Hobo spider

Taxobox
name = Hobo spider


image_width = 250px
image_caption = Hobo spider
regnum = Animalia
phylum = Arthropoda
classis = Arachnida
ordo = Araneae
familia = Agelenidae
genus = "Tegenaria"
species = "T. agrestis"
binomial = "Tegenaria agrestis"
binomial_authority = {Walckenaer, 1802}
range_

range_map_width = 250px
range_map_caption = Distribution in North America in green (native European distribution not shown)

The hobo spider ("Tegenaria agrestis") is a member of the genus of spiders known colloquially as funnel web spiders. It is one of a small number of spiders in North America whose bites are generally considered to be medically significant. Individuals construct a funnel-shaped structure of silk sheeting and lie in wait at the small end of the funnel for prey insects to blunder onto their webs. Hobo spiders sometimes build their webs in or around human habitations. Although this species of spider has a reputation for aggressiveness, they will normally avoid contact with humans. Most bites occur when the spider is accidentally crushed or squeezed by a human. The spider's venom is strong enough to cause considerable local pain and there are unconfirmed reports that it may cause tissue death (necrosis) at and near the bite.

Appearance

The female reaches sizes of 11–15 mm (.43–.6 in); the male 8–11 mm (.31–.43 in). There is no dimorphism in colour or markings. Their coloration rather subdued, being a mixture of brown and rust earth shades. They have a herringbone pattern on the top side of their abdomens. Although most Agelenidae have very "hairy"-looking legs, the legs of this spider are fairly smooth. Lookalikes have stripes on the legs and are often much larger—the hobo spider has solid light-brown-colored legs. Related species are quite similar in appearance, and it requires technical training to identify them reliably.

The web is a flat or curved sheet of dense silk, curved upwards at the edges, and slanting down to one corner where it forms a funnel-like tubular retreat; the spider rests there, and has an escape hole at the back. The web captures prey that fall on it from above, and insects that fall into it become tangled in the irregular "trip threads" over its surface; the hobo's web is not sticky. Prey includes household pests such as roaches, houseflies, earwigs, silverfish, and carpet beetles.Crawford, R. & Vest, D.K. (1988) The Hobo Spider and other European House Spiders. Burke Museum Educational Bulletin No. 1.]

Habitat and distribution

"T. agrestis" is indigenous to western and central Europe, but is now also found in the northwestern USA and southwestern Canada. It has recently spread to southern Alaska, becoming the first spider considered dangerous to humans in the state. [http://www.webmd.com/hw/health_guide_atoz/tm6500.asp Symptoms] Recently it has been found in great numbers in Denmark on a small island (Peberholm), which is on the route of the Oresund highway, and according to media it has spread from there to the province of Skåne in southern Sweden, though there have not been any reported bite cases.

It prefers moderately dry and warm environments, but in Europe does not typically live in houses, largely due to competition from house spiders such as "Tegenaria duellica", which also occurs in the Pacific Northwest. It is most commonly found in gardens, fields, hedges, and the like. It is quite rarely found in houses in Europe, where almost no envenomations have been reported, and the harmless if similar-looking "T. domestica" and "T. saeva" are far more common.

As a rule, and especially in the mild climate west of the Cascades, hobo spiders are perceived primarily as a problem in August or September, when the short-lived males are active.

Toxicity and aggressiveness

The toxicity and aggression of the hobo spider are currently disputed by scientists. Its nickname "aggressive house spider" comes from a misinterpretation of the Latin name "Agrestis", which literally translates to "of the fields", but is mistranslated as "aggressive". If a hobo spider is tending an egg sac, it may become aggressive if it perceives the egg sac to be threatened [ [http://www.washington.edu/burkemuseum/spidermyth/myths/hobo.html http://www.washington.edu/burkemuseum/spidermyth/myths/hobo.html] ] . However, they generally do not bite unless forced to protect themselves, and in the majority of cases the hobo spider does not actually inject venom when it does bite [ [http://whatcom.wsu.edu/ag/homehort/pest/hobo.html] ] .

