The house-fly has quite the typical insect form, inasmuch as there are three well defined sections of body—the head, the chest or thorax, and the abdomen; also it has three pairs of legs, each with nine joints, of which five joints constitute what may be called the foot. The twelve segments of the maggot are observable as twelve rings in the puparium, but in the fly the three which form the thorax look like one, whilst the eight which should theoretically exist in the abdomen look like four or five, until the rings of the ovipositor are counted. The illustration on page 39 will make plain how the permanence of the twelve-segment structure (conspicuous in the larval stage) has been thought to persist throughout the life-cycle, but at the same time will disclose how great is the change in the relative proportions of these segments. The prominent features of the hemispherical head are the two large compound eyes and the proboscis or trunk-like mouth. The antennÆ or horns are very short appendages with three joints; small plume-like projections, called arista, are attached to the third segment; the horns hang down over a hollow in the middle of the face, and are insignificant in size when As with many other flies and other insects, there are on the top of the head very small simple and rather inconspicuous eyes called ocelli, three in number, between the large and prominent compound eyes, The thorax is mainly occupied with the powerful muscles which actuate the attached wings, the legs, and the small appendages called halteres or balancers, which are supposed to be obsolete hind wings. There are three unequal segments in the thorax; the pair of front legs belong to the first segment, the wings and the pair of middle legs are attached to the second larger segment, whilst the third is connected with the hind legs and the halteres. The breathing apparatus of the fly is distributed in portions over the head, thorax, and abdomen; it consists of a number of internal air-sacks with membranous ducts ramifying everywhere; the largest air-sacks are in the abdomen near the waist. There is a pair of external spiracles to each segment of the body, and these lead to the air-sacks. 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 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 In comparison with the more heavily constructed wasp, with its four wings, the house-fly, with its two wings, is the more alert and active flier. The wasp is more robust than the fly and will be active in weather too inclement for the latter; however, some of the frail and slender gnats will brave cold temperatures impossible for the wasp. |