RECOGNITION
Adult female body length about 1/2" (12-13 mm) including an almost spherical
abdomen about 1/4-3/8" (7.2-9.6 mm) in diameter, with overall length including legs
of about 1 1/2-1 3/8" (38-43 mm); males about half female size. Color typically black,
abdomen on ventral or underneath side with 2 reddish triangular markings usually
joined to form a reddish hourglass-shaped marking but sometimes separated
("split-hourglass") or only a single mark; usually with red markings above
spinnerets. Females usually black (occasionally brownish black; red widow of Florida with
cephalothorax and legs usually bright orange, sometimes yellow or brick red); males
usually with color pattern on dorsal or upper surface, pattern variable but typically
consists of a median row of red spots with white lines or bars radiating out to sides.
With 8 simple eyes, 2 lateral pairs almost touching. Last tarsal segment of 4th pair of
legs with row/comb of serrated bristles on venter; all tarsi with 3 claws each. Young
spiders primarily orange and white but acquire more and more black as they mature, with
markings very similar to those of males. With I or 2 reddish markings on underneath side
of abdomen.
REPRESENTATIVE SPECIES
Identification to species is left to experts since several species are involved
and species separation is based primarily on differences in the male genitalia. Although a
very brief description based on female coloration is given below, it must be realized that
color and markings are not always reliable characteristics for separation of these species
and that males are usually colored and marked quite differently.
1. Latrodectus bishopi Kaston, the red widow. Cephalothorax reddish, abdomen
black with reddish to organish spots outlined by white, red marks on venter, and legs
reddish; occurs only in central and southern Florida.
2. Latrodectus geometricus (Fabricius), the brown widow. Color brownish with
white and black brown markings, abdomen with. red hourglass mark, legs banded/ringed;
occurs only in southern Florida.
3. Latrodectus hesperus Chamberlin and Ivie, the "western" widow.
Color blackish, abdomen with ventral red hourglass mark complete, anterior triangular part
longer and broader than posterior triangle, red spot above anal tubercle Oust above
spinnerets) usually absent; occurs in the western United States and western Canada.
4. Latrodectus mactans (Fabricius), the black widow. Color blackish, abdomen
with ventral red hourglass mark complete, posterior half more a rounded rectangle than a
triangle, often with a red spot just above anal tubercle Oust above spinnerets), and/or
with row of red spots dorsally along midline; occurs primarily from Massachusetts to
Florida and west to Texas, Oklahoma, and Kansas, but is most common in the southern
states.
5. Latrodectus variolus Walchenaer, the northern widow. Color blackish, abdomen
usually with separated red hourglass mark, having posterior mark triangular and anterior
mark rounded, dorsally with row of red spots along midline and diagonal white bands
laterally; occurs in the same area as the black widow but is also found in the northern
states and in Canada and is more common in the northern states.
SIMILAR GROUPS
(1) All other spiders lack a hourglass-shaped or split-hourglass marking on
underside of abdomen and/or comb of serrated bristles on last tarsal segment of 4th pair
of legs and 8 eyes with 2 lateral pairs almost touching.
BIOLOGY
Female black widow spiders deposit their eggs in silken egg sacs which are about
3/8-1/2" (9.5-12 mm) in diameter, can be constructed in 1-3 hours, and are white but
soon turn pale brown. Depending on the species, the maximum number of sacs per female
ranges from 6 to 21 and the average number of eggs per sac is about 185-464 (maximum 917).
The sac's outer covering is tough and closely woven. The incubation period ranges from 8
to 30 days. When newly hatched, the spiderlings undergo the first molt within the sac.
They go through 4 to 9 instars in 54 to 107 days respectively, with each instar having
different coloration and/or color pattern. Most of the spiders overwinter as immature
individuals, develop into adults in the spring, and die in late July, taking nearly a year
to grow from egg to adult. The maximum number of days survived after maturity ranges from
822 to 952 for females and 127 to 196 for males, depending on the species. These spiders
spin an irregular web and hang from it in an inverted or upsidedown position. The web is
used to ensnare prey. The female avoids light in sheltered places, and tends to work her
web at night.
In terms of poisonous bites, only the female is of concern. The male's poison/venom
sacs cease development and he does not attack prey upon maturity. Spiderlings are
poisonous when ingested until they are 18 days old, and then loose their poison; adults
have a different kind of venom. The black widow's venom is a neurotoxin. The female is
normally shy and retiring, but aggressively attacks immediately after egg laying and when
guarding her eggs. The black widow bite is not always felt, so the only reliable evidence
of a bite is a slight local swelling with 2 tiny red spots where the fangs entered.
However, pain is usually almost immediate and reaches its maximum in 1-3 hours, continues
for 12-48 hours, and then gradually subsides. The major symptoms are increased body
temperature and blood pressure, profuse sweating, and nausea. There is an antitoxin
available. Immediately call a physican or go to an emergency room if bitten. If treated,
bites are rarely fatal except occasionally in small children.
HABITS
Initially, the second instar spiderlings remain near the sac but within a few
days they climb to a high point within suitable air currents, spin silk threads and float
out on the breeze like kites. This "ballooning" provides for general dispersal
of the species.
Outside, black widows commonly live in protected places among and under stones and
pieces of wood, in firewood piles, under decks, in hollow stumps and tress, in rodent
burrows, and less often in low tree branches or shrubbery. Favorite places are dry man-
made structures including barns, outhouses, henhouses, sheds, meter boxes, brick veneer,
barrels, and woodpiles.
Indoors, they are typically found in seldom-used parts of
garages and basements and in crawl spaces, preferring the more cluttered areas because
they provide more harborage for their prey which consists primarily of insects. Their webs
are usually about 1 foot (30 cm) in diameter.
