City Creek Nature Notes – Salt Lake City

November 5, 2016

November 5th

Filed under: Orb Weaver Spider, Uncategorized — canopus56 @ 9:21 pm

A Cat’s Face on the Road

4:30 p.m. Along the road at mile 0.4, a Halloween phantasmic is crawling across the road: a spider with yellow-orange orb-shaped shell on its back that is suggestive of a cat’s face. It is a female Cat’s face orb weaver (Araneus gemmoides). My insect guide states that they are commonly seen in late October as they migrate seeking a place to hibernate for the winter. This common western orb spider looks like a crab, and a mark of its primitive evolutionary history are the hairs that cover its leg. The cat’s face spider weaves a circular web that is held in place by radial scaffolding. In the canyon, webs of the orb spider can be seen during the summer strung between thistles below the red bridge near mile 0.9. Although harmless to people, they are a fearsome predator of flying insects that feeds on wasps and bees captured in its web.

In 2007, evolutionary biologists Vollrath at Oxford and Selden at Kansas University suggested an alternative theory that predatory spiders like the orb weaver were a driving force in the evolution of flying insects because spiders and flying insects began to exponentially diversify into many new species before the arrival of flowering plants. The current generally accepted theory is that flowering plants, which began to dominate forests about 145 million years ago, were the primary driver of the evolution of wasps and bees. Ants evolved into flying insects in order to exploit a new food source: the nectar of flowering plants. According to the new theory proposed by Vollrath and Selden, the oldest spiders were ground dwelling when they first learned to spin webs, as some spiders in the canyon still do today. In response, ants sprouted wings, learned to fly, and evolved into wasps in order to avoid the horizontal webs of ground spiders. Orb spiders responded by chasing the wasps into the sky. They learned to climb thistles and other plants and to weave their vertically placed webs. Later the wasps co-evolved into bees. Vollrath and Selden acknowledge that this is an evolutionary “just so” story. Spider webs are ephemeral objects that disappear after a few days and they are not preserved in fossils. The record in the rocks is too sparse to know when the first spiders began to spin ground webs or when they took to spinning vertical webs strung between plants.


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