Spiders are a byword for industry. These diligent little engineers constantly tear down their webs and rebuild them in an effort to snare a snack. However, despite their ingenuity some spiders go hungry for days while waiting, so what effect does starvation have on a web's composition? Mark Townley explains that although a web's structure is largely derived from silk,40-70% of an orb-web's mass is composed of the low-molecular-mass compounds(LMM) that contribute to the web's adhesive coating. Edward Tillinghast and Townley decided to analyse the LMM components in orb-web glue to find out which glue compounds the spiders synthesize from scratch and what happens to the adhesive when spiders go without(p. 1463).
Knowing that labelled carbon from radioactive glucose would be incorporated in LMM compounds that the spiders synthesized, Townley offered two species of Argiope spiders a sip of radioactive glucose solution before collecting their webs to find out which LMMs were home made and which supplied by the diet. Having washed the adhesive from the webs, Townley isolated individual LMM components by electrophoresis before identifying them with NMR and found that the spiders were producing radioactive GABamide, glycine and alanine; the spiders were synthesising these compounds. But choline and glycine betaine remained unaffected by the arachnid's hot glucose drink; the spiders derived these compounds directly from their diet.
Keen to discover which adhesive components were most affected when the spiders went hungry, Townley and Tillinghast collected Argiope and Araneus spiders and divided them into two groups, fed and unfed, to compare the effects of starvation on the web's adhesive composition. But spider physiology seemed intent on confounding Townley's analysis. Townley explains that the fed spiders continued with normal physiological functions such as moulting and egg production, but most of these activities were severely reduced or even abolished in the starving spiders, making it tricky to isolate the effects of starvation from these routine physiological demands.
Collecting webs from both groups of spiders over the course of several weeks, Townley patiently analysed the adhesive's components from the vanishingly small samples. Teaming up with plant statistician Christopher Neefus, to identify consistent trends in the glue's changing composition,Townley found that the proportion of synthesised compounds in the adhesive,such as GABamide and glycine, increased, and the proportion of diet-derived compounds decreased, as the spiders became hungrier. And when Townley compared the fed spiders' adhesive composition with that of the starved spiders, both sets of spiders produced similar trends; the fed spiders also rapidly lost components derived from their diet while enriching the self-synthesised materials. Why was the glue composition varying in similar ways, even though half of the spiders were going hungry while the rest were well fed?
Townley suspects that several factors account for the similarity. He suggests that the fed spiders invested the surplus from their diet in activities that starved spiders avoid. One other factor also affected both groups equally; all of the spiders suffered from losing their webs. He explains that spiders constantly recycle their webs, devouring the old before constructing new ones. By taking away the resources invested in the web,Townley suspects that he was depriving the spiders of essential adhesive components that the arachnids normally recycle.
