ARTHROPOD MUSEUM NOTES
Number 12 2002 • revised May 17, 2003 • Jeffrey K. Barnes
Southern and northern black widow spiders
Genus and species: Latrodectus mactans (F) and L. variolus Walckenaer
Black widows can be found throughout Arkansas. The body of the female is usually shiny and black and can be up to 1½” long, including the legs. The abdomen appears fat and nearly spherical and it may bear few or many reddish spots. The most characteristic spot occurs on the underside of the abdomen, and it is usually somewhat hourglass-shaped. Males are much smaller and narrower than females, and they have white lines on the abdomen in addition to red markings. Classification and identification of black widows is notoriously difficult. There is so much variation and overlap in characters that it is not always possible to name a given specimen with certainty. It is now understood that at least 2 species occur in Arkansas.
The southern black widow, Latrodectus mactans, is probably the most common species in this state. The posterior portion of the red hourglass on the underside of the female’s abdomen may appear more like a rounded rectangle than a triangle, and there is usually a row of red spots on top of the abdomen. The northern black widow, L. variolus, is more common in the northern states. The anterior and posterior portions of the ventral hourglass are not joined at the middle, and red and white marking on top of abdomen tend to be more prominent than in the southern black widow (Kaston 1970)
Black widows build irregular webs of coarse silk, usually near the ground in dark places, and usually outdoors. Webs are often built among leaf litter on the ground in deciduous forests. They are also found under rocks or logs, in piles of rubble, in mammal burrows, and in dark corners of sheds, garages, crawl spaces, cellars, and basements. The spiders hang in an inverted position in these webs, do not leave them voluntarily, and are clumsy and completely out of their element away from them. They will eat almost any insect that wanders into the snare. In one study of southern black widows in Texas cotton fields, three quarters of the spiders’ prey consisted of fire ants (Nyffeler et al. 1988)
Mating most often occurs in late spring and early summer. Although hungry females may kill their mates, some healthy males live to mate with several females (Breene and Sweet 1985). Females can produce several egg masses during a summer. Egg sacs made of tough, papery, tan or gray silk are suspended in the snare. They are a little less than half and inch in diameter. Those of the southern black widow are spherical and have a conspicuous nipple at the top. Those of the northern black widow are more pear-shaped. Young spiderlings are much lighter in color than adults, adding more black with each molt. Half grown females resemble full-grown males. Spiderlings mature in 2 to 3 months. Females often live more than one year, and some have been known to live as long as three years (Kaston 1970).
Although the nocturnal black widows are not aggressive, females will bite when provoked sufficiently. Males, which are rarely seen, do not bite. Females may rush out and bite when their webs are disturbed or when they are accidentally touched or trapped in clothing or shoes. In the past, bites were most commonly inflicted on people using outdoor privies. Today people are most likely to suffer black widow bites while moving lumber or other clutter from infested areas, while sitting on outdoor furniture, or when putting on clothing in which a spider is trapped.
The venom of the black widow is and oily yellow fluid that causes a general release of neurotransmitters at synaptic junctions, and so the clinical manifestations of a black widow bite are primarily neurological. The insertion of the female's fangs produces mild to sharp pain. Tissue reactions at the site of the bite are minimal, but they may include redness, mild swelling, and itching. Within 30 to 60 minutes, the victim experiences severe pain in local muscle groups. Pain migrates from the site of bite to regional muscle groups. Severe cramps may spread throughout the body. In a typical case, the victim will experience unremitting, cramping pain of extremities, thorax, back, abdomen, and groin areas. Pain and cramping peaks at 2 to 3 hours and may debilitate the victim for 24 to 36 hours. Other symptoms include hypertension, profuse perspiration, nausea, fatigue, shock, and coma. Severe cramping of abdominal and thoracic musculature can lead to respiratory distress, which may be fatal. Death has been reported in less then 1% of documented black widow spider bite cases. Healthy individuals usually recover quickly and completely (Blackman 1995, Kunkel 1984, Wilson and King 1990)
First aid for black widow bites consists of cleaning the wound thoroughly and applying ice packs to slow absorption of the venom. Over-the-counter pain relievers may be used to relieve minor symptoms. The victim should be taken to a physician or hospital immediately. The physician will treat for pain and cramping with intravenous calcium and muscle relaxants. An antivenin is available for serious cases.
To reduce the occurrence of black widow spiders, rubble, scrap, lumber, disused machinery, and similar items should be removed from yards. Take proper precautions, wear gloves, and pay attention to where you are working or relaxing. Inspect outdoor furniture before sitting. Insecticides labeled for spider control can be applied around buildings and in crawl spaces to reduce spider populations. Spraying near the ground, in cracks and crevices, around windows or doors will significantly reduce spider populations and reduce the number of insects that are available as food sources
Blackman, J. R. 1995. Spider bites. Journal of the American Board of Family Practice 8 (4): 288-294.
Breene, R. G., and M. H. Sweet. 1985. Evidence of insemination of multiple females by the male black widow spider, Latrodectus mactans (Araneae, Theridiidae). Journal of Arachnology 13: 331-335.
Kaston, B. J. 1970. Comparative biology of American black widow spiders. Transactions of the San Diego Society of Natural History 16 (3): 33-82.
Kunkel, D. B. 1984. Arthropod envenomations. Emergency Medicine Clinics of North America 2 (3): 579-586.
Nyffeler, M., D. A. Dean, and W. L. Sterling. 1988. The southern black widow spider, Latrodectus mactans (Araneae, Theridiidae), as a predator of the red imported fire ant, Solenopsis invicta (Hymenoptera, Formicidae), in Texas cotton fields. Journal of Applied Entomology 106: 52-57.
Wilson, D. C., and L. E. King, Jr. 1990. Spiders and spider bites. Dermatologic Clinics 8 (2): 277-286.
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