Abstract1. Stegodyphus mimosarum, a social spider, lives in colonies which may contain hundreds of individuals. Feeding behaviour was examined with respect to feeding group size and prey size. 2. Prey were less likely to escape and were subdued more quickly when attacked by more than one spider. 3. During capture small prey were frequently bitten directly on the body whereas large prey were almost always bitten on an appendage. 4. Pulling struggles for subdued prey occurred. They lasted longest over medium sized prey. Small prey were easier to transport to the nest than medium prey and large prey were pulled by more spiders from a single retreat. 5. Spiders which had participated in a capture initially bit preferentially on the prey's head or thorax but others which joined later to feed bit at random. 6. Feeding became less efficient as group size increased and an experiment suggests that individuals injected less poison and digestive enzymes when feeding in groups.
Male tenebrionid beetles from the Namib Desert are captured by predators more frequently than females. Males exhibit several behaviors that predispose them to an increased risk of mortality from predation. In attempts to find and monopolize females, males are more active on the surface, move more frequently, and travel farther than females. One result of such activity is a bias in captures by predators and pitfall traps. These biased sex ratios differ significantly from those in censused natural populations and an assumed 1:1 sex ratio. We suggest that such differential predation is a major cost of reproduction and can constrain sexual selection.
In the green algaMougeotia, the dichroic orientation of the red-absorbing form of phytochrome (Pr) is parallel of the cell surface, whereas the far-red-absorbing form (Pfr) is oriented normal to it. The time course of the change from parallel to normal was investigated by double-flash irradiation with polarized red and far-red light. The results obtained by two different methods indicate that most of the phytochrome intermediates existing in the first 5 ms after the inducing red flash are still oriented parallel to the cell surface, similar to Pr. At increasing intervals between the red and the far-red flashes, more and more phytochrome molecules turn their transition moments to the Pfr orientation. This reaction is finished after approximately 30 ms. We conclude that the change in dichroic orientation of the phytochrome molecules inMougeotia occurs during the last relaxation steps of the intermediates on the way from Pr to Pfr. It cannot be decided yet, whether the first surface-normal phytochrome species is an intermediate or Pfr itself.
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