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Tuesday, July 23, 2013

Spiders on the Prowl

Green Lynx Spider, Peucetia viridans 
An often seen spider in our suburban yards and gardens is the black and yellow garden spider (Argiope aurantia).   True to its common name, this spider has distinctive black and yellow (and sometimes orange) markings on its abdomen and a mostly white area behind its head.  With a fairly rotund body of 1 ½ inches in length, the females of this species are twice as large as the males (common for most spiders), and can have colorful banding on the legs.  These spiders are active during the summer months, and tend to be somewhat local, staying in one place throughout much of their lifetime.  Like other members of the Argiope family, they are considered harmless to humans.  

Black and Yellow Garden Spider, Argiope aurantia
These garden spiders have three claws on each foot, unlike most spiders that have only two claws, and the extra claw helps them to spin complicated webs.  Many times these webs are built in areas adjacent to open sunny areas, often two to eight feet off the ground.  Most distinctively, the circular part of their webs are up to two feet in diameter, with a dense zigzag of silk, known as a stabilimentum, in the center.  While the purpose of this structure is disputed, it helps this spider earn its other common name, the writing spider.  It is most often thought that the stabilimentum might warn birds of the presence of the web, and only those spiders that are active during the day construct these types of patterns in their webs. 

Black and yellow garden spiders breed once a year. The males roam in search of a female, building a small web near or actually in the female's web, then court the females by plucking strands on her web.  When the male approaches the female, he often has a safety drop line ready, in case she attacks him. After mating, the male dies, and is sometimes then eaten by the female.  The female will then lay her eggs, cover them with a sheet of silk, roll them into a sac, and hang them from the center of her web, where she spends most of her time. She guards the eggs against predation as long as she is able, but as the weather cools, she becomes more frail, and dies around the time of the first hard frost.  Come spring, the tiny young spiders exit the sac and disperse, often on a strand of silk carried by the wind.

Another common but startling-looking spider is the spiny-backed orbweaver, or spiny orb-weaver (Gasteracantha cancriformis).  Its shell is shaped like a crab shell, wide, flat and variably red, white, orange, or yellow with dark oval spots, rimmed by six red or orange spines.  The males lack these distinctive spines, having only four or five stubby dark projections, and are two-thirds smaller than the females.  Their Latin name comes from ‘cancer’ meaning ‘crab’ and ‘forma’ meaning ‘shape, form, or appearance.’

Spiny Orb-Weaver, Gasteracantha cancriformis
Also called a jewel or jewel box spider, this spider ranges across the southern half of the United States and is found year-round in woodland edges and shrubby areas of Texas.  It usually adds decorations or little tufts of silk to its web, possibly to warn birds and other animals of the web’s location.  A short-lived spider, its lifespan lasts until reproduction in the spring following their birth.  In fact, the males die only six days after mating with a female.

Often found feeding on flowers, the Green Lynx Spider (Peucetia viridans) is a bright green spider and the largest lynx spider in North America. The species name is derived from the Latin viridis, meaning ‘green’, and is its signature characteristic.  Its abdomen has a series of cream-colored chevrons along its length, with white stripes accented by russet margins.  Long, thin legs are pale green to yellow, and are covered with long black spines and spots.  Gravid females are able to change color to fit their background, and depend on this ability in September and October, to help them defend their egg sac filled with bright orange eggs, from predation.  

Green Lynx Spider, Peucetia viridans 
Active during late spring and summer in a wide variety of habitats, the Green Lynx spider does not spin a web, but hunts for moths and other small insects among low shrubs and plants.  As such, this spider is of great interest for its use in agricultural pest management, but unfortunately also preys on beneficial insects such as honey bees.  Very seldom does this spider bite humans, and its bite is harmless. 

Relatively common throughout Texas, tarantulas are our heaviest and largest spiders.  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 our state.  Identification of species is difficult, however, and is often performed only on mature males under a microscope.

Tarantula crossing the road
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. 

Tarantula visiting our back porch
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, with the males actively wandering in large numbers in late summer, apparently seeking out females.  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.