Across the month of July 2016, iButtons recorded temperature every 5 minutes from the refuge of a single adult female black widow from each of four urban collecting locales (lat.-long.: 33.597256 – 112.180383 Chandler, 33.561977 – 112.180830 Tempe, 33.567907 – 112.124740 Scottsdale, 33.627612 – 112.151331 Glendale Community College) and four desert collecting locales (lat. – long.: 34.130961 – 112.088018 Great Western Trail, 34.113567 – 112.355665 Peralta Trail, 33.402360 – 111.350040 Cortez, 33.500475 – 111.458843 East Mesa). These sites were always separated by a minimum of 10 km. Urban measures/spider refuges came from residential habitat (e.g. park benches, hedgerows lining schoolyards, cracks in concrete block walls), whereas desert measures/spider refuges came from undisturbed Sonoran Desert habitat a minimum of 15 km outside of the Phoenix-area urban growth. Collection sites were found on either unprotected public lands, or private lands where the land owner had given us permission to work.

Egg/spiderling development Adult female L. hesperus were collected from 10 urban Phoenix sites similar to those described above. Spiders were housed individually in boxes measuring 10×10×12 cm. On the day of its deposition (day 0), egg sacs were collected from 36 females representing these 10 urban collection sites. No later than day 3, 50 eggs from each family were weighed (microgram, Cahn Microbalance) and egg area was digitally imaged (mm2). Eggs (n=1800) were then housed individually in boxes measuring 4×4×6 cm with a piece of cotton lining the bottom and two toothpicks crossing diagonally to provide a structure for eventual web building. These boxes were initially housed at 24 degree C on a 12:12 light-dark cycle. L. hesperus is native to desert habitats and as such can be reared in the laboratory with no augmentation to humidity. Beginning day 30 of development, each spiderling was fed two Drosophila melanogaster twice a week. While L. hesperus spiderlings are highly cannibalistic (Johnson et al., 2010), we have never seen cannibalism happen prior to day 30 (Johnson, unpublished data). Each box was checked daily for molts and deaths. On day 44 of development, each family was divided evenly into incubators simulating urban (33 degrees C) and desert (27 degrees C) temperature conditions. We waited until day 44 to begin the UHI treatment so as to allow spiderlings a chance to build web and feed before stressing them with extreme heat. Incubators were held on a 12:12 light-dark cycle. At day 105, all surviving spiderlings were weighed (mg).

Male development and behavior Given that males have a much shorter lifespan than females, we continued to follow males through their final juvenile molt (penultimate molt). Male food regimes were doubled for their penultimate molt (i.e. 4 flies twice weekly), and male feeding voracity (latency to kill prey in seconds) was scored during these feedings. Specifically, the voracity of a spider was scored as the latency between the release of prey in the middle of the web and the time a spider initiated silk wrapping. The development study ended when we recorded each male’s adult molt date, as very few males from the urban heat treatment survived this final molt (see Results).

Female web building In a separate experiment we examined the effect of urban heat conditions (27 vs. 33 degree C) on the web building behavior of 36 female black widows across their final two juvenile molts and into adulthood. To clarify, we examined these effects using lineages separate to those described above. For these new lineages, females hatched in the laboratory from urban parents were raised at 24 degree C until the antepenultimate molt, at which point they were randomly assigned to one of the two temperature treatments, urban or desert, as defined above. Spiders were fed a standard-sized House cricket (Acheta domesticus) 7 days before each web-building trial. Females were housed in a walk-in chamber for 7 days at their prescribed temperature treatment in individual boxes measuring 10×10×12 cm. At the end of those 7 days, spiders were weighed (mg) and returned to their respective temperature treatment in a large, clean web-building tub (57×38×33 cm.). These tubs were equipped with sandy substrate and a crisscrossed pair of wooden dowels that led from the floor up into a refuge made out of a crevice inside of a Styrofoam wall. After a 10-minute acclimation period, we scored spiders as web building or inactive every 10 minutes for the first 3 hours of their dark cycle. Following a trial, spiders were returned to their smaller boxes, fed a cricket, and then reassigned to the temperature treatment (chamber) they did not experience previously. Web-building tubs were wiped with alcohol before each new trial and sand, dowels and Styrofoam were replaced for each new trial. This temperature pairing was repeated 7 days following the ante-penultimate, penultimate and adult molts.

Johnson JC, Kitchen K, Andrade MCB. Family affects sibling cannibalism in the black widow spider, latrodectus hesperus. Ethology. 2010;116(8)