Slingshot spiders (Theridiosoma gemmosum) don’t just passively wait for their prey to find its way into their web. Instead, they take action. These arachnids–also called ray spiders–pull the center of their flat web back to form a cone with themselves at the tip of the cone. They then keep the net in place by holding on to a taut anchor tread and then release the thread to let the web fly. The spider then catapults forward when an insect passes by to trap the victim in a sticky spiral.
However, in 2021, biophysicists Saad Bahmla from Georgia Institute of Technology and Todd Blackledge from the University of Akron and their colleagues discovered that they could trick the spiders into releasing these ballistic nets with a simple snap of their fingers.
Now, Blackedge and University of Akron PhD student Sarah Han believe that they have proved that slingshot spiders can listen for approaching insects and wait for the victims to be in range before releasing the web and catapulting towards their next snack. The findings are detailed in a study published December 4 in the Journal of Experimental Biology.
[Related: Spider conversations decoded with the help of machine learning and contact microphones.]
For this new study, Han spent hours at a local riverbanks peeking into crevices and rocks for the distinctive cone-shaped webs with a spider perched at the tip.
“Slingshot spiders are really tiny, so they can be quite hard to find,” Han said in a statement. “It does take some time to develop the eye for them.
Han brought the spiders back to the lab and set up some twigs for them to build webs on. She then went searching for some preferred spider snacks–mosquitoes and flies. Back in the lab, she attached individual insects–with their wings free so that they could flap and make noise–to strips of black paper and weaved them close to the cone-shaped spider webs while filming.
The spiders let their webs fly when the flapping mosquitoes were nearby. However, a closer look at the footage revealed that the insects never touched the spider webs with their protruding front legs. Instead, the slingshot spiders were capable of launching the webs before the mosquito even touched it.
Han then used a tuning fork that was set to the tone that is produced by a flies’ whining wings. When she placed it in front of the web, the arachnids still released their webs. The team believes that the spiders must have been listening for the approaching insects and let their webs loose once the mosquitoes were close enough, but before it landed into it. The spiders could be using sound-sensitive hairs on their legs to listen closely for the approaching insects.
To find out just how fast these webs fly once the spiders let them go, Han plotted each spider’s trajectory as they rode the web while it ripped forward. She calculated that the webs can reach speeds of nearly one meter-per second (3.2 feet-per-second) to intercept a mosquito within 38 milliseconds. The web shoots out far too quickly for an insect to make an escape.
Han also noticed that the spiders were 76 percent more likely to release their web cones when the mosquito was in front of the web. It only tried to release the web when the mosquito was behind it 29 percent of the time. The team believes that the spiders could compare how they perceive sound transmitted through the web to their bodies with the sound vibrations that are carried through the air to their legs. This could tell them whether an insect is in front of or behind their web and help avoid a costly misfire.
