Spiders of Medical Importance
Spiders as a whole are seldom aggressive, generally
biting only when threatened or injured. Nearly all
spiders have venom glands, but the toxicity of the venom
to humans is so low as to be insignificant. The severity
of the reaction to a spider bite is influenced by the
species of spiders, the area of the body where the bite
occurs, the amount of venom injected, and depth of the
bite. Only two spiders occurring in this area can cause
poisoning in humans: the Northern Black Widow Spider
(Latrodectus variolus) and the Brown Recluse Spider
(Loxosceles reclusa).
If bitten by any spider suspected of being poisonous
you should do the following:
(1) remain calm,
(2) collect the specimen, if possible, for a positive
identification (specimens may be sent to Insect
Diagnostic Lab, Department of Entomology, Michigan State
University, East Lansing, MI. 48824),
(3) seek medical attention and contact your local poison
control center immediately (consult the inside cover of
your phone directory, or dial for operator assistance),
and
(4) if unable to get professional assistance, follow the
first aid instructions given in this section.
The Northern Black Widow is found throughout
Michigan's lower peninsula. Its preferred habitat is
marginal land with sparse vegetation: in stumps, hollow
logs, and piles of debris. Occasionally it may be found
indoors. Its relative, the Southern Black Widow (L.
mactans), is considered much more dangerous because of
its larger range and willingness to invade man's habitat.
The poisonous female Northern Black Widow is
approximately 1/2", excluding the legs. The overall
length may be in excess of 1 1/2" at maturity. It is
shiny black or brown-black with a spherical abdomen
having two red triangular shaped marks (a "split-
hourglass") on the underside. Its southern relative has
the true hourglass marking. The males are harmless, and
only approximately 1/2 the females' size. They have
light streaks on the sides of the abdomen, with no red
markings. Juveniles of both sexes resemble the male and
are harmless.
The female spins a tangled coarse web in dark
undisturbed places, and encloses her eggs in a whitish
egg sac which she guards until the spiderlings hatch.
Being shy and nocturnal, she will not leave her web
voluntarily and will attack only if her web is disturbed.
Her bite is slight, and may not be felt. It is followed
by burning, local swelling, and redness. Two puncture
points may be visible. The pain intensifies after 1-3
hours, and may last up to 48 hours. The venom is
neurotoxic, effecting the human nervous system, and
cramps in the legs, arms and chest occur concurrently
with the local swelling. Additional symptoms may include
headache, nausea, tremors, stuttering, and a slight rise
in body temperature. The severity of the symptoms depends
on the age and sensitivity of the victim, with small
children most severely affected.
First Aid for Northern Black Widow Bite:
If bitten by a Northern Widow, contact your physician
immediately. The physician may give a specific antivenin
or calcium gluconate.
The Brown Recluse, is found east of the Rockies and
as far north as Ohio, Indiana, and Illinois. No specimen
has been collected in Michigan. The males and females are
similar in appearance, being 1/4-1/3" in length and are
equally toxic. With legs extended, they may be as large
as a half dollar. Their color varies from orangish-yellow
to dark brown, and the body is covered with short hairs.
The legs are long and slightly darker than the body.
Distinguishing characteristic include six eyes (most
spiders have eight) arranged in 3 pair, called diads, and
a violin shaped marking on the cephalothorax (the head
and thorax, which bears the legs). Development from egg
to mature adult requires about 1 year, with total
lifespan years.
Though its natural home is in sheltered places
outdoors, it is commonly attracted to buildings of all
types, especially those that are dry, littered, and
contain many insects for prey. Their web is irregular,
and it is used as a resting place rather than a snare.
The Brown Recluse is a hunter, leaving the web and
actively searching for prey. Egg sacs are hung in the web
and are guarded by the female. Bites usually occur when a
person puts on shoes or clothing in which the spider is
hiding. Because of its habits, it is often inadvertently
transported by man.
The amount of venom injected and the sensitivity of
the victim are both factors determining the seriousness
of the bits. The venom is cytotoxic, damaging the flesh
of the victim. There may be no initial pain felt, or a
stinging sensation may appear immediately. Intense pain
follows, lasting 2-8 hours. A small blister forms at the
bite, and a large area surrounding the bite swells and
reddens, becoming firm to the touch. Victims sometimes
experience nausea, abdominal cramps, stiffness of joints,
and fever. The tissue in the infected area becomes
ulcerous, sloughs away, and results in a pitted area that
eventually fills with scar tissue. The wound heals
slowly, usually in 6 to 8 weeks, and leaves a permanent
scar.
First Aid for Brown Recluse Bite:
If bitten, call your local poison center immediately
or see a doctor. Apply antiseptic solution to prevent
infection, and ice packs to the surrounding area.
Corticosteroid injections may be given by a doctor to
curtail reactions to the bite. No specific antivenom is
available. The Brown Recluse bite is especially dangerous
to children, the elderly, and those in poor physical
condition.
Another spider worth mentioning is the tarantula.
There are approximately 30 species of tarantulas found in
the southern U.S. Because of their popularity as pets
there is a chance of humans being bitten. Though the
venom is rarely more poisonous than that of bees, the
local poison center should be contacted immediately
should anyone experience a bite.
General sanitation aids in preventing a spider
infestation of any type. Caulking cracks and crevices,
tight fitting screens and doors all prevent entrance into
buildings. Since many spiders can live several months
without food, simply eliminating the food source
(insects) does not bring immediate results. Should an
infestation warrant chemical control, follow all
instructions on the label and observe any stated
precautions.
For a complete listing of suggested control options
for all home, yard and garden insect pests contact your
local Extension Service, found under local government in
the phone book.
Read and follow instructions on the pesticide label.
Heed all warnings. Check with your physician if you have
any concerns regarding your personal health risk.