sufficient for refrigeration, but in insects it is. It is by

friction against the membrane that they produce the humming sound,

as we said, in the way that children do by blowing through the holes

of a reed covered by a fine membrane. It is thus that the singing

crickets too produce their song; they possess greater warmth and are

indented at the waist, but the songless variety have no fissure there.

Animals also which are sanguineous and possess a lung, though that

contains little blood and is spongy, can in some cases, owing to the

latter fact, live a long time without breathing; for the lung,

containing little blood or fluid, can rise a long way: its own

motion can for a long time produce sufficient refrigeration. But at

last it ceases to suffice, and the animal dies of suffocation if it

does not respire-as we have already said. For of exhaustion that

kind which is destruction due to lack of refrigeration is called

suffocation, and whatsoever is thus destroyed is said to be

suffocated.

We have already stated that among animals insects do not respire,

and the fact is open to observation in the case of even small

creatures like flies and bees, for they can swim about in a fluid

for a long time if it is not too hot or too cold. Yet animals with

little strength tend to breathe more frequently. These, however, die

of what is called suffocation when the stomach becomes filled and

the heat in the central segment is destroyed. This explains also why

they revive after being among ashes for a time.

Again among water-animals those that are bloodless remain alive

longer in air than those that have blood and admit the sea-water,

as, for example, fishes. Since it is a small quantity of heat they

possess, the air is for a long time adequate for the purposes of

refrigeration in such animals as the crustacea and the polyps. It does

not however suffice, owing to their want of heat, to keep them finally

in life, for most fishes also live though among earth, yet in a

motionless state, and are to be found by digging. For all animals that