Insects may seem too small and too mute to have much to say, yet the communication systems they use are among the most varied in the natural world. Sound, scent, vibration, polarised light, touch, and even electric fields all play roles, and a single species often employs several of them in combination. The modesty of the signals is part of their interest: an odour too faint for a human to detect can rearrange the behaviour of an entire ant colony, and a vibration too quiet for human ears can bring hundreds of cicadas into synchrony.

The most thoroughly studied system is chemical. Ants deposit volatile compounds along their foraging trails, and the compounds evaporate at rates precisely matched to the half-life of the resource they mark. A freshly discovered food source is marked with a short-lived chemical, so that the trail fades when the food is exhausted; a long-established nest is marked with more persistent compounds. The British entomologist Richard Howarth, working on wood ants in the Black Forest, demonstrated that a single ant can lay down a coherent chemical message at walking speed, using different glands to vary not only the intensity of the scent but the information it conveys.

Honeybees use scent for a different purpose. Worker bees returning from a valuable flower deposit a fatty acid mixture, known as the Nasonov pheromone, on the lip of the hive to help recruits orient themselves. If the source is of particularly high quality, the returning bee also performs the famous waggle dance, whose orientation codes the angle of the food relative to the sun and whose duration encodes the distance. The dance, first decoded by Karl von Frisch in 1944, is now known to involve subtle vibrations carried through the honeycomb as well as visual cues, and recent work suggests that bees deprived of the vibrational channel interpret the dance less accurately.

Sound is more common in the insect world than a quiet garden might suggest. Cicadas produce their songs with a pair of membranes called tymbals, which click rapidly when pulled out of shape by specialised muscles; the male's abdomen amplifies the clicks like the body of a violin. The mating calls of related species differ in the precise rhythm of the clicks, enabling females to choose mates of the correct species even when several species overlap geographically. Crickets and grasshoppers produce their sounds by stridulation, rubbing one body part against another, and the frequency of the resulting call in many species corresponds strictly to the temperature of the air.

For insects that live underground or inside plants, airborne sound is useless. The leafhoppers of the family Cicadellidae communicate by vibrating the stems of their host plants; a male's courtship call travels only a few centimetres but is detected by females on the same or adjacent leaves. Because such vibrations can be recorded from outside the plant without altering the insect's behaviour, they have become a useful tool for studying behaviour that would otherwise be inaccessible. A team led by the Slovenian biologist Ana Kovac has recently identified more than forty distinct signalling patterns among European species, several of which are sufficiently stereotyped to allow species identification from a recording alone.

Light-based communication is rarer but not unknown. Fireflies flash patterns that differ between species and between sexes, and some Southeast Asian species have evolved the remarkable ability to synchronise their flashes across an entire mangrove forest. The synchrony, once thought mysterious, is now explained by a simple coupling rule: each male adjusts the timing of his own flash by a small amount in response to the flashes he sees, and over many iterations the population drifts into agreement. A mathematical model of this process, developed in the 1980s, turned out to apply to a range of quite different biological systems, including the pacemaker cells in the human heart.

Electric signalling remained largely unsuspected until careful experiments in the last fifteen years. Bumblebees approaching a flower are now known to detect the weak static electric field that surrounds the flower; the field changes when a bee lands and extracts nectar, and the signal appears to help other bees avoid recently visited flowers. Whether the bees actively broadcast their own electric signature or merely respond to changes in the ambient field is an open question. Howarth has argued that such subtle channels are probably more common than has been recognised, and that the standard picture of an insect as a creature guided mainly by scent and sight will need steady revision.

The practical implications of this research reach beyond pure entomology. Pest species, including the codling moth and the diamondback moth, can be controlled by synthetic pheromones that disrupt the mating signals of the natural population. Agricultural researchers have begun to develop devices that record cicada and cricket song as a proxy for ecosystem health, because the acoustic community reacts to habitat disturbance more quickly than more conventional biological indicators. Kovac has suggested that a complete soundscape recording of a forest, taken over several years, may become a routine tool for monitoring biodiversity, provided that the insect community can be identified from the recording.

Whether these applications scale to commercial use will depend on cost and on the continued decoding of signals that remain obscure. What is already clear is that the small noises and weaker scents of the insect world add up to an intricate communication network whose full dimensions are still being mapped.