Bug o’the Week – Spinybacked Orbweaver– A Spider for Snowbirds

Greetings, BugFans,

This episode is dedicated to BugFan Tom.  Tom has a day job, practices suburban agriculture afterwards, and conducts field research by night, so the BugLady really appreciates his fitting spider studies into his off time.  He has an insatiable curiosity – and a camera – and he provided many pictures and lots of wonderful running commentary and deep thought about a cast of backyard spiders that he got to know personally (his BugFan wife suggested that he may have taken more pictures of those spiders than of his offspring’s childhood, a complaint that the BugLady’s parents would have been sympathetic to).

As Tom says, “There is a lot going on here, which is why they need eight legs.”

Ask a birder what a snowbird is and they’ll tell you it’s a Dark-eyed Junco, a bird whose idea of “flying south in winter” may only bring them from their nesting grounds in Canada to bird feeders here in God’s Country (though some do reach the Gulf Coast); 844 juncos were tallied during the Riveredge Nature Center Christmas Bird Count on Dec. 14.  Ask Chambers of Commerce of communities along the southern edge of the US what a snowbird is, and they’ll tell you that snowbirds are Northerners who fly south for the winter months.  If you’re a snowbird, check the bushes, orchards, edges, and gardens from the Deep South and the Caribbean west through southern California, and south into South America for these unique spiders; you won’t find them in Wisconsin.

They’re called Spinybacked Orbweavers (Gasteracantha cancriformis), in the orb weaver family Araneidae.  According to bugguide.net, “gaster” and “acantha” are Greek for “belly” and “thorn,” and “cancriformes” is Latin for “crab-shaped.”  Not surprisingly, they’ve picked up lots of common names – Crab-like Spiny/Crab-like Orbweaver, Jewel Box/Jewel Spider, Spiny-bellied Orbweaver, Smiley Face Spider, and Crab Spider (though they’re not related to our goldenrod crab spiders in the family Thomisidae).  Early taxonomists sometimes mistook the different color variations for separate species.

Tom’s great macro photography makes them look larger than they are – females are about a half-inch long and a little wider, and males are much smaller.  Females have six, sturdy spines on their abdomens, and males are just lumpy https://bugguide.net/node/view/1111533/bgimage.

Males court warily in fall, first hanging from her web by a thread, and then tapping in a four-tap rhythm.  He approaches her, and she responds by wrapping him in silk (her favorable response apparently starts out the same as her unfavorable response would) and, with the web vibrating, they copulate.  He staykus at her web afterwards and dies six days later.  She sits in her webs, making an egg sac that contains as many as 260 eggs, then she spins a silk pad on to the underside of a nearby leaf, and fastens the egg sac to it.  She covers it with cream-colored or yellow silk, runs a dark green silk line down the center, and finishes it with a canopy of coarse, green threads.  When the protective case is finished, she dies

The spiderlings overwinter and hatch within the silken nest, finally exiting by February (southern climes, remember?)  The BugLady found several different phenologies for Spinybacked orbweavers – they are, alternatively, present in the egg case in November through January and as spiderlings and adults for a few months after that; present as adults over winter until they reproduce in spring; or, in Florida, present all year as adults (females), or present except in December and January (males).  Tom, in Mississippi, sees eggs sacs in October and early November and doesn’t start seeing the webs until mid-spring.  The BugLady suspects that location must be the key.

Like almost all spiders, they’re (mostly) carnivorous, feeding on the small insects that fly into their webs.  The catch of the day may be eaten immediately or paralyzed, wrapped, and stored.  BugFan Tom wrote about dodging biting flies by ducking under spider webs.  The hard carapace and spines make her unappealing to predators, but her egg cases do get parasitized.

Two things about the Spinybacked orbweaver:

First – Why?  A brightly-colored spider with spines sharp enough to puncture human skin if handled roughly (her bite isn’t dangerous).  Form is function – the sturdy spines certainly discourage birds and other large predators, and one source mused that it would be tough for a solitary wasp to drag a spiny spider underground to provision its young.  But, why red, orange, yellow and white?  The “prey attraction hypothesis” says that spider prey is attracted to the bright colors, but research does not support this – spiders that were painted black captured more prey.  The bright colors are more likely “aposematic” (warning) coloration.   Some color forms may be regional.

Second – so much has been written about that wonderful web.  She spins a new one every night (Tom observed one spider at work on a new web before 7:30 AM and wrote “So I still have no idea when she dismantles the capture spiral from the previous day. I have to sleep sometime.”), and she will sometimes destroy her web in a rainstorm, gathering it into a glob that she presses to her mouth before discarding it (she may be harvesting both moisture and nutrients from it).

Here’s Tom’s description of a web in progress: “Check out the attached image. This is the right side of her web, she is moving clockwise (toward the ground on this side of the web), and has just attached a strand to the radial immediately above her and is reaching out to grab the next one, which is barely visible to her left, and toward the bottom of this shot. Notice that the radial to which she has just attach a new capture spiral strand is not straight, but is bent (stretched) downward toward her just to the right (from your perspective) of her rightmost rear leg. You can see the new length of capture spiral extending from her spinneret back to the previous radial. Now, check out the point where the radial is ‘bent’ toward the ground, as described previously. There you can see a fine strand extending vertically to the top of the slide, and downwards toward and beneath the spider, though I cannot see it clearly beyond the spider and to the bottom of the slide. This is, I believe, a pre-capture spiral (might not be the right name), and these are mainly not visible to the naked eye, or to mine at least. She moves along these and the radials as she lays new capture spiral, pulling the radial toward her, attaching the new line, then releases the radial and moves to the next.

She deposited about 40 new lines within the capture spiral, but did not continue these to the center of the web, and next attached bird visibility tufts to the otherwise bare radials extending to the center of the web.

Webs are formed at a slight angle to the ground, in vegetation or on structures (males stay near female’s webs).  At the center is a disc where she rests, and she may extend as many as 30 radii.  She leaves a gap just outside the center, and then she starts spinning the sticky “capture spiral,” working clockwise (Tom had one ambidextrous spider that sometimes worked counter-clockwise), and the capture spiral may be two feet across (half-inch spider, remember).  Females orient themselves in the web “belly up,” with their backs toward the ground, presenting a dark surface in the central disc; it’s not certain if this is for camouflage/counter-shading or for thermoregulation.

They attach little white tufts of silk to the web, and there’s much discussion about that.  Do they make the web more conspicuous (web advertisement), so larger animals can see and avoid it, like the zigzag stabilimentum in the web of a Black and yellow orb weaver/garden spider?  Or, do the tufts reflect UV rays and attract prey?

Researchers Gawryszewski and Motta painted the white tufts black and saw no change in the quantities of prey captured, and the painted webs didn’t get damaged any more or less frequently than unpainted.  They reject both hypotheses and “propose that silk tufts might be an aposematic signal.

Here’s a neat article about comparative web-spinning techniques in orb-weavers: https://www.conservation.unibas.ch/team/zschokke/pdf/zschokke1995eje.pdf

Thanks a million, Tom.

Kate Redmond, The BugLady

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