the wings of Drosophila melanogaster. Flies derived from a temperate population were raised at two mean temperatures (18 and 25°C), with either no variation or a daily variation of ±4°C. Flies developed faster at a mean temperature of 25°C. Thermal fluctuations sped development, but only at 18°C. An increase in the mean and variance of temperature caused flies to develop smaller cells and wings. Thermal fluctuations reduced the size of males at 18°C and the size of females at 25°C. The thorax, the wings and the cells decreased with an increase in the mean and in the variance of temperature, but the response of cells was the strongest. Based on this pattern, we hypothesize that development of the greater area of membranes under thermal fluctuations provides a metabolic advantage that outweighs the greater energetic cost of remodelling membranes.