Welcome to yet another edition of Spider Friday!! Emily wants to say hi, but she's too busy munching on a cockroach. She's waving one of her forelegs. It seems she approves of the process.
(Find previous episodes of Spider Friday here--especially if you want to find out who Emily is, and why she's munching on a cockroach.)
Today, both by request and in accordance with prophecy, we are going to deal with the third-most-discussed poisonous spider that can afflict denizens of the United States--the Hobo Spider.
Kingdom:
Animalia (Animals)
Phylum:
Arthropoda (Arthropods)
Class:
Arachnida (Arachnids)
Order:
Araneae (Spiders)
Suborder:
Araneomorphae (true spiders)
Family:
Agelenidae (Funnel Spiders)
Genus:
Tegenaria
Species:
T. agrestis
The Hobo Spider belongs to the Agelenidae family of spiders--often called Funnel Spiders or Grass spiders. These spider are not to be confused with the Hexathelidae spiders native to Australia, which are much larger mygalomorph spiders. They are called Funnel Spiders because they weave webs in the shape of a funnel and sit in them until prey happens onto it--see the following picture of a spider in the Agelenopsis genus of this family:
The Hobo Spider is originally native to Europe, but was brought by shipping lines to the Pacific Northwest many decades ago via the port of Seattle. From there, it has spread throughout the Pacific Northwest, and has now turned up throughout the country, with confirmed sightings all the way from Southern California to the East Coast--although whether those are hitchhikers or established populations has yet to be determined.
Although there is a general consensus that the venom of the Hobo Spider is toxic, it has actually been a source of confusion and consternation among the arachnological community: no hobo spider envenomations have ever been reported in Europe--or Canada, for that matter--as opposed to the several hundred envenomations reported cumulatively in the United States. In addition, toxicology studies of Hobo venom have produced inconclusive results, with some showing the necrotic lesions commonly associated with Hobo bites, but others showing no more than minor irritation.
There are a two explanations for this phenomenon. The first is that irritations and lesions commonly attributed to the Hobo Spider are in fact the product of other pests or secondary infections. The second possible explanation for the lack of envenomations in Europe could have to do with the fact that other, larger, non-venomous species in the Tegenaria genus that enjoy Hobo spiders as snacks have prevented the infestation of Hobo Spiders in urban areas, and have kept them confined to rural areas ("agrestis" is Latin for "from the field"). Regardless, the Hobo Spider should definitely be treated with caution.
So, how do we recognize a Hobo Spider? Well, unfortunately, T. agrestis looks very similar to another well-known venomous spider, the Brown Recluse:
(Photo courtesy of Dr. Lee Ostrom of Family Protection)
Nevertheless, there are a few discernible differences between a Hobo and a Recluse. While the body shape and coloration are similar, Hobo Spiders lack the traditional telltale markings of Recluses, such as a fiddle marking on an otherwise uniform body. Hobo spiders also tend to have a herringbone pattern on their abdomen, as can be discerned from this lovely picture:
(Photo by Maxence Salomon of Simon Fraser University, who is working on a dissertation I'd be very interested in reading.}
Another prime distinguishing characteristic--indeed, the most fundamental distinguishing characteristic with regard to most spider morphology--is the eye arrangement. The eye arrangement of Agelenidae spiders is completely different from the eye arrangement of the Sicariidae. The Hobo spider has eight eyes arranged in a trapezoidal fashion, as you can see from this outstanding closeup picture.
LIFECYCLE:
There is no sexual dimorphism among Hobo spiders. The males look exactly the same as the females, with the exception of the specialized palps characteristic of all male spiders:
(Photo courtesy of Dr. Lee Ostrom of Family Protection)
Unlike Recluse spiders, females stay stationary in their web, which forces male Hobos to go wandering in search of the females. Since females will automatically tend to think that any vibration in the web is a prey item, the male taps at the web with a specific pattern--if he doesn't get the pattern right, he becomes a prey item. (We male humans have it easy--the worst thing that can happen from using the wrong pickup line is a little embarassment.)
Breeding season occurs in late summer and early fall--this is when the males will start wandering in search of females, and it is during this time that the majority of reported Hobo envenomations occur. After mating, the female produces a few egg sacs, each containing between 50-100 eggs, which she will attach to vegetation, usually with debris between two layers of silk:
(Photo © Darwin K. Vest; website)
BITES:
50% of Hobo spider bites are dry--often, the spider will inject no venom in a defensive bite. When envenomation does occur, Hobo spider bites have symptoms similar to Brown Recluse bites, except not as severe. The reaction is usually localized, with tissue necrosis. Certain system effects, such as disorientation, may occur, but are rare. No fatalities have ever been reported as a result of a Hobo bite, and most bites aren't serious at all.
Hobo spiders are not aggressive. Bites generally occur when wandering males get somehow trapped against skin. It is very important, if you think you've been bitten by a Hobo spider, to preserve the spider--or what remains of it--so that a qualified expert can attempt a diagnosis.
This concludes Spider Friday! I have plenty of spiders on my plate--figuratively speaking--and I will make a determination on next week's spider later on. Until then, you'll have to make do with my rantings and ravings about the Constitution.