2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Spider portrait Deinopis spinosa.
Deinopid spiders have been found to be able to distinguish a wide range of sound frequencies using sensory organs located on the limbs. Thanks to this, they manage to grab flying insects, focusing on the noise of their wings. In addition, sensitivity to high frequencies can save a spider's life by allowing it to hide from birds in time. The results of the study were published in an article for the journal Current Biology.
Spider hunting tactics are extremely varied. Some species weave a web and passively wait for the prey to fall into it, while others actively pursue potential victims. Deinopidae spiders are somewhere in the middle of this spectrum. They are suspended from a spider web, look out for the prey passing below with a pair of enlarged eyes and cover it with a rectangular net stretched between the four forelimbs. Usually these spiders hunt at night, and during the day they hide, relying on camouflage coloring.
However, sometimes Deinopids hunt differently: they make an attack over their heads and grab a net of insects flying on them, which they could not notice with their forward-looking eyes (you can see it in the video below). Scientists suggest that in such situations spiders rely on hearing, but there is still no solid evidence for this idea.
A team of researchers led by Jay A. Stafstrom from Cornell University decided to investigate this issue. To do this, they conducted a series of experiments with the spiders Deinopis spinosa, which live in the United States, as well as in certain regions of South America and the Caribbean.
First, the authors recorded the electrical activity of the spider's brain in response to acoustic signals ranging in volume from 55 to 90 decibels and a frequency of ten to thirteen thousand hertz, which were played using a device two meters away. Then the experiment was repeated with the legs separated from the body (it is here that some other species of spiders have special hairs that catch sounds).
The study confirmed the sensitivity of deinopid spiders to a wide frequency spectrum of sounds (from one hundred to ten thousand hertz). Moreover, scientists were able to identify specific areas of the limbs responsible for acoustic perception. They turned out to be lyre-like organs located at the ends of the legs and sensitive to deformations of the exoskeleton. They also help spiders from some other families to hear. In addition, elongated hairs, trichobothria, also located on the extremities, can play an important role in capturing sounds in D. spinosa. It is possible that trichobothria record low-frequency sounds, and lyre-like organs - high-frequency ones.
In the next step, the authors conducted field experiments with D. spinosa in their natural habitat (in this case, in Florida). Having found 25 hunting spiders, they played sound impulses with a frequency of 150, 400, 750, 2300 and 4400 hertz, as well as white noise (the volume of the recordings was 70-80 decibels, and the speaker was located a meter from the spider).
Thirteen individuals responded to at least one stimulus with an overhead attack. Interestingly, only lower frequencies, from 150 to 750 hertz, elicited the response; the spiders ignored stimuli with a higher frequency and white noise recordings. At the same time, not a single recording caused a classic forward attack in the experimental individuals. Repeating the experiment in a laboratory with 51 spiders, the authors obtained similar results.
The wings of moths and dipterans, which often fall prey to deinopid spiders, move in the same range to which D. spinosa reacted in experiments. According to the authors, this indicates that during the hunt for flying prey, these spiders are guided by the noise of their wings. Why do deinopids need the ability to distinguish sounds with a frequency of more than a thousand hertz, scientists do not yet know. In the experiments, the experimental individuals did not react to them - in addition, the potential prey does not emit sounds of such a frequency. Perhaps, the sensitivity to high frequencies helps these night spiders to notice the dangerous daytime predators, primarily birds, in time.
Scientists recently described an unusual hunting tactic for the spider Hyptiotes cavatus, which pulls the fibers of the web in anticipation of a suitable target, after which they detach with their hind limbs and, together with the web, make a sharp dash towards the victim, making it entangled in the net. This is the first example of storing the energy of multiple muscle contractions using an auxiliary object in nature.