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unpublished data).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)
