Hobo spider

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Hobo spider
Male hobo spider
Male hobo spider
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Arachnida
Order: Araneae
Family: Agelenidae
Genus: Tegenaria
Species: T. agrestis
Binomial name
Tegenaria agrestis
{Walckenaer, 1802}

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 has been reported to sometimes cause tissue death (necrosis) at and near the bite (below).

Contents

[edit] Appearance

Female hobo spider
Female hobo spider

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. Related species are quite similar in appearance, and it requires technical training to identify them reliably.

[edit] 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 state [1]. 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 from 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.

[edit] Toxicity and aggressiveness

In the United States, the hobo spider has been considered to be a dangerous species, based on a toxicology study on rabbits (wherein venom was harvested from the hobo spider and injected into the rabbits) produced necrotic lesions [2], although attempts to replicate the original study have all failed. It is also unknown whether rabbits and humans have similar reactions to hobo spider venom; they do not react the same, for example, to the venom of Sydney funnel-web spiders, and even in the original study, two different breeds of rabbits reacted differently, one failing to develop lesions. Therefore, even if the original study was correct, it would not constitute proof that hobo spider venom can cause necrotic lesions in humans. This same author further used his toxicological study as the basis for claiming that in some parts of the U.S. nearly all bites imputed to Loxosceles reclusa, the brown recluse spider, are in reality hobo spider bites[3] — recluses are not found in the range of the hobo spider. These two references are the foundation for the belief that hobo spiders are dangerous to humans.

While the purported effects of hobo spider bites can go so far as to include necrosis, they are not known to be fatal to healthy humans. The necrosis in the reported cases 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 reported symptoms include intense headaches that may last from a couple of days to a week, and in some cases there were vision abnormalities and/or a general feeling of malaise.

There have been numerous documented cases where suspected hobo spider bites produced such symptoms in humans, but in virtually none of these cases was the spider positively identified by an expert and directly observed by the victim in the act of biting. There is in fact only one documented case of a verified bite by a hobo spider that resulted in a necrotic skin lesion, and this was in a person who had a pre-existing medical condition that also leads to necrotic skin lesions [4]. Further, even in a healthy human, any puncture wound has the potential to become necrotic through sepsis, and given enough bites over a long period of time, any spider that bites humans could be expected to eventually produce a necrotic lesion in at least one victim. The true percentage of bites that fail to produce necrotic symptoms is unknown, as non-symptomatic bites are very rarely reported by victims, if they are even aware they have been bitten.

It is also by no means certain that the hobo spider deserves its reputation as an aggressive 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), and many scientists question the claim that the hobo spider is dangerous to humans. One line of evidence is that although the same spider occurs in Europe, bites by this species are completely unknown there. This fact is sometimes explained by claiming that T. agrestis changed its habits after moving to the U.S. becoming a much more "domestic" species, i.e., it is found primarily in houses. This explanation is, at present, pure speculation, without any supporting evidence or a proposed mechanism by which such behavioral evolution would occur. If a hobo spider is tending an egg sac, it may become aggressive if it perceives the egg sac to be threatened. 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.

Recent studies by scientists have confirmed that the venom of both European and American hobo spiders is chemically identical, so geographical differences in venom would not explain the difference in reputations of the spiders. Attempts to duplicate the rabbit skin toxicology experiment mentioned above have failed (more recent runs only produced red marks on the rabbits, rather than necrotic wounds). There are scientists who claim that at least in Canada no spider bites leading to dermal necrosis are ever caused by Tegenaria agrestis [5]. There are also many other medical conditions (other than bite wounds), some more common than spider bites, that involve tissue necrosis ([1]). Furthermore, identification of spiders is a specialist skill and an average hospital doctor cannot be relied on to make an accurate identification 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.)

[edit] Bite treatment

Main article: Spider bite

In case of being bitten by a Hobo spider, it is best to capture it immediately. 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. It is especially important to remember that if a bite mark was noticed before an encounter with this 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[6]. If afterwards major discomfort is experienced and medical assistance is required, then a spider expert may be needed to determine the actual cause of the bite. Though such experts are not often (if at all) on hospital staff, this step would best determine the course of treatment.

Treatments for more minor bites should be as for any puncture wound. The wound should first be encouraged to bleed to wash out any foreign material and debris. (Many wounds will not bleed because they are so small in diameter that they seal immediately.) Then a topical antiseptic such as Isodine should be applied on the off chance that the bite introduced a virus or microbe beneath the skin level and that the antiseptic will penetrate to that depth. The bite should be observed for a couple of days so that medical attention can be sought if signs of infection appear.[7]

[edit] 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 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.

[edit] Notes

  1. ^ http://www.webmd.com/hw/health_guide_atoz/tm6500.asp
  2. ^ Vest, D. K. (1987). Envenomation by Tegenaria agrestis (Walckenaer) spiders in rabbits. Toxicon 25(2):221-4.
  3. ^ 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.
  4. ^ Vetter, R. S. and G. K. Isbister. (2004). Do hobo spider bites cause dermonecrotic injuries? Annals of Emergency Medicine 44:605-607.
  5. ^ 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.
  6. ^ 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
  7. ^ Wilkerson, James A., M.D. (1967). Medicine for Mountaineering, Seattle. p. 50

[edit] References

[edit] External links

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