Nephila Golden Orb Weaver

My interests in magnetic resonance imaging and spectroscopy of small live critters brought me to become acquainted with the Golden Orb Weaver spider.  There are numerous species of these large and beautiful spiders in the various warm, humid corners of the world, in this case it was specifically Nephila Clavipes, which is found in the American Southeast.  Folks in Texas, Florida and Louisiana know them well as ‘Banana Spiders’, presumably because they are large and yellow.  They make enormous round webs of particularly strong silk, and these webs are referred to as ‘golden’ because they contain silk fibers that are pale yellow in color.

I found out about these guys because of a lab at Arizona State University that had the same kind of high field NMR spectrometer I was using for MRI, which they were using to investigate the structural properties of spider silk.  Further they were interested in the process of imaging the spider itself while ‘silking’ (making and secreting silk strands), but lacked the MRI probe for their spectrometer (mine being the only one in existence).

http://ednieuw.home.xs4all.nl/Spiders/Info/webthread.html

I went to visit their lab and had a great time playing with the large and beautiful spiders, who are not harmed by the process of collecting silk.  They are anesthetized briefly and gently restrained in an inverted position (on their back).  As they are handled a double fiber of silk trails behind them, and this silk line can be attached to a spool.  As the spider wakes up, this spool can be turned and the result is a circular loop of many-layered silk.  Snipping it in one spot gives you a strong white fiber resembling dental floss which can be easily handled, and in doing so you will quickly observe that it is strong stuff, and I would say, very aesthetically pleasing as well.

 

Infovisual.com
Under the microscope, the spider can be seen to have 8 different silk glands (as I recall), and part of the study of the silk is in determining the different material properties and structures of the multiplicity of silk the spider produces.  A great difficulty in understanding the process of silk production is that the glands are long, tapering structures apparently correlating with a progressive chemical process that starts with a liquid ‘dope’ at one end and POOF! produces an exceptionally strong and specific type of solid proteinaceous fiber at the other end.  When you dissect out one of these glands, you destroy the delicate chemical process you wish to study.  Thus the impetus to image the glands live, in an intact spider while silk is being collected, and, ideally, perform in vivo spectroscopy on the gland during the process.

I was very much in favor of this elegant application of my MRI microscope.  Unfortunately, by the time we got the whole thing set up, including shipping the equipment to Arizona and flying myself and the SMRL facilities manager out as well and staying the night in a hotel next to campus, the probe broke.  In fact within an hour of starting our work in the morning, the Y-axis gradient went dead (open circuit).  Knowing what I know now (the Y-axis gradient connection had been cobbled together in a most ugly fashion by my probe-building contractor) I would have just torn down the probe and fixed it on the spot.  At the time though we decided to scrap the whole mission, fly home, and send the probe back to the company that constructed it for me.  What a shame!

On a bio-geek tangent, however, it did get me thinking about that golden silk.  Why and how does this golden silk happen?  This beautiful and seductive spider lured me in.  It led me to cool hypothesis which I will paste below from my notes:

orb weaver golden silk hypothesis:
In examining the orb weaver, it appeared to me that the yellow markings on the abdomen are translucent patches of cuticle, with yellowish internal organs visible underneath.  My suspicion is that sunlight, shining through these patches, elicits a photochemical synthetic pathway to form the golden pigment of the characteristic yellow-tinted silk these spiders produce.  My first hypothesis regarding the golden web material was that its value to the spider is as a visual camouflage.  I am told that the spiders kept in the lab produce little or no golden silk, showing that some factor is missing in the lab environment, such as a food source.  Not all the spiders silks are tinted, nor are the tinted ones always tinted.  I suspect that the position of the translucent cuticle patches may correspond in some way to the position of the silk glands in the abdomen that make the golden silk.  The spider then sits in the center of its web for long hours in sunlight (they are native to sunny southern states such as Florida and Texas, as well as Northern Australia, Senegal, etc.), exposing these patches to sunlight for optimal photochemistry initiation.  The legs also show golden bands, and the naked cuticle areas of these bands are interspersed by black areas that are heavily bristled.  Possibly this is another way that the photochemical formation of pigment is selectively activated to effect yellow banding in the body.  Based on collected images and text found on the internet, it seems the spider’s egg sac is normally entirely golden in color, and is made in late summer to autumn, when there is a great deal of sunlight.  Potentially under this hypothesis there could be a connection between the position of translucent cuticle patches, angle of the sun, position and orientation of web design, and position and activity of the spider to maximize pigment photochemistry.  In other words, the spider may sit in the web orienting these putative solar collectors toward the angle of the sunlight in late summer to autumn.

