Quick Facts...
- Widow spiders rarely, if ever, bite when not within a web. Bites may
be more likely if the female is tending an egg sac in the web, which
she will defend.
- Widow spiders produce a toxin that affects the nervous system. Muscle
and chest pain or tightness are some of the most common reactions to
the widow toxin.
- Widow spiders prefer to nest near the ground, in dark, undisturbed
areas. Outdoor nest sites include holes produced by small animals or
around construction openings and wood piles.
The “widow” spiders are a group of related spiders in the genus
Latrodectus. Several species occur in the United States, but the
western widow, Latrodectus hesperus, is the overwhelmingly dominant
species throughout Colorado. (The “true” black widow, Latrodectus
mactans, is more common in eastern and southern areas of the country.)
Identification
|
|
|
Figure 1. Western widow female in web.
|
Mature females of the western widow spider are generally round in form
with a bulbous abdomen. They usually reach a length of 1/4 to 1/3 inch.
Adult females are distinctively shiny and dark colored, generally black
or occasionally dark-brown.
The distinguishing feature of all widows (Latrodectus spp.) is the presence
of a red or red-orange “hourglass” pattern on the underside
of the abdomen. However, this pattern can be highly variable with the
western widow. The pattern may appear as two unconnected spots, as a roughly
rectangular area, or be so faint as to be barely visible. The color of
the “hourglass” may fade or intensify in color during the life
of any individual widow spider.
The immature stages of both sexes and adult male widow spiders may have
red or red-orange or yellow spots and stripes on the top of their abdomen.
Immature females can be colored gray or pale brown, with banding patterns.
Darker coloration increases as they get older. The presence of an “hourglass”
pattern on the underside of the abdomen occurs throughout their development.
Males of the western widow are one-half to one-third the size of females,
and they have a more elongate body form. (The body of the males typically
range from 1/8 to 1/4-inch in length.) They are not usually black in overall
color, instead appearing light brown or gray and banded. Male widows may
have the characteristic hourglass pattern, but coloration is often more
orange and sometimes yellow. When mature, they have large knob-like structures
(pedipalps) originating from the cephalothorax, or first body segment.
Aside from this last feature (conspicuous pedipalps) they are similar
in appearance to immature females.
Widow spiders build sticky irregular mesh-type webs, typical of other
spiders in the cobweb spider family (Theridiidae). Widow spiders often
live in ready-made holes in dark, undisturbed sites. Therefore, widow
spiders are most often found in abandoned rodent burrows, loose stone
or wood piles, or the corners of rooms, garages, and outbuildings. They
do not produce the symmetrical web typical of orb weaving spiders (Araneidae)
or the distinctive dense and funnel patterned web of the common funnelweb
spiders (Agelenidae).
Spiders of Similar Appearance. The combination of the dark
black color, the irregular web shape, and the distinctive spherical abdomen
are useful characteristics for separating widow spiders from other spider
species. However, related, but harmless, combfooted/cob web spiders produce
similar webs and are often the most common spiders found in homes.
These harmless cobweb spiders are the same general shape as widow spiders,
but are smaller and are either mottled brown or are dark brown or black
with a white band around the front of the abdomen. At quick glance one
can confuse these with immature black widows, but they invariably lack
the underside hourglass pattern or bright coloring of the immature and
male widow spiders.
Cobweb spiders that are most easily confused with widows are in the genus
Steatoda. These cob web spiders are similarly black and have the
same enlarged abdomen as widow spiders. However, they lack the orange-red
hourglass on the underside of the abdomen, have prominent white markings
on the top of the abdomen, and often have a white band around the anterior
edge of the abdomen.
 |
|
 |
| Figure 2. Western widow male. |
|
Figure 3. Underside of female western
widow.. |
Life History and Habits
|
|
|
Figure 4. Male and female widow
spider.
|
Eggs of widow spiders are laid in an egg sac, attached to the web of
the mother. Egg sacs produced by the western widow are pear shaped and
light whitish yellow to brown. About 200 eggs may be laid per egg sac.
Under favorable conditions, females may produce several egg sacs in their
lifetime.
Eggs hatch in the egg sac about 2 weeks after they are laid. The newly
hatched spiders, known as "spiderlings," remain in the sac for several
days and molt inside the egg sac. After this first molt, they leave the
sac by cutting an opening with their fangs. Spiderlings may remain clustered
near the female in or near her web until the yolk leftover from the egg
stage is completely digested. At that time, they disperse and leave the
egg sac, often by a process called "ballooning." Ballooning spiderlings
release small silk strands that allow them to catch wind currents and
be carried in the air.
If young spiders find a suitable spot they begin to produce a web; however,
the majority die before successfully producing a web. Survivors feed on
insects that become trapped in the web. They grow, molting several times,
with females typically becoming mature in about 4 to 6 months. Males develop
more rapidly because they go through fewer developmental stages (instars)
and molt fewer times than the females. Developmental rates are greatly
influenced by temperature and the availability of food. In laboratory
conditions, mature females can live over a year after becoming mature.
Males typically live only a few weeks. Under optimum conditions, widow
spiders can live two years or more. The normal life cycle is one year.
Black widows survive winter as either mature or immature forms. During
cold weather, spider development outdoors ceases and the spiders seek
refuge under stones or other cover. Black widows can develop year-round
indoors and many move into homes by autumn for winter protection.
Mating
|
|
|
Figure 5. Male widow mating with
female.
|
Although females of many spider species, including widow spiders, occasionally
feed on males if the female is hungry, this cannibalistic behavior is
not the norm despite the common misconception.
