Spider Crossing

Jeff Mitton

Natural Selections (appeared Dec 23, 2005 in the Daily Camera)

            At the end of the day, I was driving east in the Guadalupe Mountains of New Mexico, and I had the road to myself.  The sun was low, and at my back, so visibility was good. As I drove along, I saw black smears on the road, emanating from coarse debris. I wondered what the smears were. Then the answer appeared in the distance, walking slowly across the road: tarantulas. The black smears on the road were tarantula road kills.

            I downshifted, and stopped at the nearest tarantula. I approached tentatively on foot, for I did not know whether it would charge, or flee, or assume a defensive posture. But it ignored me, and continued, slowly and purposefully, across the road. I put my hand on the pavement, several feet in front of the spider, directly in its path. The tarantula did not alter its speed or its course. I withdrew my hand just in time to keep the tarantula from marching over it.

            The spider continued across the road and into the grassland. Gazing down the road, I could see more tarantulas, perhaps a dozen, crossing with the same slow-motion determination.

The Texas brown tarantula, Aphonopelma hentzi, is an arresting sight, for it is big and hairy. As it trundles along, it is six to eight inches across and over three inches tall.

            Spiders are often viewed with dread (arachnophobia is the fear of spiders), for their body plans are so different from the familiar birds and mammals that many people consider attractive. The body of a spider has two parts, a cephalothorax in front, and an abdomen behind. Eight legs are attached to the cephalothorax. Eight eyes, two in the center and three on each side, are tightly clustered on the top of the cephalothorax. Below and in front of the eyes are four mouthparts.  Tarantulas have two prominent chelicerae tipped with poisonous fangs. In most spiders, the chelicerae close from the sides, but in the more ancient spiders, including tarantulas, the chelicerae close by moving vertically. Pedipalps are mouthparts that look like legs but are used like antennae to sense objects that the tarantula encounters. Males also use their distinctively large pedipalps to transfer a packet of sperm to a receptive female.  The abdomen is larger than the cephalothorax, and it contains the spinnerets to weave a blanket of silk for baby tarantulas. Like cats, tarantulas have retractable claws.

            The Texas brown tarantula is predominantly brown, but it has many shades. The legs are dark brown, furry with grey hairs. The cephalothorax is light tan. The abdomen is dark brown, with long, reddish-brown hairs.

            Tarantulas do not weave webs, but wait in their burrows for a cricket or bug or grasshopper to venture near. They pounce on it, injecting venom from with their fangs. They inject digestive enzymes into their immobilized prey, wait for the innards to liquefy, and suck it dry.

            Tarantula venom is no more dangerous to most humans than bee or wasp venom, though some people are allergic to tarantula venom.

            Tarantulas defend themselves by plucking urticating hairs from their abdomen and hurling them at threatening creatures. The hairs are sharp and barbed, and cannot be pulled out. Urticating hairs cause intense itching and burning.

            The most common predator of tarantulas is the tarantula hawk, a three-inch long wasp with a metallic blue-black body and orange wings.

            Males live about eight years, but females can live three times as long.

            Why do the tarantulas cross the road? The females are coy; they wait in their burrows, wafting inviting pheromones from their silken blankets. Like males of many species, male tarantulas know that the grass is greener on the other side. Convinced that more females wait on the other side, the males hit the road.

 Late in the day in August and September, male Texas brown tarantulas, Aphonopelma hentzi, migrate across roads in search of receptive females

Photo by Jeff Mitton

This Texas brown tarantula is looking right at you--its eight eyes are clustered in the small, prominent structure on the tan cephalothorax. Directly below the eyes are the pedipalps--they look like a pair of small legs, but they are actually mouthparts.

Photo by Jeff Mitton