Hungry Spiders and Sticky Webs

This week’s article is all about glue. Specifically, the glue spiders use to trap prey in their webs, and how spiders alter the chemical structure of their glue when they aren’t getting enough nutrients. The paper, Nutrient Deprivation Induces Property Variations in Spider Gluey Silk, was written by a team of researchers from Tunghai University in Taiwan and Akron University in the United States and published in 2014. Whereas the articles I have looked at in the past have focused on learning new and interesting aspects of spider biology and how their findings could affect our understanding of how spiders interact within their environment, the team behind this paper focused their efforts on exploring the variations in gluey silk as a way to understand biological adhesives and possibly help develop new methods of synthesizing adhesives.

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Left: Western Black Widow (Photo: Bryan E. Reynolds) Right: Golden Orb Web Spider (Photo: Ianare Sevi)

The team of scientists looked at two different spiders: the golden orb web spider Nephila clavipes, and the western black widow Latrodectus hesperus, two species that spin very different webs. Golden orb web spiders spin (believe it or not) orb webs, with a main scaffold of structural silk and a central spiral of silk covered in droplets of glue that captures prey that happen to fly, jump, or fall into it. Western black widows spin tangle webs, wild masses of silk with little to no organization (sometimes called cobwebs), that have several gumfoot threads that extend from the bottom of the web to the ground or another nearby surface. These gumfoot threads have droplets of glue along them, along with a large droplet at the base that holds the thread under tension. When a prey item disturbs the glue holding the silk (usually by walking into it) the thread acts like a spring trap, snapping back towards the web and taking the prey with it.

Samples of these spiders were captured and allowed to spin webs in a lab at Akron University for several days while being fed a sugar solution (to make sure the spiders had completely digested whatever their last meals were prior to being caught). The researchers collected samples of gluey silk from each of the spider webs before dividing each species of spider into two groups: one which received a sugar solution with protein added to it, and one group which received just the sugar solution. After several days of feeding the spiders protein rich or protein poor solutions, the scientists took more gluey silk samples.

The properties of the gluey silk samples were tested two different ways. First, they looked at the size and spacing of the droplets using a polarized light microscope. in doing so, the research team found two significant differences in the glue droplets of the golden orb web spiders: the droplets of the protein-deprived spiders were significantly larger and had a yellow tint. Western black widow glue droplets did not have any significant differences in terms of appearance or size between the two groups.

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Photographs of glue droplets produced before treatment (A,D), by protein-deprived spiders (B,E), and by protein fed spiders (C,F). A, B, and C were produced by golden orb web spiders, while D, E, and F were produced by western black widows. Note the yellow coloration of droplet B. (Photo: Blamires et al. 2014)

The second method used to examine the properties of the glue droplets was measuring the stickiness of the glue by placing the silk on a piece of glass and using a machine that measures force to slowly pull on the silk until it was removed from the glass. This method revealed that the protein-deprived groups of both spider species had significantly stickier webs (i.e. they required more force to remove the silk from the glass) than their protein-fed counterparts.

While the research team was not able to analyze the chemical composition of the glue droplets, they note that another research group were looking at the composition of various orb web glues and had found that differences between glues consisted mainly of variations in salt content, rather than changes in the protein content. The researchers note that changes in salt composition could explain the differences in color (different compositions may have different optical properties) and droplet size (salts help the droplets take in water from the atmosphere, increasing the water content and size) found in the golden orb web spider glue. They suggest that similar changes in salt content are occurring in the gumfoot threads of the western black widows without affecting the size or coloration of the droplets.

From an ecological perspective, these findings are very important, as they suggest that these spiders are altering the properties of their webs to increase their ability to capture prey when they are protein starved, although the researchers note that it is important to test whether these changes will translate to increased prey retention in webs. Testing this may seem like a waste of time, but consider the possible outcomes and what those outcomes mean in for the spiders’ survivability. If prey retention is increased, why would the spiders only produce these stickier droplets when they are starving? Are they using difficult to find nutrients as a last-ditch effort to survive or does it take more energy to produce stickier droplets? Conversely, if prey retention isn’t increased, what are the spiders accomplishing by altering the properties of their glue droplets? Are they conserving nutrients that are usually fairly expendable? If so, which ones?

This experiment resulted in more questions than answers, but in the end, that’s what scientists strive for. We live for the thrill of learning something no one else knows, and we get joy from spreading that knowledge around.

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