The songs of cicadas and crickets are some of the most characteristic sounds of summer. The volume they can reach is surprising and, as in our case, has probably led you to wonder how insects sing or why they do it.
Which insects communicate with sounds?
Sound communication occurs in the following groups: Orthoptera (crickets and grasshoppers), Hemiptera (bugs, cicadas and aphids), Lepidoptera (butterflies and moths), Diptera (flies), Hymenoptera (wasps, bees and ants) and possibly in Blatodea (cockroaches). However, it is only a predominant and widespread form of communication in some groups of Orthoptera and Hemiptera.
In insects, sound communication has a primarily reproductive function. It is especially useful for informing about sex, species, "quality" and location of the individual from a great distance. Normally, it is the males who "sing" and they do so to announce their presence to a potential mate, to court her and to compete sexually with other males. Each species has its own song, and in cases where both sexes sing, it is also different for males and females. Typically, the female responds by moving towards the singing male, without producing a sound. However, in some grasshoppers and cicadas, the female also responds with sound, and they may move towards each other, or seek each other out.
As for competition between males, they may use their songs in a kind of duel for territory. Generally, the one who sings the loudest wins. However, if there is no clear winner, the singing duel also escalates to physical aggression.
When several males are calling females in the same place and hear each other, they may change the frequency, duration or timing of their calls, synchronising or alternating according to the females' preference. In addition, males may also use the intensity of their calls to distance themselves from each other until they find their own "calling zone", thus reducing competition for females. Finally, there are cunning males that remain silent near those that do sing in an attempt to mate with the females attracted by their mates. In this way, they save energy and avoid attracting unwanted predators and parasites.
Once the pair has been attracted by the location songs, courtship songs begin, which include short- and long-distance sounds. These may be accompanied by other vibrational, visual or olfactory signals. In both crickets and flies, differences in song have been seen between males that successfully mate and those that do not, suggesting that females have sound preferences.
In the case of social insects, honeybees also use sound communication, but not for reproductive purposes. Workers accompany their dances with sounds to indicate to other workers where the resources they need are located.
How do insects sing?
To produce signals that are dispersed over long distances, such as homing songs, insects often use "stridulation", producing sound by rubbing specialised rough surfaces on their bodies. One part of the body with a defined edge "scratches" the other part, which has small teeth or undulations. Crickets have these structures on their hind wings, which sound when rubbed, while grasshoppers use their hind wings, or rub their hind wings with their hind legs.
Another way of producing sound is through the use of structures called tympani, which are found in hemiptera, such as cicadas, and many moths. These membranous structures have a pattern of channels, and contract and expand to generate sound. However, in the case of many moths, we cannot hear them because the sounds have a frequency that humans cannot detect.
Finally, in the case of flies and bees, they produce their characteristic buzzing sound by flapping their wings. The frequency of wing beats gives rise to different tones, which also allows them to differentiate between species.
Differentiating species by their song is not only useful for insects to find mates, it is also useful for other animals interested in locating them. Among them are scientists who use the sounds of insects to monitor them. Bioacoustics has many topics to explore, so we'll come back to it soon!
References
Balakrishnan, R. (2016). Behavioral ecology of insect acoustic communication. Insect hearing, 49-80.
Córdoba-Aguilar, A. et al. (2018). Insect behaviour: from mechanisms to ecological and evolutionary consequences. Oxford University Press.
Greenfield, M. D. (2016). Evolution of acoustic communication in insects. In Insect hearing (pp. 17-47). Cham: Springer International Publishing.
Gullan, P. J., & Cranston, P. S. (2014). The insects: an outline of entomology. John Wiley & Sons.
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