Hobo spider
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| Hobo spider | ||||||||||||||
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| Missing image Male-hobo.gif Male Hobo spider Male Hobo spider | ||||||||||||||
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| Tegenaria agrestis Walckenaer, 1802 |
The hobo spider (Tegenaria agrestis) is a member of the genus of spiders known colloquially as funnel web spiders. 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. If a spider is out on its web, for instance, it normally will retreat if it senses a large animal coming near. Perhaps a fairly high number of unhappy interactions occur because these spiders may build dwellings in or near human habitations, and so naturally come in more frequent contact with humans than would spiders that favor prairies or forests. Another cause of problems may be the fact that male spiders go in search of females to mate with, and while so doing may unintentionally get in bed with a sleeping human. If the human rolls over on the spider and squeezes it, the spider will quite naturally deliver a defensive bite. The spider may die, but, unfortunately for the human the spider's venom is strong enough to cause considerable local pain and also to cause tissue death (necrosis) at and near the site of the bite. Since the human is unlikely to realize the motivation for the bite, the spider will very likely get the blame. It does not help the spider's case that this particular species is not especially attractive.
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Appearance
The female reaches sizes of 11-15 mm (.5 - .66 inch); the male 8-11 mm (.25 - .33 inch). There is no dimorphism in colour or markings. The appearance of hobo spiders is not unattractive, but 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.
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 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 rare in Europe, where almost no invenomations have been reported and the harmless if similar looking T. domestica and T. atrica are far more common.
Toxicity and aggressivity
It is by no means certain that the Hobo spider deserves its reputation as an agressive and venomous spider. (its Latin name Agrestis means 'of the fields' and is related to agriculture, not to aggressive). 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. Are the two species really the same? Some authors have argued that in some parts of the US nearly all bites imputed to Loxoscles reclusa, the brown recluse spider, are in reality hobo spider bites1. However, they base this purely on finding such spiders in the home of the victim. The spiders are common in some areas and are found in nearly every home. On the other hand, there are authors who claim that at least in Canada no spider bites leading to dermal necrosis are ever caused by tegenaria agrestis 2. 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.
While the 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 spiders. 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. In the past, hobo spider bites have been mistaken for those of the somewhat more dangerous brown recluse spider, whose bites have initially similar effects.
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.
Avoiding bites
The best protection against hobo spiders in your 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.
For more information
The website maintained by the University of California at Riverside gives much good information concerning these spiders: [1] (http://spiders.ucr.edu/dermatol.html)
Notes
1)Vest DK. Necrotic arachnidism in the northwest United States and its probable relationship to Tegenaria agrestis (Walckenaer) spiders. Toxicon. 1987;25(2):175-84. 2) 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.de:Feldwinkelspinne nl:Veldtrechterspin