Spiders, scorpions, mites, ticks, and harvestmen are arachnids, not insects. They are arthropods: invertebrates with molting exoskeletons (external skeletons), segmented bodies, and jointed limbs.
Except in Antarctica, spiders exist worldwide. They may dwell in houses, trees, or underground. Significant features are eight legs, mostly four pairs of eyes, venomous fangs, and two body segments.
Females sometimes eat their counterparts (males are usually smaller) before, during, and after mating. To survive, some males choose mates weakened from molting. Yet, it seems cannibalism is not typical—even with black widow spiders.
Dozens to hundreds of eggs are laid in one or more silk sacs which are carried, dragged, or safeguarded in nests or webs. Mothers generally feed and carry their offspring. Females live longer; lifespan is up to two years.
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| Spiny-backed Orb Weaver - Charlie Corbeil |
Silk is emitted from spinnerets (silk-spinning organs) for weaving webs which offer protection and trap nourishment. Webs with sticky or fluffy silk ambush insects which may be numbed and wrapped in fine silk. Non-sticky silk allows resident spiders to travel in certain web areas. Silk threads provide safety lines to escape danger. Spiderlings colonize by parachuting (flying) hundreds of miles on threads.
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| Banana spider - Brett Pigon |
These arachnids, are mostly solitary, but some species build communal webs—a four-acre web in a Baltimore building housed 107 million spiders. Webs facilitate arachnid classification. For example, the orb weaver builds the spiral web, and the sheet web spider forms a sheet. At maturity, some males abandon web weaving to pursue mating. Webs may be eaten for energy. Some are rebuilt, repaired, or abandoned as cobwebs. However, certain species do not utilize webs.
