Insects rely on chemical, tactile, visual and mechanosensory information. Of all the Arthropods, insects depend on sound more than most - to find prey, avoid predators, defend territories, and reproduce.
A great deal of work has been done on Drosophila. Male D. melanogaster sing to attract females; they have two distinct song types, one a hum and one a purring pulse sound, both made by vibrating one wing. They also vary these song types to make them more attractive to the fly ladies. Both sexes rely on Johnston's organ to detect the sounds.This is a collection of sensory cells found at the base of the antennae and it detects motions of the antennae and also other vibrations. The Johnston's organ also helps a flying insect maintain stability during flight. Mosquitoes buzz as part of courtship; females have about 7,500 sensory neurons in their Johnson's organs while males have about twice as many, and they match their buzzing frequencies when mating.
Image by DrosophilaDrawings; reproduced with permission
Johnston's organ and the cochlear hair cells in mammals have a common evolutionary origin; homologous genes control the development of both.
Some insects, including cicadas, crickets, grasshoppers, and katydids among others, have evolved specialized hearing organs called tympanal organs. These have a tympanal membrane or eardrum - basically a stretched area of the cuticle - covering an air filled sac and innervated by a sensory organ. These organs are found in a variety of shapes, sizes and even places on the insect's body - on the legs, mouthparts, abdomen, thorax, even the wings. There are usually but not always two.
The purposes of these organs vary. Some use them for intraspecific communication; these include the cricket cousins and some butterflies and moths. A few, such as parasitoid flies, use them to find prey. Some use their tympanal organs to avoid predators. For instance, insects that fly at night often use hearing to detect nocturnal predators - notably bats - and this has been frequently cited as an example of an evolutionary arms race. As the bats have gotten better at locating prey using echolocation, the prey - for example, moths - have gotten better at detecting and avoiding the bats.
Katydids are especially for noted making noise themselves - and for their hearing. Their tympanal organs are typically on their legs. You can listen to a local katydid, Neoconocephalus robustus. For some amazing photos of katydids by Piotr Naskrecki, click the PowerPoint link below.
Studies show that the katydid's tympanal hearing organ has the same basic functions of mammalian hearing. Some insect ears are quite remarkable.
QUESTION: did bats invent butterflies? When bats evolved echolocation, moths fought back! Some developed ways to detect the bats' ultrasound pulses (and the Bat vs. Moth Wars continue today). Other moths began to be active during the day - and became butterflies! Jayne Yack of Carleton University in Ottawa studies insect hearing in butterflies and moths.
Unique Functions of Sound
The Madagascar hissing cockroach, Gromphadorhina portentosa, uses its specially adapted spiracles to make hissing sounds. When threatened it hisses loudly to startle predators and give it a chance to escape. Males hiss in agonistic encounters with other males, to warn off rivals or establish dominance. They also use soft hisses, along with gentle stroking with their antennae, to try to seduce females. (But the females have the last word; if they are not interested they simply walk away.) (Photo by Piotr Naskrecki.)
The caterpillar of the sphinx moth Amorpha juglandis also uses enlarged spiracles to produce whistling sounds when threatened. As with G. portentosa, this presumably startles the would-be predator enough to scare it away. The caterpillars also whistle for safety.
Other caterpillars use sound in still different ways. Hooktip moths, for example, spin protective shelters for themselves and if another caterpillar approaches they start drumming and scraping leaves. Some of these vibrations can even be heard by humans!
Bark beetles in the group Scolytinae are economically very important because they are serious pests in forests worldwide. They produce a variety of sounds that apparently function in defense, aggression, courtship, species recognition - and possibly in finding the trees they want to infest. Jayne Yack and her associates at Carleton University in Ottawa are studying several bark beetle species to discover how and why they produce the sounds that they do.