Necrotic Arachnidism, Life-cycle, Identification & House Spiders

Hobo spiders (Tegenaria Agrestis) were introduced into the Northwestern USA1 from Europe by commercial vessels carrying agricultural products. They were probably introduced during the early 1900s to the port of Seattle and have since then spread to the states surrounding Washington, which is Oregon, Idaho, Nevada, Montana, and Utah.

Hobo Spider Identification

Giant house spiders and many other spiders are often misidentified as hobo spiders. However, there are a number of characteristics that can help determine whether a spider is a hobo or not. Unfortunately, some of these identification characteristics require a microscope, as neither size nor color can be used as identification parameters.

Characteristics you can use to identify a hobo spider are given below. You can see the answer to whether the spider in the picture is a hobo spider at the end of this article.

  • If you can see dark rings around the spider's leg, it is not a hobo spider.
  • If its legs are shiny and missing fine hair, it is not a hobo spider.
  • If its palp are long and pointed, it is not a hobo spider.
  • If it has stripes on its cephalothorax, it is not a hobo spider.
Not a hobo spider but a house spider - Tegenaria Domestica aka the aggressive house spider

                                                              







                                                              

Appearance

The hobo spider is a hairy spider with hairy legs and cephalothorax. Males have two pedipalps that are enlarged at the top, which makes them appear similar to boxing gloves. It is 1½-2 inch or 4-5 cm in diameter. In the section about identification (further down), you can read what to look for to find out if the spider you are studying is a hobo spider.

Misconception About Hobo Spiders

Many people believe that hobo spiders are extremely dangerous. Fortunately, all evidence suggests that this requires more scientific scrutiny to be confirmed. Indeed, a hobo spider bite may be dangerous—the scientific community just needs further proof. Such proof can only emerge if people capture the spider that bit them and bring it with them to the hospital.

Funnel-web Spiders

They are a member of the funnel-web weaver spider family, Agelenidae. Funnel-web spiders can move rapidly, and the hobo spider should be capable of running at a velocity of almost four feet per second. It produces a flat web with a funnel-like lair in the middle, where the spider is waiting for prey.

Hobo Spider Geographic Distribution

The hobo spider can be found in Oregon, Idaho, Wyoming2, Montana, Utah, Washington, and Southern British Columbia in Canada. Its nature makes it uncomfortable in very dry regions, so encountering a hobo spider south of Utah is probably impossible.

Note: The cephalothorax is the first part of the spider's body where the legs are attached.

For an exact identification, it is necessary to study reproductive structures under a microscope.

Hobo Spider Bites

Hobo spider bites have been linked to necrotic arachnidism and systemic poisoning. However, uncertainty exists to how well documented the link between necrotic arachnidism and the hobo spider really is. Has it been hobo spiders in all cases?

In a study by Darwin K. Vest (1987)3 giant rabbits were subjected to forced hobo spider bites. The rabbits were followed to study how their lesions evolved, and their organs were investigated subsequently. The study showed that male hobo spiders caused both dermal lesions and organ hemorrhage, while female bites mainly caused dermal lesions. In this study, however, only a few rabbits were tested.

Necrotic Arachnidism

Consequences, or sufferings, due to poisoning by a hobo spider are called tegenarism; the name is related to the name tegenaria. Tegenarism is probably the leading cause of spider envenomations in the northwestern USA and Canada, as black widows are not abundant that far north. The effects of hobo spider's bites are very similar to those of the brown recluse spider, and often people bitten by hobo spiders think they were bitten by a brown recluse spider, although those two species do not coexist in the same geographic regions.

Life-cycle

The life-cycle of males is particularly interesting. The male will retreat to the female’s burrow to mate for an extended period of time (1-2 weeks). The female does not eat him after mating, but he dies automatically due to the seriousness of his efforts. When the male hobo is dead, the female eats him.

Lesser House Spider or Common House Spider

Both named are used to describe Tegenaria Agrestis, a close sibling to the hobo spider. In contrast to its venomous cousin, the house spider is not venomous. It is found throughout most of the U.S. and Europe. It is very difficult to discriminate between this spider and the hobo spider as their size is approximately the same, and they have the same chevron pattern on their abdomens.

Giant House Spider

Until the mid-nineties this spider was known as Tegenaria Gigantea although it was later shown to be identical to Tegenaria Duellica. It can move extremely fast, and it is probably the quickest spider in the world, as it can reach a speed of 0.53 m/s or 1.73 feet/s. It is almost as fast as the camel spider. The giant house spider is venomous, although it is not known to envenomate humans. The giant spider is truly gigantic, as it can reach a length of ten centimeters.

Tegenaria Atrica

This spider was until recently only found in Europe and Northern Africa. Now it also spreads fear in the United States.

Tegenaria Duellicas

This giant spider is found in Europe and in the United Kingdom. It is a Tegenaria though. European hobo spiders are all considered benign4.

The fact that European variants of Tegenaria are considered rather benign makes it difficult to believe that their American counterparts are as dangerous as people believe.

References

1. Exline, H. Tegenaria agrestis (Walck.), An European spider introduced into Washington State. Ann. ent. Soc. Amer (44) pp. 308-310 (1951)
2. Baird, C.R. & Stoltz, RL. Range expansion of the hobo spider, Tegenaria agrestis, in the northwestern United States (Araneae, Agelenidae). J. Arachnol (30) pp. 201-204 (2002)
3. Vest, D.K. Envenomation by Tegenaria agrestis (Walckenaer) spiders in rabbits. Toxicon 25(2) pp. 221-224 (1987)
4. Goldfrank, L.R & Flomenbaum, N. Goldfrank's toxicologic emergencies. (2006)

Web links

Pest notes
Links
Burke Museum

Note: The picture in the top of the article is not a hobo spider but an aggressive house spider.

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