The last time you walked through a sticky spider web, you might have brushed it off in annoyance. Building these webs, though, is no small feat. It requires precious time and energy that could be spent eating or mating, so spiders are quite picky about where they string their silk.
If you’re a spider, better real estate means better hunting; it’s all about location, location, location. Stealing someone else’s web in a prime spot might pay off, but only if you’re confident you’re much bigger than your opponent. Web-building spiders, however, have notoriously poor eyesight. So, how do they size up their rivals?
Like human noses, spiders’ hairy legs can pick up chemical cues from their environment. Researchers from Miami University of Ohio found that one species of cellar spider, Pholcus manueli, can tell a web builder’s size by smelling chemicals left on the silk. And, this sizing-up ability might give P. manueli an edge on a closely-related competitor, the long-bodied cellar spider (Pholcus phalangioides). While both are considered invasive, P. manueli is challenging its longbodied counterpart's century-long dominance in the midwestern United States.
To determine how the species differ in their chemical sensitivity, the team first had a set of “builder” spiders from both species construct webs, then traded the builder out for another, “focal” spider. They measured each spider’s size and recorded how quickly the focal spider invaded the builder’s web. To ensure the spiders’ reactions were due to chemicals signals and not simply web design or structure, researchers repeated their experiment, adding an ethanol “web washing” control step before introducing focal spiders.
They found that P. phalangioides invaded webs faster than P. manueli no matter the size of the builder. P. manueli were flexible in their strategy, though, invading webs made by larger spiders more cautiously. Because this behavior disappeared after the web wash, the researchers concluded that P. manueli was definitely exploiting chemical clues.
What does this mean for upstart P. manueli? Despite P. phalangioides’ more aggressive approach, by picking their battles carefully, P. manueli just might win the war.