2011-07-29 / Columnists

Drawing On Science

Orb Weaver Spider
Commentary By Stephen S. Yaeger

Orb Weaver (Barn) spider. Orb Weaver (Barn) spider. Once an insect is ensnarled in the web, the spider will rush down toward the prey and investigate it as a possible candidate for a meal. If the prey is not satisfactory it will be cut from the web or ignored. If it’s deemed delectable she will proceed to wrap it in a cocoon of silk to immobilize it. Once she has completed the wrapping she will bite and inject venom into the prey. The venom consists of proteins, amines and polypeptides some of which disrupts the prey’s nervous system and paralyzes it. In other cases molecules may kill cells (necrosis) outright leading to death. When the prey is immobilized the spider releases enzymes from its mouth into the victim dissolving (digesting) the contents inside. Once the contents are liquefied the spider proceeds to suck out its food.

In the cases where the spider ingests food larger than its own body volume, the digested proteins are stored in a special place. The spider can live off this food for many weeks. Waste material is converted to crystals and stored in specialized cells called guanocytes. There are reports of some species of large orb weavers able to trap and dine on hummingbirds or frogs. I have photographs of an orb weaver feeding on a cicada (about twice its size) unlucky enough to become trapped in its web. No Nathans’ hot dog contest eater could match this; eating enough hot dogs equal to or twice the size of the contestant.

Most everyone who’s come into contact with an orb weaver will have backed away in disgust and fright. But they’re very docile and, in fact, will run from any threat a person makes; even drop off the web with a line of silk. They are not dangerous under normal circumstances and keep in mind that they do eat insect pests and so are beneficial to us.

If gently touched they’ll rarely bite and I’ve been lucky so far. However, if I’d try to pick one up, the spider would then feel threatened and, well, you can guess the result. The bite is likened to a bee sting and for most people not lifethreatening. However, one would be better off just watching an orb weaver do its thing … maybe even photographing it and its beautiful web (especially in the morning with dew droplets gracing the silk threads).

The male spends most of his time seeking a mate and is generally on the move in its quest for love. It’s much smaller than the female, which has a large abdomen and small head, and he rarely constructs a web. He’s also rarely seen for the same reason. That’s why, throughout this column I’ve been referring to the spider as “she.” The female waits on or near her web for the arrival of a suitable mate. After mating, the male typically leaves the web and dies or immediately becomes his ex-lover’s next meal – having the same result. In the fall the female lays clutches of several hundred eggs, all encased in an egg-sac, then dies with the first frost. The eggs over-winter even under freezing conditions; in the spring they hatch and a new generation does its thing.

Probably the most famous Orb Weaver is named Charlotte. What’s she famous for?

Questions? E-mail Steve: Drawing onscience@aol.com.

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