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Black widow spider

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and
Fish

                iBlack widow
         Scientific classification

   Kingdom: Animalia
   Phylum:  Arthropoda
   Class:   Arachnida
   Order:   Araneae
   Family:  Theridiidae
   Genus:   Latrodectus
            Walckenaer, 1805

                                   Species

   L. mactans Fabricius, 1775
   L. hesperus Chamberlin & Ivie, 1935
   L. variolus Walckenaer, 1837

   The black widow spider (Latrodectus spp.) is a spider notorious for its
   neurotoxic venom. It is a large widow spider found throughout the world
   and commonly associated with urban habitats or agricultural areas.
   Although the common name 'black widow spider' is most commonly used to
   refer to the three North American species best known for their dark
   coloration and red hourglass pattern, it is occasionally also applied
   to several other members of the Latrodectus (widow spider) genus in
   which there are 31 recognized species including the Australian
   red-back, brown widow spider (sometimes called the gray widow), and the
   red widow spider. In South Africa, widow spiders are also known as the
   button spiders.

   Currently, there are three recognized species of black widow found in
   North America: The southern black widow (L. mactans), the northern
   black widow (L. variolus), and the western black widow (L. hesperus).
   As the name indicates, the southern widow is primarily found (and is
   indigenous to) the southeastern United States, ranging from Florida to
   New York, and west to Texas and Oklahoma. Specimens have been found in
   California as well. The northern widow is found primarily in the
   northeastern US and southeastern Canada (only on the Bruce Peninsula),
   though its ranges overlap that of L. mactans quite a bit. The western
   widow is found in the western half of the United States, as well as in
   southwestern Canada and much of Mexico. Prior to 1970, when the current
   taxonomic divisions for North American black widows were set forth by
   Kaston, all three varieties were classified as a single species, L.
   mactans. As a result, there exist numerous references which claim that
   "black widow" (without any geographic modifier) applies to L. mactans
   alone. As common usage of the term "black widow" makes no distinction
   between the three species (and many laypersons are unaware of the
   differences between them), and as the three species have much in
   common, this article treats all three species of black widow equally.
   Except where otherwise indicated, the remainder of the article applies
   to all three of the above species.

Description

   Adult female black widow spiders are shiny black with an hourglass
   shaped marking on the underside of its abdomen which, although most
   commonly red, may range in colour from white to yellow to various
   shades of orange and red. They also bear a small, usually red (colors
   vary) dot near the spinerettes, which is separate from the hourglass.
   In L. variolus, the two halves of the hourglass shape may be separated
   into two separate patches. A large female black widow spider can grow
   to 5.0 inches (51 mm), counting legspan. The body is about 1.75 inches
   (20 mm). Male black widow spiders are half the size of the female or
   smaller. They have longer legs and a smaller abdomen in relation to
   their body size. They are also usually dark brown with varying colors
   of stripes/dots, with no hourglass mark. Adult males can be
   distinguished from juvenile females by their more-slender body, longer
   legs and large pedipalps typical of most other male spiders. Juvenile
   black widow spiders start white, molting to dark brown to black
   exoskeletons with white, yellow, orange and red stripes and/or dots on
   their backs.

   As with many venomous creatures, the brightly colored markings serve as
   a warning to predators. Eating a black widow will normally not kill a
   small predator (birds, et cetera), but the sickness that follows
   digestion is enough for the creature to remember that the bright red
   means not to eat. Because the adult female black widow typically hangs
   and moves about its web upside down, its hourglass is on its front.
   However, juvenile widows (female) spend a large quantity of time in
   search of an optimal environment. Once an optimal location is found,
   adult female widows often spend their entire lives in one place.
   However, because juvenile females must first find this optimal
   location, they bear brightly colored marks upon their backs, so that
   they may be seen by predators when the widow is traveling upon its
   legs. Males bear similar marks to the females to serve as warning while
   they are searching for mates, however, the marks are not as prominent
   (not as brightly colored, or as large). Males, being less venomous, are
   less of a threat to predators, so having similar marks not as prominent
   helps predators to better judge their prey (some large birds can eat
   male widows without adverse effect, and so only avoid female widows.
   Those who cannot eat any widow without adverse effect eat nothing with
   the marks). Female juveniles develop an hourglass before the dorsal
   markings are shed. As is characteristic of all arthropods, black widow
   spiders have a hard exoskeleton composed of chitin and protein (5).
   {The Unknown.}
   Black Widow with a fly.
   Enlarge
   Black Widow with a fly.

