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You Are What You Weave

The giant wood spider

The giant wood spider (Nephila pilipes) builds different webs depending on whether it is attempting to trap crickets or flies.

By Sean Blamires

A spider web’s architecture and the properties of its silk are a consequence of environmental conditions and the nutrients that the spider extracts from its prey.

Spider webs are intricate constructions made from silk. Their primary function is to ensnare flying insects for the spiders to consume.

The architecture and properties of spider webs and the silks from which they are made have fascinated researchers for centuries. Nevertheless, only recently have we been able to look closely enough at webs and their constituent silks to begin to untangle the interplay between spider evolution, nutritional physiology, web architecture and silk biochemistry and biophysics.

The 40,000 or so extant species of spiders use silk in a multitude of ways to capture and consume their invertebrate prey. The most well-known is the spider web, of which the most readily recognisable is the orb web. Until relatively recently the orb web was thought to be the pinnacle of spider web evolution. However, recent molecular evidence shows that this is not the case. In fact, it appears that many lineages of spiders have repeatedly lost and reinvented the orb web over evolutionary time.

The orb web is the only animal-built structure capable of catching aerial insects in flight. At face value this might not seem much of a feat, but if we scale up the materials to sizes we are more familiar with, it is the equivalent of manufacturing a trap capable of stopping an aeroplane in full flight using highly elastic threads with a 12mm diameter.

Furthermore, spiders produce the raw material for their webs under environmentally benign conditions using only water as the solvent. Accordingly, the development of materials that mimic the physical properties of spider silk is considered the “holy grail” of bioengineering.

To construct an orb web, spiders produce five types of silk, each named after the gland from which it is derived. They are: the major ampullate silk, which is extensible and extremely strong; the minor ampullate silk; the highly extensible flagelliform silk, which is deployed as the axial threads holding the orb web’s capture spirals; the viscous aggregate glue that coats the flagelliform threads; and the pyriform silk cement, which is used to attach the threads to each other and to the substrate on which the web is built.

But how do these silks interact with the environment to enable the orb web to perform the feats it does? And what physiological costs do the spiders encounter when they produce their remarkable silks? My colleagues and I have recently examined these questions by examining how factors such as wind and diet interact with the properties of the different silks to affect web properties.

In all of our experiments we housed spiders in frames in which they could build orb webs, and then manipulated their environment over about 21 days. We then counted the number of radial threads or “spokes” of the orb, the number of spiral threads intercepting a single radial along the four cardinal directions (up, down, left and right), and we measured the upper, lower, left and right hub and web radius . We then used these measurements to calculate the web capture area, the spiral and total silk lengths, and the average width between spiral threads (the “mesh height”)(see figure).

We also used a tensile testing machine to determine the stickiness of the spiral threads, and measured the thread diameter, the glue droplet volume and surface area, and counted the number of droplets per millimetre of thread.

We also collected major ampullate silk from the spider’s spinnerets and used a tensile testing machine to pull the thread until it broke to determine its strength, elasticity, toughness and stiffness.

For testing the influence of wind on spider webs and silks we used the orb web spider Cyclosa mulmeinensis, which aggregates in large numbers at extremely windy locations on Orchard Island, Taiwan.

We found that, when exposed to strong wind, C. mulmeinensis builds webs with smaller capture areas, shorter and more widely spaced capture spirals and larger glue droplets. These results suggest that the spiders use less silk overall in strong wind. We expect that its larger glue droplets are a buffer against spiral thread dehydration.

At the same time, our examination of the tensile properties of the major ampullate silk of C. mulmeinensis showed that spiders exposed to strong wind produce stronger, more elastic and tougher silk.

These results confirm that spiders use wind as a cue to adjust the properties of their silks and webs to maximise the efficiency of the orb web as a trap in windy conditions.

A simultaneous increase in strength and extensibility is difficult to achieve when producing synthetic materials. We therefore expect that the process by which C. mulmeinensis changes its silk properties in wind will be extremely interesting to bioengineers.

Next we examined the interaction between the spiders’ diet and its silk and web properties using the giant wood spider, Nephila pilipes. The reason for choosing this species is that earlier research found that when fed different prey (e.g. crickets as opposed to flies) they built webs that differed in architecture and produced major ampullate silk that differed in its tensile properties.

We followed up these previous studies by examining whether the nutrients contained in the different prey and the different vibratory stimuli that each prey applies to the web induces the spiders to build webs with different architectures and/or produce silks with different properties. To do this we fed N. pilipes crickets or flies that were either coupled or uncoupled to the vibratory stimuli that the spiders might expect from such prey.

We found that spiders fed on live flies built larger webs with more radii that were more compliant. However, spiders fed crickets that were coupled to fly vibrations built stickier webs.

We believe that the variations in web architecture occurred because the spiders were altering the geometry of their webs to enable them to capture flies when cricket cues were entirely absent. We suspected that the reason that the spiders focused on cricket cues to adjust their web architecture is because crickets are more nutritionally profitable prey.

We were, nevertheless, unable to explain the variations in web stickiness that we found.

Interestingly, other studies have found that wolf spiders – which chase their prey down rather than building webs to capture them – extract different nutrients from different prey in different foraging contexts (e.g. if the prey are encountered dead or alive). Moreover, we have conducted studies in collaboration with researchers at the University of Akron, USA, and found that orb web spiral threads may vary in stickiness when the spiders are deprived of protein. We considered this response to be primarily a consequence of changes in the biochemical composition of the gluey aggregate silk.

These findings prompted us to perform a further experiment hypothesising that the construction of stickier webs by N. pilipes when fed crickets while receiving fly vibratory stimuli is a consequence of them extracting different nutrients. Our results confirmed this.

These and other studies that we have undertaken, such as examining how major ampullate silk properties change in response to variations in the spiders’ diet, show that the balance of nutrients in the diet is strongly associated with the quantity and quality of silk used to build webs, with consequences for the constituent web architectures. Such silk property and web architecture co-variation may significantly affect the web’s performance, and thus influences future foraging gains.

This negative reinforcement might explain why trap building by predators is exceptionally rare. Further, it suggests that changes in the nutritional environment may have forced some evolutionary lineages of spider to repeatedly change their silk investment and/or foraging strategies.

This might explain why many spiders have perpetually changed web forms over evolutionary time.

Sean Blamires is a DECRA Postdoctoral Fellow in the Evolution & Ecology Research Centre of The University of NSW.