1-12. SPIDERS
Spiders are cosmopolitan in distribution, but the majority is found in the
temperate and tropical zones. Many spiders use their venom to paralyze their prey.
Although their venom is sufficient for this purpose, humans are not affected seriously by
most spiders. Very few spiders have mouthparts that can penetrate the skin of a
person; most of those that do have venom that can produce only local symptoms or an
occasional allergic reaction. Spiders have eight legs, no wings, and no antennae. They
have an unsegmented abdomen attached to the cephalothorax by a short pedicel or
stalk. The eyes are simple, vary in number, and are grouped together or separated
across the head. After hatching from eggs, immature spiderlings pass through several
molts before reaching sexual maturity. The Iife span of individual spiders varies with
such factors as food supply, natural enemies, temperature, and humidity. Two common
poisonous spiders of North America are the black widow and the brown recluse.
a. Black Widow Spider, Lactrodectus mactans. The black widow spider is
known to occur in virtually every state in the US, with the exception of Alaska.
(1) The adult female is glossy black to dark brown. In the US, it is usually
completely black dorsally, although an irregular red (rarely white) stripe or pattern is
sometimes present. The characteristic crimson hourglass marking on the underside of
the abdomen (rarely, altogether absent) varies among individual spiders from the
distinct hourglass marking (see figure 1-27) to a design comprising two or more distinct
triangles or blotches, or sometimes only an irregular longitudinal area. The abdomen is
globe-shaped. The average width of the abdomen is 6 mm, and the overall length of
the female spider is about 40 mm. The abdomen of the gravid female often measures 9
to 13 mm.
Figure 1-27. Black widow spider showing hourglass
marking on center of the abdomen.
MD0170
1-22
Previous Page