simple expt:  expose lab spiders to broad spectrum light.

if this effects golden silk, paint over or occlude the translucent patches to see if pigment level is then affected.

if this is fruitful, obviously, there are lots of corollaries, follow up expts to determine the key light wavelength, predict and identify the pigment forming mechanisms, and so on. “

There are few things quite so elegant as a quick and decisive scientific experiment. And if it involves things of beauty, so much the better. The golden silk experiment seemed like a good one. Get some spiders, see if you can control the coloration of their silk with light.

So I ordered some spiders (found one place online to get them). They turned out to be in limited supply, and when they did come, were either dead or dying. The deal is, the retailer of terrestrial invertebrates (bugs and critters) pays someone to go out in the swamps and so forth of the Southeast and grab these hapless adult female spiders and, presumably, ship them to the retailer who is another state, who then keeps them in a greenhouse or whatever before shipping them again, to me. The spiders don’t like it much, and they are on a limited life budget to begin with. This is not some fat tarantula you can keep as a pet for many years. These critters die each year. So at their biggest and most catchable, they are already on their way out. I did however also receive one slightly golden-colored egg case. And it did in fact become a swarm of tiny spiderlings, silking like mad. Suddenly I was in the spider rearing business.
   

I built them a little structure on which to make webs, which they quickly silked up in a disorderly tangle of single silk lines, like a house being TP’d by teenagers, and then they clung together in a big mass upon it. Efforts to feed them were largely unsuccessful. I had access to a bounty of laboratory fruit flies, which were larger than the spiderlings, and it wasn’t clear that the spiderlings had much interest in capturing at eating food anyway. Within a few days their interest changed from clustering to dispersing: I took them outside in the sun on a clear day and removed the web support structure from the enclosing jar for a better look. Immediate ‘kiting’ ensued. Kiting is when a baby spider releases a long strand of silk from its abdomen, catches the wind, and takes off like an errant paraglider. I believe it was described most familiarly in Charlottes Web. It was with terror and awe that I watched one then another and another of my tiny spiderlings take flight and begin to drift away on gentle zephyrs. I didn’t want to lose them, and I didn’t want to be responsible, however unlikely, for introducing a foreign species into the Western California ecosystem, nor did I want my poor, vulnerable little charges to be lost, alone and afraid to the certain doom of a habitat that has no place for them. I was after all trying to raise test subjects for future use. So I stuffed them back into the jar as quickly as possible. After that it was basically a long and tedious process of watching them outgrow one web structure/enclosure after another, eat each other, die off, escape into the rafters of my basement, and generally fail to flourish in culture, as I struggled to keep up with their needs for space, food, warmth, and humidity, without great success. They never made it to my test enclosures fitted with different spectra of lights.

At the end of all this it was painfully clear that a likely much simpler way to do this experiment would be to go to the spiders’ habitat and simply work on them there. I think they proliferate into their full golden glory some time in the spring to early summer in places like the everglades, and in fact much of the American Southeast, not to mention many other parts of the world (Asia, Australia, Africa) so perhaps one of these days the variables will align for me to be in the right place at the right time to glue little blinders on golden spiders and see if I can whiten their webs, or not.

Bibliography:
Got Silk. – New York Times
Spider silks: recombinant synthesis, assembly, spinning, and engineering of synthetic proteins by Thomas Scheibel*
Amyloidogenic nature of spider silk by John M. Kenney, David Knight, Michael J. Wise and Fritz Vollrath
Lab Methods for Maintaining Spiders
Mechanical Differences of Major Ampullate Silk Fibers from Nephila clavipes and Argiope aurantia
MIT lab works to mimic spider silk – MIT News Office
Nephila Anatomy
Nutritional requirements for web synthesis in the tetragnathid spider Nephila clavipes
Solvent Removal during Synthetic and Nephila Fiber Spinning
Use of spider silk fibres as an innovative material in a biocompatible artificial nerve conduit
VARIATION IN THE CHEMICAL COMPOSITION OF ORB WEBS BUILT BY THE SPIDER NEPHILA CLAVIPES

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