Crab Spiders

Crab spiders are so named because they hold their legs to the side in a crab-like fashion. They commonly are 5 mm to 10 mm long. These spiders do not spin webs, but wait in ambush on flowers and foliage for their insect prey. Crab spiders such as Misumenoides spp. are often extremely well camouflaged, blending in perfectly with the flowers they live among.

The females of this species are quite large and can match the color of various white, yellow, and orange flowers quite closely. Thomisids are best distinguished from running crab spiders (Philodromidae) by their rear legs, which are much shorter and thinner than their front legs. There are around 40 species in around 8 genera in California.

These spiders do not build webs, but wait motionless for prey to come within close range. Many species hunt on flowers for insects, such as bees, flies, and butterflies, using their fast-acting, powerful venom to quickly paralyze them. Many crab spiders are capable of camouflaging themselves to match the flower’s color by changing their color to white, yellow, or pink over several days. One of the best ways to find these spiders is to look on flowers for immobile insects situated at an odd angle. Upon closer inspection you will see the insect in the jaws of a crab spider.

Life Cycle : Although males have been found in every month except December and January, they are most common in October and November. Females, which are found as adults throughout the year, are most common from October through January. Mixed-mesophytic woodlands and citrus groves are where they are most frequently found. Males hang by single threads from the females.

Ovate egg sacs, 20 to 25 mm long by 10 to 15 mm wide, are deposited on the undersides of leaves adjacent to the female’s web from October through January. The egg mass consists of 101 to 256 eggs, with a mean of 169 (based on 15 egg masses). After the eggs are laid on a white silken sheet, they are first covered with a loose, tangled mass of fine white or yellowish silk, then several strands of dark green silk are laid along the longitudinal axis of the egg mass, followed by a net-like canopy of coarse green and yellow threads. Eggs take 11 to 13 days to hatch, then spend two to three days in a pink and white deutova stage before molting to the first instar.

After another five to seven days, the spiderlings acquire dark coloration. Spiderlings dispersed within a week later in disturbed laboratory colonies, but remained in the eggsacs an additional two to five weeks in the field. Spiderlings make tiny, inconspicious orb webs or hang from single strands. In the late summer and early fall, significant increases occur in both body and web size. The larger webs have 10 to 30 radii. The central disk where the spider rests is separated from the sticky (viscid) spirals by an open area 4 to 8 cm wide. There may be as many as 30 loops of the viscid spiral, spaced at 2 to 4 mm intervals. The catching area of the web may be 30 to 60 cm in diameter. Conspicuous tufts of silk occur on the web, primarily on the foundation lines. preventing the birds from flying into and destroying the webs. The webs may be less than 1 m to more than 6 m above ground. The spiders prey on whiteflies, flies, moths, and beetles that are caught in the webs.

Crab spiders can bite, yet never seek out humans. They will run from people or drop off of flower petals to avoid us. Their bite is not dangerous to people, and is not particularly painful, but they should not be picked up with bare hands. If you find one indoors the best way to get rid of it is to place a cup over it, slide a piece of paper under the cup and move the spider outdoors.

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