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Exploration of the Month

June 1999


Recently, I found a spider in my basement. She was hunched up over her egg sac, but not moving. I wondered what a spider really looked like up close so this month we take a closer look at spiders.

Nursery Spider Nursery Spider
View looking downView from underneath

Adult spiders range in size from less than a millimeter to over 10 inches (that's counting the leg span). Most are dark in color, often brown, although this summer I saw a bright yellow Crab Spider. - can you think why it might be helpful for the spider to be yellow? Some spiders, especially the few that are active during the day, are wonderfully camouflaged, some as pieces of straw or bird droppings, others resemble ants or caterpillars.

Spiders have two main body divisions, the head (cephalothorax) and the abdomen, connected by a slender waist and eight legs. Insects have three body divisions -head, thorax and abdomen -and only six legs. Also unlike insects, spiders have no antennae. The first image below is the cephalothorax, with the top of the legs just visible.

Body of spider Spider eyes Higher magnification

Whereas many insects have compound eyes arachnids (the scientific name for all spiders) have simple eyes. Some have two eyes, others eight, the in-betweens either four or six, except for the ones who have none--like those who live in pitch-dark caves and have no use for them. Spider eyesight serves mainly to distinguish large movements and tell light from dark. The second two images above shows the spider eyes. Notice that they are not all the same size. How many can you count?

Spider legs are covered in thousands of tactile hairs, or setae. Some are hollow and detect chemicals; others vibrate in a special pit to detect movements of air currents; still others detect mechanical movement or even moisture.

Depending on its size, a spider typically molts (sheds its cuticle) six to eight times between the time it leaves the egg sac and when it reaches adulthood. Molting is a dangerous process. The spider is vulnerable for as much as a whole day, until the new exoskeleton hardens. If a leg is broken off as the spider develops, a new but shorter leg generally appears at the next molt. Spiders can live anywhere from less than a year to more than twenty.


Spider leg joint Spider knee joint Spider foot

P.S. The yellow color of the Crab spider helps it hide on yellow flower petals to catch its prey.



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Last Update: 9/8/99