The country people are well aware of the nature and cause of these bots. They know that each one contains a worm, that this worm comes from a fly, and that later it will be transformed into a fly itself. Each of these bots has in its interior a cavity, occupied by a larva, which, as well as the bot, increases in size as the larva becomes developed. It is generally on young cows or young bullocks--in fact, on cattle of from two to three years of age--that these tumours exist, and they are rarely to be seen on old animals. The fly, which by piercing the skin occasions these tumours, always chooses those whose skin offers little resistance. Each tumour is provided with a small opening, by which the larva breathes. In order to examine the interior of the cavity, Réaumur opened some of these tumours, either with a razor or a pair of scissors. He found them in a most disgusting state. The larva is lodged in a regular festering wound, matter occupying the bottom of the cavity, and the head of the worm is continually, or almost continually, plunged in this liquid. "It is most likely very well off there," says Réaumur; and he adds that this matter appears to be the sole food of the larva. "The position of a horned beast," observes the great naturalist, "which has thirty or forty of these bumps on its back, would be a very cruel one, and a terrible state of suffering, if his flesh were continually mangled by thirty or forty large worms. But it is probable they cause no suffering, or at least very little, to the large animal. Besides," continues Réaumur, "those cattle whose bodies are the most covered with bumps, not only show no signs of pain, but it does not appear that they are prejudicial to them in any way." [Illustration: Fig. 46.--Bumps produced on Cattle by the larvæ of the Bot-fly.] Réaumur tried to discover how the larva, when arrived at its full growth, succeeds in leaving its abode, as the opening is smaller than its own body.