In the United States, the hobo spider has been considered to be a dangerous species based on a toxicology study on rabbits where lesions appeared after spiders were induced to bite the rabbits [Vest, D. K. (1987). Envenomation by "Tegenaria agrestis" (Walckenaer) spiders in rabbits. "Toxicon" 25(2):221-4.] , although attempts to replicate the study (by injecting venom to ensure envenomation) have failed to produce necrotic lesions, and there is even question as to whether the lesions observed in the original study were necroticVetter, R. S. and G. K. Isbister. (2004). Do hobo spider bites cause dermonecrotic injuries? "Annals of Emergency Medicine" 44:605-607.] . Due to this one laboratory study, it has been proposed that in some parts of the U.S. nearly all bites imputed to the brown recluse spider are in reality the hobo spider's bite [Vest, D. K. (1987). Necrotic arachnidism in the northwest United States and its probable relationship to "Tegenaria agrestis" (Walckenaer) spiders. "Toxicon" 25(2):175-84.] . The CDC and other U.S. government agencies (e.g. [http://www.fs.fed.us/r1/fire/nrcg/Committees/Operations/ims/ims_web_site/Hobo%20Spider.htm] ) have also used this same study as the basis for a report claiming that the hobo spider bite causes necrosis in humans [ [http://www.cdc.gov/mmwr/preview/mmwrhtml/00042059.htm] Centers for Disease Control and Prevention (CDC). Necrotic arachnidism-Pacific Northwest, 1988-1996. MMWR Morb Mortal Wkly Rep 1996;45:433-6.] , despite the absence of any confirmed cases. In Canada, there are scientists who claim that no hobo spider bites lead to dermal necrosis [Bennett, R. G. and R. S. Vetter. (2004). An approach to spider bites: erroneous attribution of dermonecrotic lesions to brown recluse and hobo spider bites in Canada. "Canadian Fam. Physician" 50: 1098-1101.] . Hobo spiders are common in Europe, though bites are relatively unknown, and there are no confirmed reports of them causing necrosis despite hundreds of years of coexistence there. Nonetheless, it is believed by some that hobo spiders are dangerous to humans. The only documented case of a verified hobo spider bite leading to necrotic skin lesions involves a person who had a pre-existing medical condition (phlebitis) that can also cause the appearance of skin lesions.

Hobo spider bites are not known to be fatal to healthy humans. The necrosis in purported cases is similar to, but more mild than, that caused by the brown recluse spider, and in severe cases can take months to heal. Other reported symptoms include intense headaches, vision abnormalities, and/or general feelings of malaise. These symptoms are not confirmed for the hobo spider bite specifically due to lack of positive identification of the spider by an expert, and the Oregon Poison Center (affiliated with the Oregon Health & Science University) is attempting to gather definitive evidence regarding the validity of these reports as of September 2007 [cite news|url=http://www.portlandtribune.com/news/story.php?story_id=119032341162984200
title=Spider bite? Drop the critter in the mail|work=Portland Tribune|date=2007-09-21
author=Peter Korn|accessdate=2007-11-02] [cite web|url=http://www.ohsu.edu/ohsuedu/newspub/releases/090507spider.cfm|title=OHSU Wants Your Spiders, Dead or Alive|date=2007-09-05|publisher=Oregon Health & Science University|accessdate=2007-11-02] .

Diagnosis and identification

If one is bitten by a spider, it is best to capture it immediately, as the specimen is necessary to determine its identity. One way of doing this is by dropping a drinking glass over the spider, sliding a card or a sheet of stiff paper under the glass, then flipping it over and securing the top. If, after a known bite, major discomfort is experienced and medical assistance is required, then a spider expert may be needed to confirm the identity of the specimen. Though such experts are not often (if at all) on hospital staff, this step would best determine the course of treatment. It is especially important to remember that if a bite mark was noticed before an encounter with a spider, this is not evidence that the spider was responsible; this is the most common source of false reports, which - most significantly - have often led to misdiagnosis and mistreatment, with potentially life-threatening consequences. [Vetter, R. S. (2000). Myth: Idiopathic wounds are often due to brown recluse or other spider bites throughout the United States. "Western Journal of Medicine" 173:357-358] Spider bite is often confused with other clinical conditions, and it is common but inappropriate to identify skin lesions of unknown origin as "spider bites", as is the case with most reports of hobo spider bites.

Avoiding bites

The best protection against hobo spiders in one's home is probably the presence of other spiders, such as the giant house spider ("Tegenaria duellica"; formerly "Tegenaria gigantea") that will compete with them for territory and food and often kill them in confrontations. Killing all spiders by using insecticides can result in the proliferation of a single species when its competition disappears, and since spiders consume many household pest insects, eliminating spiders can have serious consequences. Chemical control of spiders is not recommended and should be considered only as a last resort. [http://www.nature.nps.gov/biology/ipm/manual/ipmmanual.pdf]

Normal care should be exercised when disturbing webs or moving long-unused boxes. Spiders do not perceive humans as prey, and the only reason they have for biting is self-defense. If it becomes necessary to remove a spider's funnel web, it is prudent to do so by gently sweeping it away with a broom. If a spider is in or on a box, it will usually leave as quickly as possible when the box is disturbed. [http://www.cepis.ops-oms.org/bvsapud/i/fulltext/spiders/spiders.pdf]

Notes

References

* [http://www.lclark.edu/~biology/Binford/Toxicon2001.pdf Binford, J. G. (2001). "An analysis of geographic and intersexual chemical variation in venoms of the spider "Tegenaria agrestis" (Agelenidae). "Toxicon" 39:955-968]
* Bolzern, A. & Hänggi, A. (2006). Phylogeny of "Tegenaria" (Araneae, Agelenidae), with special focus on the human-biting "Tegenaria agrestis"-complex: a revision using morphological and molecular data.

External links

* [http://spiders.ucr.edu/dermatol.html Website maintained by the University of California at Riverside with information concerning these spiders]
* [http://hobospider.org/ Website maintained by the family of the author of the rabbit toxicology study]
* [http://www.msnbc.msn.com/id/16160932/ An example of a necrotic wound blamed on the HS without evidence] and [http://seattlepi.nwsource.com/local/295595_hobos12.html a rebuttal from a different newsfeed of the preceding article]