Taking up this question, I thought: "Well, everything is for sure known about ants - and what, and how, and how they hear!" It turned out - nothing of the kind! For myrmecologists (as they call ant specialists), only one thing is certain: ants can communicate using sounds. And if so, it means that they definitely have hearing and organs that (with a rather big stretch) can be called ears.
And the "ears" of ants are not at all like what we are used to calling this beautiful word. And there are several types of "ears". And hearing is by no means their only function. And they are located not only on the head, but also … Okay, about everything in order.
As you know, sounds can spread not only through the air, but also through liquids (for example, water) and even over solids (for example, soil, tree trunks and leaves). And if for people the most important thing is "air" sounds, then for ants who crawl on the ground, trees and other solid things all their lives, "hard" sounds are of great importance. (In principle, a person is also able to hear "hard substrate" sounds. Remember Vasilisa the Beautiful, who puts her ear to the ground to hear how far Kashchei the Immortal is galloping on his heroic horse.)
And in order to recognize such "solid" sounds, you need to be able to perceive vibrations, vibrations of the substrate. And for this, two ears on the head are not enough - the organs of hearing should be located wherever only the body comes into contact with the "sounding" surface, that is, practically throughout the body.
Figure: 1. The structure of the chordotonal organ. Scolopidia are like strings stretched between the cuticle and the flexible membrane. When the cuticle moves, it pulls the scolopidium along with it and causes excitation of the neuron located in this scolopidium. Image from what-when-how.com
In structure, these organs are also not in the least like the ears of people or, say, hares. Since they should not perceive waves flying in the air, they do not need that external "catcher" in the form of a shell, which we are used to calling the ear. And these auditory organs consist of peculiar "strings" (they are called scolopidia) stretched between the cuticle (external skeleton of an insect) and a special flexible membrane. Each scolopidium consists of three cells, one of which is nervous. If the surface that the ant touches begins to vibrate, then the cuticle will begin to pull the scolopidia. When the scolopidium is stretched, the nerve cell under the influence of tension is excited and sends an impulse to the corresponding nerve node. Thus, surface vibrations are transformed into nerve impulses, and the ant hears the sound. The organs described above are called chordotonal and are involved not only in distinguishing sounds, but also in proprioception - that is, they feel muscle stretching and determine the position of the body in space.
So, we figured out the "hard" sounds. But does the ant also hear "air" sounds? There is no definite answer to this question yet, but it is possible to extrapolate to ants the data obtained on other insects, for example, mosquitoes and flies.
And flies and mosquitoes are able to hear "air" sounds with the help of special bristles located on the antennae. A sound wave moves such a bristle, the bristle pulls on the scolopidium, from which the neuron located in the scolopidium is discharged and sends an impulse to the nerve node. These hearing organs are called johnston's organs. They are a subtype of chordotonal organs and are sensitive only in the near field (usually at a distance of no more than tens of centimeters). It is easy to understand that they will sense not only sounds as such, but also any vibration in the air - for example, the wind caused by an approaching fly swatter.
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And besides, insects have another type of sense organs capable of perceiving sounds - trichoid sensilla. This complex phrase refers to the tiny bristles on the body of an insect. These bristles are directly (and not through the scolopidium, like Johnston's organs) are connected to the nerve ending, and when a sound wave (or simply wind) vibrates the trichoid sensilla, the nerve ending is excited and generates an impulse, and as a result, information about the vibrations reaches the corresponding nerve node … Ants have trichoid sensilla, but whether they are sensitive enough to perceive sounds is still not completely clear.
Figure: 2. Antennae of an ant (electron micrograph). The antennae carry Johnston's organs as well as many trichoid sensilla, but it is not known if they are sensitive enough to hear sounds. The length of the scale bar in the upper figure is 500 µm, in the lower one - 200 microns. Photo from the article: R. Hickling and RL Brown. Analysis of acoustic communication by ants // Journ. Acoust. Soc. Amer. 2000. V. 108, No. 4. Pp. 1920-1929
But something is known about how ants use sound alarms.
For example, kamponotus ants, or carpenter ants, gnawing their nests in wood, strike the walls of the nest with their jaws or abdomen in order to summon congeners to protect it.
And still many ants are able to chirp, rubbing their abdomen on special "graters" on the stalk between the chest and abdomen (Fig. 3). The chatter is barely audible, the human ear can barely distinguish it even at close range. However, this volume is enough for ants, and they can perfectly communicate with each other using chirping.
Figure: 3. Most ants make sounds by rubbing the abdomen (Gaster) against the stalk (Postpetiole). Photo from the article: R. Hickling and RL Brown. Analysis of acoustic communication by ants // Journ. Acoust. Soc. Amer. 2000. V. 108, No. 4. Pp. 1920-1929
For example, this chirping is transmitted through the soil. Kindred can dig up an ant buried in the sand, having heard its "cries for help."
And through the leaves and branches of trees, vibration from chirping is also transmitted. Some ants use it in a very unexpected way. It turned out that in leaf-cutting ants, the vibration of the abdomen is transmitted to the jaws (mandibles). When the mandibles cut the leaf, they vibrate at a frequency of about 1 kHz (a thousand times per second!). Thanks to this, the sheet is cut, if not faster, then smoother and more accurate.
And later it turned out that ants chirp more often when they cut not harder, but more delicious leaves! It turned out that while doing this, smaller workers run up to the larger worker ants. Then a large worker drags the cut leaf into the anthill, and the small ones climb onto the leaf and ride it. But they do not just ride, but, for example, protect porters from flies that try to lay their testicles on the bodies of large workers.
Recently, it turned out that sounds for communication are used not only by ants, but also by their parasites. Hundreds of other insect species usually live in an anthill. Among them are the caterpillars of some blue-eyed butterflies. These caterpillars are similar to the larva of a certain species of ants in appearance, and most importantly, in smell. Working ants, finding such a caterpillar, drag it to the nest. Caterpillars of some species continue to mimic larvae so well that worker ants feed them like their own little sisters (worker ants are sterile females, and the larvae are their sisters).
Recently it was revealed that the caterpillars and pupae of the "cuckoo-pigeons" make sounds, imitating adult ants. At the same time, as it turned out, in the host ants (one of the species of the genus Myrmica), the queens and worker ants chirp differently. If you play the sounds emitted by the uterus to workers, they surround the sound source and assume characteristic "protective" postures, as if guarding the real uterus. The sly caterpillars and pupae of the pigeons imitate the sounds of the uterus, and the worker ants rush to guard them!
This example shows that sounds can play an important role in the life of the ant family: in particular, the royal "well-placed voice" helps the uterus to occupy the highest level in the hierarchy. This means that ants are well aware of different sounds of their relatives - no matter what they hear …
The author is grateful to N. G. Bibikov, A. A. Zakharov, and Vera Bashmakova for advice and assistance in preparing the answer.
Author: Sergey Glagolev