Exposure to high temperatures is fatal for eggs and suppresses growth in a false widow spider, Steatoda grossa

Ectothermic organisms are highly sensitive to ambient temperature, and exposure to extreme heat can induce a range of lethal and sublethal effects. Spiders are abundant and ubiquitous arthropod predators in most terrestrial ecosystems and studying their responses to different temperature regimes provides valuable insights into effects of temperature on this important group of organisms. Here, we exposed the false widow spider (Steatoda grossa), either as eggs or spiderlings to different temperature regimes and monitored their survival and development. The regimes were set at 22/12 °C (day/night), 27/17 °C, 32/22 °C, or were simulated as heatwaves in which the days at 32/22 °C were restricted. Egg and spiderling survival decreased with higher temperature exposure. The growth trajectories of female spiderlings exposed to thermal stress after hatching were stronger affected by the various regimes than those of males. Only the males, which develop much faster than females, reached adulthood and those reared at lower temperatures grew slower but attained significantly more adult body mass. Male spiderlings and, to a lesser extent, eggs and female spiderlings, were able to withstand transient exposure to 32 °C during simulated heatwaves. Our study shows that eggs and young spiderlings are highly sensitive to high temperatures and that female spiderlings are more sensitive than males. Female spiders of this species are mostly sedentary and spend their entire lives in relatively small webs, whereas adult males wander in search of females. Given the higher sensitivity of females and eggs to high temperatures, we argue that females are under strong selection to build webs and place egg sacs at sites where temperatures exceeding 30 °C are avoided.