FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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EDGEWOOD’S ARTHROPODS By Paul Heiple With the possible exception of the Bay checkerspot butterfly, many people think only of the large vertebrate animals when they think of the wildlife of Edgewood. In fact, the majority of animal species in the park are insects and other arthropods. They are also the most numerous. In this first installment in a series of articles about Edgewood’s arthropods, I will begin with two species of large spiders that are now common in Edgewood’s grasslands. They are Argiope aurantia, the common garden spider, and Argiope trifasciata, the silver or transverse Argiope. The genus Argiope is one of the genera of orb web weavers, in the family Araneidae. The distinguishing feature of this genus is that the spiders weave a zigzag pattern in the center of the orb web. The purpose of this structure is not entirely known but some research indicates it confuses the prey items by making the web less visible. Another proposed explanation is that it keeps birds from going through the web. The black and yellow garden spider Argiope aurantia is a species I have known for a long time. It is found from coast to coast. In Edgewood, the black may be replaced by brown in some individuals, a variation I did not see in the eastern individuals. Full-sized females can be found as early as August 1. Nearly an inch long, this is the largest Argiope in California. The male spiders are not often seen; they are about a quarter of the length of the females and much thinner. To find them, look around the upper edges of the female’s web in early August. They can be found in their own webs close to the edge of the female’s. Eggs are encased in brown silk pouches which are round to tear shaped and located within a foot or two of the web. The silver Argiope, Argiope trifasciata, is nearly as large as the black and yellow garden spider. In Edgewood it may even appear to be larger in some specimens. Also occurring from coast to coast, this species tends to live near marshes in the east. It matures later, usually around September 1. The egg cases are light gray with one side flattened. This species tends to open up a space in the grass where the web is spun, like a ball-shaped cavity. To distinguish between these species, look at the upper surface of the rear-most body segment (the opisthosoma). On A. aurantia, a dark band (black or brown) covers the full length of the segment with four yellow spots in it and many branches of the dark band dividing the yellow area. With A. trifasciata the dark band alternates with bands of silver or yellow. Editor's note added 2/6/2002: Paul Heiple notes that a preferred common name for A. trifasciata may be Transverse Argiope. Another common name could be Banded Argiope.
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