They are fast asleep and out of sight. The reason there are so few houseflies in winter is that their eggs will hatch only at temperatures between 75 and 90 degrees Fahrenheit. Meanwhile, their numbers steadily diminish owing to insecticides and natural causes. Since flies are often carriers of diseases, it is not a bad thing that their numbers diminish during the long sleep. During the period of suspended existence, not only does the hibernator's temperature drop to almost half of normal, but the heart slows down, and all functions of alimentation and excretion cease.
Respiration is faint, and in some sleeps appears to wholly cease.
It is not only by hibernating that animals are able to survive the winter. Listen to this amazing fact. Everyone knows that when the temperature drops, fingers, toes and ears freeze first. But it has been only within the last 200 years that we have understood that this is due to the ratio between the amount of body surface through which heat can be lost, and the body's bulk and heat supply. About 100 years ago, physicists working on the kinetic theory of gases found that heat travels very slowly through still air.