Hobo spider

Hobo spider
Female Hobo spider missing one leg
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Arachnida
Order:	Araneae
Family:	Agelenidae
Genus:	Eratigena
Species:	E. agrestis
Binomial name
Eratigena agrestis (Walckenaer, 1802)
Distribution in North America in green (native European distribution not shown)
Synonyms
Philoica agrestis Tegenaria agrestis Tegenaria alpestris Tegenaria magnacava Tegenaria osellai Tegenaria rhaetica Tegenaria trinacriae

The hobo spider (Eratigena agrestis, formerly Tegenaria agrestis) is a member of the genus of spiders known colloquially as funnel web spiders, but not to be confused with the Australian funnel-web spider. The medical significance of its bite is still poorly understood and debated. 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.

Habitat and History

Europe

The Hobo spider is a resident of fields, rarely entering human habitations due to the presence of major competitors, particularly the giant house spider (Eratigena atrica) which is a common resident of houses and other man-made structures in Europe. As a result, human contacts with the hobo spider are uncommon in Europe.

It is recorded in the checklist of Danish spider species.^[1]

The species was first described in 1802 by naturalist C.A. Walkenaer,^[2] under the name Aranea agrestis, in reference to its western European habitat in fields, woods, and under rocks.

The hobo spider, initially named Tegenaria agrestis, was transferred to a new genus Eratigena, an anagram of Tegenaria.^[3] Several species of closely related taxa were synonymized under E. atrica including a second Pacific Northwest inhabitant, the giant house spider,^[3] which has been referenced in the literature under the names of T. gigantea and T. duellica, causing confusion among arachnologists due to the use of various species names.^[4]

Identification

Spiders, including the Hobo spider, vary considerably in appearance, and identification can be difficult. Identification relies on an examination of the spider’s anatomy. Like many species of spider the positive identification of Eratigena agrestis requires microscopic examination of the epigynum and pedipalps (the female and male sex organs respectively) and is best done by an arachnologist. However, the following characteristics can help in identification of hobo spiders in order to prevent misidentification and eradication of beneficial species with a similar general appearance:

• Hobo spiders lack the colored bands found on many spiders of the Agelenidae family where the leg joints meet.^[5]
• The abdomen has chevron (V-shaped) patterns (possibly many of them) down the middle, with the chevrons pointing towards the head.
• Hobo spiders have a light stripe running down the middle of the sternum. If the spider instead has three or four pairs of light spots on the lateral portions of the sternum, then it is one of the other two related Eratigena species. However absence of spots is not conclusive proof that the spider is a hobo spider, since the spots on other Eratigena species may be extremely faint and not readily visible.^[5]
• Hobo spiders do not have two distinct longitudinal dark stripes on the top side of the cephalothorax, instead showing indistinct or diffused patterns. Washington spiders with distinct dark stripes include spiders from the genera Agelenopsis and Hololena and possibly some wolf spiders. (These spiders do not have common names.)^[5]

Toxicity and aggression

The toxicity and aggression of the hobo spider are currently disputed by arachnologists. One common name, the aggressive house spider may arise from a misinterpretation of the Latin name agrestis, (lit. "of the fields") as "aggressive". If a Hobo spider is tending an egg sac, it may become aggressive if it perceives the egg sac as being threatened.^[6] However, they generally do not bite unless forced to protect themselves.

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.^[7] This laboratory study has led to the proposal that in some parts of the U.S. nearly all bites attributed to the brown recluse spider are in reality the hobo spider's bite.^[8] The CDC and other U.S. government agencies^[9] have also used this same study as the basis for a report claiming that the hobo spider bite causes necrosis in humans,^[10] despite the absence of any confirmed cases. Subsequent attempts to replicate the study by injecting sufficient 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 necrotic.^[11]

In Canada, there are scientists who claim that no hobo spider bites lead to dermal necrosis.^[12] Hobo spiders are common in Europe, though bites are relatively uncommon, and other than unconfirmed reports there is no evidence of them causing necrosis despite hundreds of years of coexistence there. 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.^[11]

Hobo spider bites are not known to be fatal to healthy humans. The necrosis in purported cases is similar to, but milder 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.^[13][14]

Notes

References

• Binford GJ (July 2001). "An analysis of geographic and intersexual chemical variation in venoms of the spider Tegenaria agrestis (Agelenidae)". Toxicon 39 (7): 955–68. doi:10.1016/S0041-0101(00)00234-8. PMID 11223084. 

morphological and molecular data.

• Isbister GK, Gray MR (August 2003). "White-tail spider bite: a prospective study of 130 definite bites by Lampona species". The Medical Journal of Australia 179 (4): 199–202. PMID 12914510. 
• Vetter, R. S. 2001. Hobo spider. Univ. Calif. Pest Notes #7488, 3 pp. (http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7488.html)

External links

Wikimedia Commons has media related to Tegenaria agrestis.

• Website maintained by the University of California at Riverside with information concerning these spiders
• Website maintained by the family of Darwin K. Vest, the author of the rabbit toxicology study
• An example of a necrotic wound blamed on the HS without evidence and a rebuttal from a different newsfeed of the preceding article