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The tubular burrow of a trapdoor spider may be six to ten inches deep (15 to 25 cm) with walls lined with a smooth, silken web. The entrance is capped with a very tight-fitting lid which is hinged on the uphill side. The spider holds the door shut with its chelicerae and then suddenly opens the door and grabs a hapless passerby. Trapdoor spiders prefer to build their nests on sunny, south-facing hillsides, preferably in adobe-type soils. Very few spiders are known to actually dig their nests in the ground. Unfortunately, the coastal sage scrub habitat in southern California where trapdoor spiders live is rapidly being converted into housing developments.

In order to grow, tarantulas must shed their exoskeleton (molt). The exoskeleton is composed of chitin, a long-chain nitrogen-containing polysaccharide. It is similar to cellulose except one hydroxyl group on each monomer is replaced with an acetyl amine group. It is covered with several different types of hairs (bristles) that are very different from mammalian hairs. New World tarantulas of the family Theraphosidae have microscopic, easily detached, urticating hairs that are released from the abdomen (opisthosoma) in the face of danger. By briskly rubbing its abdomen with its hind legs a cloud of hairs are released in the direction of a would-be predator. If the barbed hairs pierce the skin or lodge in the cornea the results can be excruciating and debilitating.

Note: Finger nails, horn, mammalian hair, wool, claws, beaks, scales, and feathers are composed of keratin, a fibrous protein with a high percentage of the amino acid cystine. Unlike other proteins, including collagen of connective tissue, tendons and cartilage, keratin is resistant to hydrolysis by most solvents (including weak acids and bases), and by proteolytic enzymes of the gastrointestinal tract. The stability of keratin is due to numerous primary disulfide (-S-S-) cross-linkages within the molecule and secondary hydrogen bonding between neighboring polypeptide chains.

The following image shows an Aphonopelma molt and urticating hairs from a tarantula I rescued from Owens Peak near Palomar College. The hairs appear more "plumose" (a botanical term) than other urticating hair images I have observed on tarantula websites. The hairs are very minute and require at least 400x magnification with a compound microscope. Apparently the effects of Aphonopelma plumose hairs on humans are not as severe as the more harpoon-shaped, barbed bristles of other genera.

The cobweb spider (Pholcus phalangioides) is a common domestic spider introduced from Europe. It is the major contributor of unsightly cobwebs under the eaves of homes and in the ceilings of rooms. When disturbed, it gyrates in its web, presumably to scare away potential predators. In San Diego County, a graveyard of dead Argentine ants in the corner of a room is evidence of a cobweb spider perched high above on the ceiling. Cobweb spiders belong to the order Araneae along with most spiders. The true "daddy long-legs" or "harvestmen" belong to the order Opiliones. In southern California, our two common genera of harvestmen are Protolophus and Leuronychus. An insect that is incorrectly called a daddy long-legs is the common crane fly (Tipula planicornis)

Mimicry: One insect (called a mimic) that is perfectly palatable to its predator resembles another insect (called the model) that is quite disagreeable to the same predator. Mimicry in which the mimic is essentially defenseless is called Batesian Mimicry. A harmless moth (Aegeria) is a Batesian mimic because it is incapable of stinging another animal, but yet it resembles the yellow jacket wasp (Vespula). To a larger predator (preying mantis or bird) this small spider is essentially defenseless. If it resembles a powerful stinging insect like the red velvet ant (a wasp), the predator might be reluctant to grap it. The problem here is that it is not a very good mimic, at least in my humble opinion. Maybe this is an example of "Imperfect Batesian Mimicry."

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