Search Nature Watch

Sunday, August 9, 2015

Traveling Tarantulas

Relatively common throughout Texas, the Tarantula (Aphonopelma sp.) is Texas’ heaviest and largest spider.  Typically, the head and legs are dark brown, and the abdomen is brownish-black. Coloration varies between individuals as well as between the 14 different species found in Texas. Identification of individual species is difficult, however, and is often performed only on mature males under a microscope.

Tarantulas are typically found in grasslands and semi-open areas, and use burrows, natural cavities under stones or fallen logs, spaces under loose tree bark, and even old rodent holes as shelters.  They are also capable of digging their own burrows, and often line them with webbing, placing a few strands across the front to help detect passing prey.  Laying several hundred eggs in a hammock-like web constructed inside the burrow, females will guard them until they hatch. Females have lived in captivity for over 25 years, while males rarely live over two or three months after reaching maturity.   
Like many animals, tarantulas molt their exoskeletons several times as they grow.  The skin on the hard upper shell and abdomen splits, and the tarantula begins the process of squeezing through the opening.  Most of the time, tarantulas molt while positioned on their backs, twitching, stretching, and kicking until the entire exoskeleton has been cast off.  After they have wriggled free of their old skin, this discarded exoskeleton is a perfect replica of the spider, minus its head and fangs. 

Other insects such as crickets, beetles, grasshoppers, cicadas, and caterpillars form the basic diet of the tarantula.  They inject their prey with a poison when they bite, which liquefies the prey’s insides, making it easier to ingest.  While they can climb, they are usually restricted to the ground, where the majority of their prey is found.  The hairiness and large size of tarantulas often evokes concern, but the bites of Texas species are not serious to humans.  Tarantulas maneuver quickly to face whatever disturbs them, often raising up on their hind legs and stretching out their front legs in a threatening posture.  They have also been observed rapidly brushing the top of their abdomen with their hind legs to dislodge hairs that can be used to irritate the attacker’s eyes or skin.

For a few weeks late in the summer or early fall, one of the most spectacular spider events occurs in Texas.  Not well understood, this phenomenon is often called a migration, but it may be related more to mating rather than seasonally motivated movement.  Males actively wander to seek out females, and can travel 50 miles in search of a mate.  Populations seem to follow a boom and bust cycle, depending on weather patterns and the availability of food, but a good year can be a sight to behold if summer rains have been plentiful.  While the males are out searching, females wait in their burrows for a suitor to appear.  Larger and more robust, the female does not always accept any male that comes along, and will kill and eat males that are deemed unsuitable. 

Tarantula Hawk
As formidable as they may seem, tarantulas are not without their own enemies.  In fact, in late spring and early summer they are routinely hunted by female Tarantula Hawks as food for their larvae.  Belonging to a group of spider wasps in the genera Pepis and Hemipepsis, tarantula hawks are large, 2-inch long wasps with iridescent blue-black bodies and bright, rust-colored wings.  This vivid color combination is a form of aposematism or warning coloration, a type of advertising signal to both predator and prey that these species are potentially harmful.  These wasps have the ability to deliver a powerful sting, and their long legs have hooked claws for grappling their victims.  

Flying low over the ground, the female tarantula hawk will find a tarantula and sting it, which paralyzes the spider but does not kill it.  She then drags the inert tarantula into her burrow or transports it to a specially prepared nest, where she lays a single egg on the spider’s abdomen, then seals the opening to the burrow as she leaves.  When the wasp larva hatches, it creates a small hole and enters the spider’s abdomen, where it feeds voraciously, avoiding vital organs to keep the spider alive as long as possible.  After several weeks the spider dies, the larva pupates, and then it emerges from the spider’s abdomen to continue its lifecycle.