Hobo spider
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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, spiders 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 also to 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 is 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.
Bite Ranks
Depending on the amount of venom injected, ten different bites may appear.
First, there is the least harmful bite, in which they will not inject poison. This acts like any regular bite.
Second, there is a bite that will get plenty of black necrosis and pain, mainly worse when on the hand or leg.
Third, there is the same bite as the second rate except the necrosis will break and the bite will constantly bleed and need cleaning.
Fourth, the necrosis will develop and eventually burst, leaving a pool of blood to eventually close up and leave a life-long mark.
And finally, Fifth, the worst that can occur is if the bite's venom spreads about the body and becomes infected, your doctor will have to cut into the bite and clean it out throughoutly, leaving life-long scabs and really horrible bite patterns that will always look worse than they feel.
The ranks can be found on some sites to be ten different bites, however, this is not true, they're just simply different effects. The five ranks are the main possibilities.
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 dangerous to humans in the state1. 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.
It prefers moderately dry and warm environments, but in Europe does not typically live in houses, largely due to competition by house spiders such as Tegenaria gigantea. It is most commonly found in gardens, fields, hedges, and the like. It is quite rarely found in houses in Europe, where almost no invenomations have been reported, and the harmless if similar-looking T. domestica and T. saeva are far more common.
Toxicity and aggressivity
Many authorities consider the hobo spider to be a dangerous species. A toxicology study on rabbits (wherein venom was harvested from the hobo spider and injected into the rabbits) produced necrotic lesions, and there have been numerous documented cases where suspected hobo spider bites produced similar symptoms in humans. Some authors have argued that in some parts of the U.S. nearly all bites imputed to Loxoscoles reclusa, the brown recluse spider, are in reality hobo spider bites2 — recluses are not found in the range of the hobo spider.While the reported effects of hobo spider bites can go so far as to include local tissue death (necrosis), they are not known to be fatal to healthy humans. The necrosis is similar to, but less serious than, the necrosis caused by the brown recluse spider. If such a lesion is severe it may take months to heal. Other symptoms include intense headaches that may last from a couple of days to a week, and in some cases there are vision abnormalities and/or a general feeling of malaise.
If a hobo spider is tending an egg sac, it may become aggressive if it perceives the egg sac to be threatened. Humans may have unfortunate interactions with these spiders on account of careless contact with them in their lairs, or unintentional contact while in bed sleeping when the males wander by searching for mates. 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.
However, it is by no means certain that the hobo spider deserves its reputation as an aggressive and venomous spider (its nickname, "aggressive house spider," came from a misinterpretation of the Latin name Agrestis, which means rural, literally 'of the fields,' and is unrelated to aggression). Some scientists are starting to question the belief that the hobo spider is dangerous to humans. One argument is that although it is reputed to have originated in Europe, bites of this species are completely unknown there. This is sometimes explained by claiming that T. agrestis changed its habits after moving to the U.S. and becoming a much more domestic species, found primarily in houses.
Recent studies by scientists have confirmed that the venom of both European and American hobo spiders is chemically identical, so geographical differences may not adequately explain the difference in reputations of the spiders. Attempts to duplicate the toxicology experiment above have failed (more recent runs only produced red marks on the rabbits, rather than necrotic wounds). There are authors who claim that at least in Canada no spider bites leading to dermal necrosis are ever caused by tegenaria agrestis 3. There are many other medical conditions involving tissue necrosis without involvement of any spiders at all. Furthermore, identification of spiders is a specialist skill and an average hospital doctor cannot be relied on to make an accurate diagnosis of a spider species without a good microscope, some specialist spider literature, and considerable experience. A diagnosis at-a-glance by a non-expert, comparing a spider with a photo, is almost completely worthless. (For a comparison of the toxicity of various species, see the list of spiders having medically significant venom.)
Bite Treatment
If you get bitten by a spider it is best to capture it as you might capture a bee in a water glass. Just drop the glass over the spider, slide a card or sheet of stiff paper under the glass and then flip the whole thing over. Put something more substantial on top to secure the spider within. If you experience major discomfort and require medical treatment, then the hospital can determine the species of spider that has bitten you and procede appropriately.
Treatments for more minor bites should be as for any puncture wound. The wound should first be encouraged to bleed. Then a topical antiseptic such as Isodine (r) should be applied on the off chance that the bite introduced some virus or microbe beneath the skin level. The bite should be observed for a couple of days so that medical attention can be sought if signs of infection appear. (It is obviously difficult to get antiseptic to penetrate to the bottom of such a puncture.
Avoiding bites
The best protection against hobo spiders in one's home is probably the presence of other spiders that will compete with them for territory and food. Killing all spiders by using insecticides can result in the proliferation of a single species when its competition disappears.
Normal care should be exercised when disturbing webs or moving long-unused boxes. Spiders do not perceive humans as prey, and the only other reason they have for biting is self-defense. If it becomes necessary to remove a spider's funnel web, it would be prudent to do so by gently sweeping it away with a broom. If a spider is in or on a box, it will ordinarily leave as rapidly as possible when the box is disturbed.
Further reading
- The website maintained by the University of California at Riverside gives information concerning these spiders: [link]
- The website maintained by an independent organization that has done substantial research on these spiders provides another source of information: [link]
References
- 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. [PDF]
Notes
- http://www.webmd.com/hw/health_guide_atoz/tm6500.asp
- Vest DK. Necrotic arachnidism in the northwest United States and its probable relationship to Tegenaria agrestis (Walckenaer) spiders. Toxicon. 1987;25(2):175-84.
- Bennett, R. G. and R. S. Vetter. An approach to spider bites: erroneous attribution of dermonecrotic lesions to brown recluse and hobo spider bites in Canada. Canadian Fam Physician 2004; 50: 1098-1101.
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