Normally the presence of animals in a house will be confirmed either by directly seeing them or by finding their tracks. Sometimes, however, we can use our sense of hearing, as well as of smell.

The sounds made by animals are very varied and naturally these come mainly from the larger species. Most of us will have suffered insomnia due to a persistent mouse trying to gnaw its way through the skirting-board, or will have heard it playing hide-and-seek with its mates up in the attic.

On account of their size rats tend to make even more noise. They have a very disturbing habit of dragging large objects around at night, which has given rise to stories of poltergeists. The fact that rats are involved is betrayed by the shrieks and squeals which accompany their petty squabbles.

Beech martens in a loft also make noises, particularly if there is a female with a litter of large, playful young.

Bats can often be heard during the day as they move around in their roosts and disturb each other. In the evening they also make a certain amount of noise when they wake up and prepare to fly out.

In addition to these relatively large animals, some of the much smaller ones also betray their presence by the sounds they make. One has only to think of the irritating hum of a gnat in a darkened room, or of blowflies and wasps which are usually heard before they are seen.

The nests of wasps, bumble bees and feral honey bees produce a continuous humming sound throughout the day and night, and this sometimes sounds like the snoring of a large mammal. The sound increases in volume when the insects are disturbed by vibration.

The noisiest sound made by an insect indoors is undoubtedly the singing of a house cricket. The males will perform for hours at a time from their favourite corner, and unfortunately mostly at night. However, the song has the advantage that it betrays their position, so that one can deal with the matter.

The larvae of many pests of timber can also be heard as they gnaw the wood. This has been likened to the sound of a nail being scraped across a piece of rough timber.

The larvae of wood wasps sometimes gnaw so loudly as to sound like a mouse.

Among the cerambycid beetles the larvae of Callidium violaceum and Phymatodes testaceus can often be heard gnawing beneath the bark.

Callidium is particularly common in the bark edges of, for example, roof timber made of softwood, while Phymatodes occurs in fire logs of birch and beech. The larvae of the house longhorn can also be heard from time to time, particularly on warm summer days when they are most active and their appetite reaches a peak. Normally a larva will feed for 5-10 minutes each hour and rest for the remainder of the time. The sounds of gnawing may be a great help in confirming that an attack is still active, for this is often very difficult to detect visually. Special apparatus, comparable to a stethoscope, can also be used for this purpose.

Many of the furniture beetles that live in timber also make their presence known by special sounds. These are the ticking sounds heard at night when the house is at rest. They are made, for instance, by death-watch beetles. The sound is produced by the adult beetle striking its thorax, in rapid succession, against the substrate. Both males and females make use of these sound signals, which probably serve to lead them to one another.

Some of the species of booklice that live indoors can produce tapping sounds, and this applies particularly to the winged booklice. Here again both sexes are calling to one another, and they produce the sound by tapping the abdomen against the substrate. They often appear to favour a position, such as an area of loose wall- paper, where the substrate is resonant, so that the sound produced is louder.

There may be a multitude of different sounds to be heard in most houses, and these can usually be traced quite quickly. Sometimes, however, these occur with such regularity that one is convinced that they must be made by some animal. Naturally the sound may emanate from one of the animals mentioned above, but in many cases it may be due to strains or tension set up by temperature changes in the central heating system or to the creaking of timber in furniture or elsewhere caused by periodic fluctuations in temperature and humidity.