For the week of May 25, 1999  thru June 1, 1999  

Venomous spiders share the Wood River Valley

How to avoid the sneaky biters and what to expect if you don’t

Express Staff Writer

y26spider.jpg (6549 bytes)Two venomous spiders—the notorious black widow and the lesser known hobo spider—make their homes in dark crevices near you, and in spring and summer they like to roam.

Neither of these Wood River Valley dwellers is aggressive and usually bites humans only as a defensive measure.

In the first three months of this year, there were only 23 spider bites in Idaho—one black widow, four hobo, and 18 unidentified spiders, according to the Rocky Mountain Poison Control Center in Denver, Colorado.

In 1998, there were a total of 189 spider bites reported in Idaho—23 from the widow, 16 from the hobo and 150 from unidentified spiders.

"Unidentified" means that a bite from a spider was confirmed but that the spider was not seen or caught by the victim.

It is probable that most of the reported bites were inflicted either by widows or hobos, according to Gerald O’Malley, clinical toxicology fellow at the poison control center in Denver.

The numbers reported to the center "under-represent what’s going on" because victims and physicians are not required to report spider bites to any poison control center, O’Malley said.

"It’s nothing to get panicked about," O’Malley said. "But people should be aware of what spiders they’re living with."


The black widow is ubiquitous, inhabiting most warmer regions of the world.

"The black widow is as common as dirt," said University of Idaho entomologist Bob Stoltz. "If you dig a hole in the desert, you’ll have a black widow in it in no time. We live with the black widow every day."

Black widows are common around wood piles, and are frequently encountered when homeowners carry firewood indoors. Widows also like to dwell under eaves, in outdoor toilets and in other quiet places.

Adult male black widow spiders have shiny, jet black, rounded abdomens with two reddish or yellowish triangles on the underside, which form a characteristic hourglass marking. Adult female widow spiders are also shiny black or brown-black with two reddish triangles on the underside, resembling a split hourglass.

Female widows are about one-half inch long, not including the legs. Adult males are harmless and about half the female’s size.

Widows spin tangled, erratic webs of coarse silk in dark places, usually outdoors.

Webs are usually built near the ground wherever a web can be strung.

"Black widows stay in cryptic places," Stoltz said. "They don’t wander around, unless they’re nest building, which they do in spring and fall. When we move woodpiles or brick piles around, we disturb their nests."

When black widows bite, it’s almost always a defensive move.

If poisoned by the black widow, the victim usually experiences pain at the bite site. A round, reddish ring forms around the bite and the skin will sometimes sweat slightly. Hairs around the site usually stand straight up. A cramping sensation begins at the bite site and usually migrates to other parts of the body.

"Abdominal cramping is usually why most people report to physicians," said O’Malley.

That cramping, which can be severe, occasionally prompts a misdiagnosis of appendicitis, according to O’Malley

"You won’t die, but you might wish you were dead," O’Malley said.

No deaths from a black widow bite have been reported for five years, O’Malley said.


The adult hobo spider is a large brown spider that would fit perfectly on a quarter. The abdomen has a herringbone or chevron pattern. The male has two antennae-like protuberances that look like boxing gloves, which are actually the male genitalia. The female has a slightly larger abdomen and no "boxing gloves."

For its dwelling, the hobo likes to spin flat webs in low-growing shrubs. Like the black widow, the hobo prefers the cryptic life provided by the undersides of rocks, wood, lawn ornaments and debris.

The hobo typically enters homes when it is in search of a mate. Males tend to wander with this yearning in August.

"Many hobo spider bites occur in bed, when a sleeping person accidentally rolls on to the spider," Stoltz said.

This can be prevented by eliminating the spider’s means of getting onto the bed, such as by elevating the bed spread from the floor, according to Stoltz.

Hobos are not adept at scaling slick surfaces.

Luckily, hobos rarely bite without provocation and are more likely to incorporate venom into a food-getting bite than in a defensive bite.

The bite symptoms of the hobo are almost identical to that of the brown recluse spider, a spider that has never been positively identified in Idaho.

After poisoning by a hobo spider bite, a large area of redness forms around the bite, which usually disappears. Within 24 to 48 hours, blistering may occur. Within an additional 24 hours, these blisters can rupture, leaving an open ulceration.

Within a few days of ulceration, a scab forms. After three weeks, this scab becomes more pronounced, giving the lesion a target and bulls-eye appearance. Then, the scab is sloughed and the lesion generally heals—leaving a scar—within 45 days of the original bite.

Depending on the severity of the poisoning, hobo-spider-bite victims may experience other symptoms like headache—which usually does not respond to over-the-counter pain relievers, dry mouth, nausea, weakness and lethargy, dizziness, hallucinations, and joint pain.

Less than 15 percent of the hobo’s victims are poisoned severely enough to require hospitalization.


To prevent bites:

 Wear gloves when working in areas where the spiders might be established—such as in gardens, around woodpiles, in crawl spaces, in sheds.

 In the home, clean out potential widow and hobo habitat by removing the areas where they might hide and by knocking down their webs.

 Shake out blankets and clothing that have been stored in attics or basements.

 If you keep shoes in a mud room or garage, shake them out before putting them on.

 Use aerosol insecticides to control the spiders in difficult to clean locations.


If you think you have been bitten by a venomous spider, seek medical attention immediately.

 Wash the bite thoroughly with soap and water.

 If possible, try to catch the spider for identification by a qualified entomologist or physician.

 Never discard a spider that has definitely bitten a human.


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