Theridiosoma gemmosum catapults its web to passing prey
A Theridiosoma gemmosum female does not wait for a mosquito to fly into her web. If she hears one approaching, she strikes her web at it, Sarah Han and colleagues write. The mosquito cannot escape.
Most female spiders that construct a web, feed on the insects that have flown into it. But Theridiosoma gemmosum takes a different approach: this spider catapults her web at a passing prey, usually a mosquito, to capture it.
If she did not, such a mosquito would escape the web. A mosquito flies with its front legs extended forward, and as soon as the legs touch a spider’s web, the mosquito reverses its flight and the spider misses her prey. But Theridiosoma gemmosum is ahead of this avoidance strategy by taking action when she hears a mosquito coming close, Sarah Han and colleagues show.
Theridiosoma gemmosum belongs to the ray or slingshot spiders (Theriodiosomatidae), small spiders that use their web as a catapult. Theridiosoma gemmosum is only a few millimetres in size. The web is also small, and it is difficult to find one. The species is widely distributed in wet environments such as river banks and swamps in the northern hemisphere.
Ready to strike
A slingshot spider makes a planar orb web, spins a thread from the centre and attaches the end to a twig. Then she sits in the middle of the web, grabs the centre with her four back legs and the anchor thread with her four front legs. By letting her front legs run over the thread, she stretches the web a few centimetres; the web becomes cone-shaped and is now ready to strike. The spider, sitting on top of the cone, holds the loose piece of thread between front and back legs with her pedipalps (the ‘boxing gloves of spiders).
And then she waits until a flying insect passes the base of the cone. If this happens, she releases the thread; the web snaps back, the spider whizzing backwards with it. The more the web was stretched, the more powerfully it shoots back. If it hits the unfortunate passer-by, the spider has captured her prey; the insect sticks to the threads and cannot escape. Otherwise, she immediately picks up the thread to tighten the catapult again.
Sound
The researchers wondered what exactly made the spider release her web. They conducted experiments in which they tethered a mosquito to a paper strip, in such a way that it could make normal flying movements. They moved it towards a web of Theridiosoma gemmosum. High-speed camera footage shows that the spider shoots its web at lightning speed at a mosquito when it is within reach, but before it touches it with its front legs and realizes the danger.
How does a slingshot spider perceive that a mosquito is within reach? Not with her eyes: the spider does not see sharply and, moreover, she is facing away from the mosquito. But she has special long hairs on the hind legs that sense the airborne vibrations caused by the wing beats. Moreover, the vibrations propagate over the threads of the web, and she detects that too. From the combination of this information, she probably infers where a mosquito is.
There is another piece of evidence that slingshot spiders respond to sound: they shoot their webs also at the snap of a finger or at the sound of a tuning fork. This does not produce any result, but apparently, slingshot spiders take every chance.
Willy van Strien
Photo: Theridiosoma gemmosum. ©Portioid (via iNaturalist, Creative Commons CC BY-SA)
The catapult in slow motion on YouTube
Sources:
Han, S.I. & T.A. Blackledge, 2024. Directional web strikes are performed by ray spiders in response to airborne prey vibrations. Journal of Experimental Biology 227: jeb249237. Doi: 10.1242/jeb.249237
Alexander, S.L.M. & M.S. Bhamla, 2020. Ultrafast launch of slingshot spiders using conical silk webs. Current Biology 30: R928-R929. Doi: 10.1016/j.cub.2020.06.076
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