Spiders: It’s all about the glue

Many of us are familiar with spider webs in our gardens. You may have even heard that their silk, which makes up their webs, is among the strongest materials found on Earth. But we never think about how the web is actually covered in tiny sticky droplets (Figure 1).  These droplets of glue have been the fascination of Dr. Sarah Stellwagen, a scientist at the Army Research Laboratory.

Dr. Stellwagen has used several properties of these droplets to characterize them and measure their performance. Each droplet has a core of glycoprotein, a protein with sugar attachments, and an outer aqueous coating (Figure 2).  She uses measures such as the size of the droplets as well the ratio of glycoprotein to aqueous coating. She also measures their toughness through the angle of thread deflection created when sticking a probe to the droplet and pulling the silk (Figure 3). The angle is then correlated to tension, which can be used to calculate toughness.  Using these characters and the knowledge that humidity affects the droplets, Dr. Stellwagen began to wonder how other environmental conditions such as temperature, and ultraviolet B (UVB) radiation could change the glue droplets’ performance.

Stellwagen and colleagues (2014) knew that temperature and humidity fluctuate on a daily basis. Temperature tends to be the lowest at night when humidity is high and the highest in the afternoon when humidity is low. The inherent hygroscopic quality (ability to attract and hold water) of the glue means that it swells with increased humidity, which makes it stretch farther but reduces the adhesion or stickiness, depending on the species and its habitat. Lowering temperature causes the viscoelastic protein to become stiffer, similar to a jar of refrigerated honey. She hypothesized that the effects of temperature and humidity are working counteractively to each other to stabilize droplet function. Dr. Stellwagen measured droplet toughness in Argiope aurantia, an orb weaving spider, under three different temperature and humidity conditions and found that indeed temperature was acting as moderator to changes in humidity, and vice versa. Afternoon conditions of high heat and low humidity resulted in the best glue. Interestingly, afternoon is when these spiders capture the bulk of their prey, which corresponds well with the glue being optimally sticky.

Many orb web building species prefer open, sunlit areas, so in addition to temperature and humidity, their webs are exposed to ultraviolet radiation, which can cause molecular damage. This inspired Dr. Stellwagen and colleagues to examine the impact of UVB radiation on the glycoprotein glue of orb-weaving spider capture thread (Stellwagen et al. 2015). They hypothesized that species which spin webs in sunny habitats will produce glue droplets that are less susceptible to UVB damage than those that spin webs in shaded forest habitats or those by nocturnal species. To test this idea, they collected thread samples built by five different species that are representative of five distinct habitats with varying degrees of sunlight (full sun, partial shade, mostly shade, full shade and nocturnal). Fresh threads were exposed to two treatments: UVB energy equivalent to a full day of summer sun and three times that amount. Droplets were also placed in a dark chamber allowing evaluation of post-production changes and aging (Figure 4).

The authors found that some material properties of glycoprotein web glue, such as the ability to absorb moisture from the air, were unaltered by the UVB treatments. However, for full shade species, the amount of energy the droplets could dissipate was reduced after UVB exposure. Thus as predicted, species that spin webs in sunlit habitats produce glue droplets that are better adapted to cope with UVB radiation and are therefore less vulnerable to its damaging effects.

 Dr. Stellwagen’s research on the various environmental factors that affect spider glycoprotein glue; namely temperature, humidity and UVB radiation, not only increases our understanding of spider biology but also holds promise for bioengineering efforts to mimic the amazing adhesive properties of spider glue.
 
References
 
Stellwagen, S. D., Opell, B. D. and Short, K. G. (2014). Temperature mediates the effect of humidity on the viscoelasticity of glycoprotein glue within the droplets of an orb-weaving spider's prey capture threads. The Journal of Experimental Biology 217, 1563-1569.
 
Stellwagen, S. D., Opell, B. D., & Clouse, M. E. (2015). The impact of UVB radiation on the glycoprotein glue of orb-weaving spider capture thread. The Journal of experimental biology, 218(17), 2675-2684.
 
 
About the authors:
 
Jessica Grant is a master’s student in Dr. Bill Lamp’s lab. Her work looks at the cold tolerance and phenology of kudzu bugs and how that impacts the range expansion and integrated pest management of the bug. For more information visit mdkudzubug.org
 
Lisa Kuder is a PhD student in Dennis vanEngelsdorp’s Bee Lab. Working with the MD State Highway Administration she is evaluating sustainable pathways for supporting highway rights-of-way pollinator initiatives. Click here for details.