The lines on the wings of the house-fly called nervures have already been alluded to in Chapter II. These nervures are strengthening ribs to the transparent tissues of the wings. The tissues are double (top and bottom) enclosing the nervures, which are so united to the connections called trachæ of the air-sacks, that the newly emerged fly helps to extend its limp and crumpled wings by a process of inflating the nervures. The stomach is located partly in the thorax and partly in the abdomen. A passage from the gullet passes through the neck into the lower part of the thorax, where are the entrances to two long capacious chambers, of which the upper one is the true stomach and the lower one a store pouch, which latter may be likened to the honey bag of the bee. The fly habitually regurgitates liquid food stored in this pouch, and, somewhat after the manner of the cow chewing the cud, passes the same back into the true stomach, whence it proceeds onwards through the digestive track. The abdomen holds all the other ordinary internal organs including that which may be called the heart, and which lies above the stomach; it consists of a long muscular tubular vessel with four contractile chambers. Although the organ called the brain is located in the head, and although that called the heart is in the abdomen, yet some sense of control over bodily motions curiously exists separately in the ganglions of different parts of the body. This fact seems to make it possible for one extremity of the body to continue performing a pleasurable action (say, the head drinking honey) after the other extremity has endured a painful catastrophe (say, amputation of the abdomen). However, it may be fairly surmised that no creatures of a lower grade than warm-blooded vertebrate animals feel pleasure and pain in any way at least after the manner of mankind. The most vital part of the fly is not the head but the thorax. A severe squeeze on the thorax will effectually paralyse and kill the creature. Muscular movements of different parts of the fly's body, which continue after severance or other fatal injury, cannot be regarded as visible proof of a slow death and prolonged sensibility. Possessed of six legs, each with nine joints, the fly exercises a unique capability of walking; the legs are moved three at a time, a front and a hind leg on one side advancing simultaneously with the middle leg on the other side; thus the fly proceeds most securely always poised on three feet, which are so well furnished with pads, claw-like hooks, and hairs, that it can walk over polished glass and can even walk upside-down along comparatively smooth surfaces.