Prey

   Black widow spiders typically prey on a variety of insects, but
   occasionally they do feed upon woodlice, diplopods, chilopods and other
   arachnids (McCorkle, 2002). When the prey is entangled by the web, L.
   mactans quickly comes out of its retreat, wraps the prey securely in
   its strong web, then punctures and poisons its prey (Foelix, 1982). The
   venom takes about ten minutes to take effect; in the mean time, the
   prey is held tightly by the spider (Foelix, 1982). When movements of
   the prey cease, digestive enzymes are released into the wound (Foelix,
   1982). The black widow spider then carries its prey back to its retreat
   before feeding (Foelix, 1982).

Reproduction

   When a male is mature, he spins a sperm web, deposits semen on it, and
   charges his palpi with the sperm (3). Black widow spiders reproduce
   sexually when the male inserts his palpus into the female's
   spermathecal openings (3). The female deposits her eggs in a globular
   silken container in which they remain camouflaged and guarded (3). A
   female black widow spider can produce four to nine egg sacs in one
   summer, each containing about 100-400 eggs (1). Usually, eggs incubate
   for twenty to thirty days. Rarely do more than one hundred survive
   through this process, due to cannibalism. On average, thirty will
   survive through the first molting, due to cannibalism, lack of food, or
   lack of proper shelter. It takes two to four months for black widow
   spiders to mature enough to breed, however full maturation typically
   takes six to nine months. The females can live for up to five years,
   while a male's lifespan is much shorter. Very rarely after mating does
   the female eat the male. Usually if that happens it's because she
   mistakes him for the prey. Lifespans depend upon environment, with
   shelter being the greatest determining factor and food the second
   greatest.

Natural Enemies

   There are various parasites and predators of widow spiders in North
   America, though apparently none of these have ever been evaluated in
   terms of augmentation programs for improved biocontrol. Parasites of
   the egg sacs include the flightless scelionid wasp Baeus latrodecti,
   and members of the chloropid fly genus Pseudogaurax. Predators of the
   adult spiders include a few wasps, most notably the blue mud dauber
   Chalybion californicum, and the spider wasp Tastiotenia festiva. Other
   species will occasionally and opportunistically take widows as prey,
   but the preceding all exhibit some significant specific preference for
   Latrodectus.

The Venom

Components and effects

   Although their venom is extremely potent, these spiders are not
   especially large. Compared to many other species of spiders, their
   chelicerae are not very large or powerful. In the case of a mature
   female, the hollow, needle shaped part of each chelicera, the part that
   penetrates the skin, is approximately 1.0 mm (around .04 inch) long,
   sufficiently long to inject the venom to a dangerous depth. The males,
   being much smaller, can inject far less venom and inject it far less
   deeply. The actual amount injected, even by a mature female, is very
   small in physical volume. When this small amount of venom is diffused
   throughout the body of a healthy, mature human, it usually does not
   amount to a fatal dose (though it can produce the very unpleasant
   symptoms of Latrodectism). Deaths in healthy adults from Latrodectus
   bites are relatively rare in terms of the number of bites per thousand
   people. Only 63 deaths were reported in the United States between 1950
   and 1989 (Miller, 1992). On the other hand, the geographical range of
   the widow spiders is very great. As a result, far more people are
   exposed, world-wide, to widow bites than are exposed to bites of more
   dangerous spiders, so the highest number of deaths world-wide are
   caused by members of their genus. Widow spiders have more potent venom
   than most spiders, and prior to the development of antivenom, 5% of
   reported bites result in fatalities.

   Improvements in plumbing have greatly reduced the incidence of bites
   and fatalities in areas where outdoor privies have been replaced by
   flush toilets. "Nearly ninety percent of the black widow bites reported
   in the medical literature of the first 4 decades of [the twentieth]
   century were inflicted on the male genitalia by spiders lurking
   underneath the seats of outdoor toilets." ^1 In Sweden there have been
   incidents with black widow spiders being found in cars imported from
   southern USA. Old cars standing unused are an attractive habitat for
   the spider.

   For more information on toxicity, etc., see Venomous spiders.

   There are a number of active components in the venom:
     * Latrotoxins
     * A number of smaller polypeptides - toxins interacting with cation
       channels which display spatial structure homology - which can
       affect the functioning of calcium, sodium, or potassium channels.
     * Adenosine (1)
     * Guanosine (2)
     * Inosine (3)
     * 2,4,6-trihydroxypurine (4).

   Notes: 1. James A. Wilkerson, M.D., Medicine for Mountaineering, p.
   149.

More photos

   Female black widow from the upper rear, showing pattern

   Female black widow showing red "hourglass" marker

   Female black widow showing mouthparts

   Dorsal view. Note that the red dot is formed by this spider's
   spinneretes.

   Black widow spider with its prey.

   Immature female black widow - brownish coloring (near black to the eye)
   with yellowish markings
   Retrieved from " http://en.wikipedia.org/wiki/Black_widow_spider"
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   with only minor checks and changes (see www.wikipedia.org for details
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