Male spiders wander and seek females for mating. When a female is encountered,
the male cautiously approaches the female, periodically vibrating the
web in a species-specific manner. If the female is receptive, she will
respond to the male’s signals by sending her own vibrational signals.
The male then approaches the female and may engage in contact courtship
behavior. In successful matings, the male may finally reach the female
in about 30 minutes. However, aggressive females, those that are particularly
hungry, may charge the approaching male and cause him to delay or abandon
his attempt.
Once touching the female, courtship begins as a tapping of the legs.
The male then inseminates the female with sperm stored in his palps. At
any time during the mating process, the female may interrupt and attack
the male. However, males are rarely attacked and eaten if the female is
well fed. Under natural conditions, the males often live around the periphery
of the female web and may feed on food that she has captured. These males
may live longer than those not associated with a female’s web. If
eaten by the female, the male spider provides a good source of nutrients,
particularly proteins, which are used to help produce and mature eggs.
Widow spiders get their common name because they are shiny black when
full grown, and black is a traditional color of mourning.
Widow Bites
All members of the genus Latrodectus are potentially harmful to humans.
However, the western widow is not aggressive and bites are very infrequent
even when large numbers of spiders occupy an area. The adult female spiders
usually remain in their webs unless forced out by adverse temperatures
or destruction of their web. They do not forage for food and the insects
they eat are caught in the webs and eaten at the site. Widow spiders rarely,
if ever, bite when not within a web. Bites may be more likely if the female
is tending an egg sac in the web, which she will defend.
Widow bites are nearly always from female widows. Male widows, are less
likely to be encountered than females. Being smaller than the females,
they have smaller venom glands and smaller fangs; thus, it is unlikely
that males, even if encountered, could penetrate the skin of an adult
human. Even female spiders are timid and not likely to bite unless seriously
provoked. Most bites occur when people inadvertently press down on a spider
resting under a rock, under wood in a wood pile, or, in the days of outdoor
privies, when people used the facilities and did not see a spider resting
on or near the seat. Widow spiders have to be provoked in a highly specific
manner in order to induce them to bite a human.
Widows, and other spiders, bite using a pair of fangs on their jaws,
or chelicerae. The bite can go unnoticed, although it most often produces
an immediate sharp, pin-prick pain. A slight swelling and redness may
soon develop at the bite site.
Widow spiders produce a toxin that affects the nervous system (neurotoxin).
Muscle and chest pain or tightness are some of the most common reactions
to the widow toxin. The pain may also spread to the abdomen, producing
stomach cramping and nausea. Other general symptoms include restlessness,
anxiety, breathing and speech difficulty, and sweating. Swelling may be
noticed in extremities and eyelids, but rarely at the bite site. A sense
of burning in the soles of the feet is often noted.
Often there is a general sense of discomfort shortly after the bite,
and acute symptoms increase in severity during the first day. Symptoms
usually decline after 2 to 3 days but some may continue for several weeks
up to a month after the bite.
Other mammals vary in their reaction to widow toxin. For example, horses
are highly susceptible whereas rabbits are more resistant. Cats may be
sensitive to a widow bite while dogs reportedly suffer only mild symptoms.
Management
Many insects prey on widow spiders including mud dauber wasps, other
spider-hunting wasps, and other spiders. A species of frit fly (Chloropidae)
and parasitic wasps develop on the eggs of the spider. Small rodents may
feed on black widows, although the sticky silk that the spider produces
often discourages these predators.
Periodically check areas in and around the home where widows may likely
occur. Widow spiders prefer to nest near the ground, in dark, undisturbed
areas. Nest sites are often near holes produced by small animals or around
construction openings and wood piles. Low shrubs are also common sites
for widows. Indoors, widows similarly occur in dark, undisturbed sites
such as behind furniture or under desks. Undisturbed basement areas and
crawl spaces of homes are also commonly colonized by widow spiders.
When discovered they can be most effectively destroyed by crushing or
vacuuming the web and spider, using protective methods (e.g., wear heavy,
leather gloves). Spiders can also be discouraged by increasing the lighting
of darkened corners, such as by appropriate furniture arrangement or the
use of artificial lighting. Encourage everyone in the family to learn
about black widows so that they can be readily identified and avoided.
It is also a good idea to wear gloves and a long-sleeved shirt when working
in spider infested areas.
Insecticides can be useful for adult spider control but are not particularly
effective against the eggsacs. Physical search and removal of widows is
the most effective treatment. Insecticides can be used to reduce spider
migrations into homes by spraying around the exterior foundation and lower
story windows. This should be done before cold weather forces spiders
(and many insects) into homes to seek shelter. However, the benefit from
such treatment will be short-lived.
Presently, various pyrethroid insecticides (bifenthrin, cyfluthrin, permethrin,
tetramethrin) are sold for general control of spiders around a home. These
have some residual effectiveness of several weeks, particularly if not
exposed to light and moisture. For control of the western widow these
should be applied to specific areas most likely frequented by widows—dark,
undisturbed sites where flying insect prey may occasionally pass. However,
physical removal or killing of the adults and crushing or freezing eggsacs
(leave in the freezer for several weeks) are more effective treatments.
A large number of controls are sometimes promoted for control of spiders,
including widows. These include various chemical repellents (e.g., mock
orange, garlic) or ultrasonic frequencies -- neither of which spiders
perceive or respond to. Because of their complete lack of demonstrated
effectiveness, the promotion for sale of such products is considered fraudulent
and illegal under Colorado law.
|
