The Eyes Have It: Keep Yours Open

Do you stand clear of spiders, kill them with your shoe, or remove them from the house by carefully holding them in a cup or a wad of tissues? We should have respect for spiders because many are beneficial in reducing populations of undesirable insects, such as flies, mosquitoes, ants, and cockroaches. However, some species should be avoided altogether due to the poisonous compounds delivered with their fangs.

Spiders belong to the class Arachnida and are defined by a hard outer skeleton consisting of only two body parts: a cephalothorax (a combined head and thorax) and an abdomen. Four pairs of jointed legs are attached to the cephalothorax—there are no wings or antennae. Scorpions are also arachnids, sharing a similar anatomy and are poisonous (see In the Shape of a Question Mark). Most spiders have eight eyes, although some have fewer. In North America, there are only two groups of spiders that are clearly medically significant: the eight-eyed widow spiders (Latrodectus spp.) and the six-eyed brown spiders (Loxosceles spp.).

The best-known poisonous spider is the Black Widow (Latrodectus mactans), characterized by its large black abdomen and distinctive red hourglass-shaped marking. The female is usually sedentary, hiding under outdoor woodpiles or in sheds and garages, but can also be found in undisturbed spots in basements and crawl spaces. However, the female widow spider will bite humans if disturbed, whereas the smaller male, if he bites, is unable to envenom humans.

Spider bites typically exhibit two distinct puncture marks where the fangs penetrate the skin, allowing venom to be delivered. Lactrodectus bites may or may not be initially painful. The Black Widow produces a potent neurotoxin, known as alpha-latrotoxin, which is sequestered in its venom glands. Pain progresses from the bite site to the abdomen and back, accompanied by severe cramping in the abdominal muscles. Patients experience nausea, tremors, difficulty breathing, increased sweating, hypertension, and fever. It’s advised to apply a cold pack over the bite area after thoroughly cleaning the wound. Victims should seek medical attention, especially children and the elderly, who are more likely to be affected. A Black Widow antivenom exists and can be given in life-threatening circumstances.

The Brown Recluse (Loxosceles reclusa) was first discovered in North America in the 1940s, but its more formidable relative, Loxosceles laeta, was known in South America much earlier. They are sometimes referred to as violin spiders or fiddlebacks due to the violin-shaped pattern on the top of their cephalothorax. These light brown spiders hide behind furniture or in undisturbed areas, such as the back of a closet, on the floor, or on clothing. The bite initially appears as a red zone on the skin, followed in some cases by a systemic reaction (loxoscelism) within 24-36 hours, which consists of fever, chills, nausea, joint pain, and hemolytic anemia (the destruction of red blood cells). Dermonecrosis (dermonecrotic arachnidism) or local tissue injury may develop over the next several days, resulting in a necrotic black eschar. The spider’s venom is cytotoxic and hemolytic, containing proteins that break down cell membranes (sphingomyelinase D), enzymes that break down substances that bind cells together (hyaluronidase), and enzymes that break down proteins (proteases). When essential tissue components are targeted, skin, blood vessels, and organs can fail.  Antivenom is available only in certain South American countries. As with the widow spiders, death from Loxosceles is possible but rare.

Of note, smaller white spiders, known as the yellow sac spider, Cheiracanthium spp., are commonly found in North America but are not considered medically significant. They dwell in houses on the ceiling, in corners of walls, or under windowsills. They create small spun sacs to hide in during the day. Yellow sac spiders can bite, and while the bite is usually mild, swelling and redness may occur at the site.

So, if you're about to spring into cleaning closets, garages, sheds, basements, or attics, wear long-sleeved shirts, pants, gloves, and closed-toe shoes. Inspect and shake out any clothing that’s been stored before wearing, and handle firewood with care. Use your two eyes to spot those six- and eight-eyed creatures before they spot you.