Tuesday, August 2, 2016

Orbweavers in the Woods


Traveling along narrow trails or in the woods this time of year means getting a face full of spider webs.  Watch closely and you will find that they are predominately made by orb weavers, small spiders that specialize in that particular niche between 2-6 feet off the ground.  The webs are rebuilt morning and evening, suspended from a single silk strand between branches of neighboring shrubs.  They are always perpendicular to the ground, perfect for catching small flying insects that are active during those hours. 

Spidermobile
Holding a broken branch in front of you as you walk will sweep most of the webs away but the spiders tend to cling to the stick with a sturdy strand of silk.  I use this technique on my ATV while riding on our trails through the woods.  At the end of a recent ride I had over 30 orb weavers on the sticks, the ATV and me, 25+ Arrowheads and 5 Spiny Orb Weavers. 

The most common species on the trails is the Arrowhead Spider, aka triangular orb weaver, Verrucosa arenata.  Most spider species are hard to identify, requiring close inspection of their eyes, palps, etc.  The female Arrowhead Spider is not only extremely common but quite distinctive with its sharp edged triangular abdomen colored white or yellow, its tip pointing away from the head.  The males have no distinctive characteristics, don't build webs, and are only found on the females' webs during courtship and mating.  As with other spider species, mating is frequently fatal for the male.

The Arrowhead's web is a large circular spiral, one to two feet in diameter.  It is rather loose, billowing like a sail in a slight breeze.  The female spider hangs in the center with her legs tucked in and her head up, unlike the other orb weavers which hang upside down.* 
M. gracilis - Wikimedia
Spiny Orb Weaver - Richard Hover


CD Web - Wikimedia

The Spiny Orb Weaver, Micrathena gracilis, is a common species which also builds its web in the 2-6 foot range above the ground.  It is also called the CD spider as its tightly woven circular web the size of a CD which can look like one when the light is right.

The spider's large spiky abdominal structure is disproportionate to its thorax and is frequently confused for something that it has captured and is carrying away.  Although this growth seems cumbersome and energy wasting, it apparently serves its unknown purpose.  The male has much smaller spines, is usually only seen while mating and often ends up being eaten, giving a whole new meaning to a "dinner date."
Spiny Orb Weaver female - "Does this silk make me look fat?"
In both species only the females build webs; circular structures quite different in size but identically suspended vertically from strong silk lines stretched across trails and openings between woody vegetation.  These are pathways frequented by small flying insects and incidentally, humans.  They specialize in tiny flying insects like mosquitoes, gnats and flies.  After the prey hits their sticky, closely woven silk, the spider bites it, injecting its venom which digests it internally.  The spider then returns later to suck up its predigested juices.

Other side of the branch - Spiny Orb Weaver being sucked dry by the Arrowhead Spider.
I was surprised by the Arrowhead's prey in the picture on top.  After taking the picture, I went to the other side of my spider shield and photographed the prey.  It was only when I enlarged the picture that I could identify as a Spiny Orb Weaver.  Ordinarily it would not be trapped by another spider.  In this case the two species were wildly swinging together from silk threads on a windy ride and once I stopped they came in contact and the Arrowhead won.

* More research on V. arenata's head up position and its effect on speed and web construction is at this Pubmed article. 

 Now photographing their webs has become an obsession.  Stop me before I click again.......

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