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 compared with those of other kinds of insects; but their structure viewed under the microscope is intricate, and they may be efficient organs of sense perception, probably in part auditory. The really unique feature is the retractile and suctorial proboscis, which is often incorrectly regarded as the tongue; it is normally held doubled up and withdrawn towards a hollow under the head, whence it is from time to time extruded. The structure of this member is characteristic of the entire tribe to which the house-fly belongs; it is a fusion or combination of mouth parts, which in other insects are used more or less separately for the various functions of inspecting, biting, masticating, drinking and swallowing. In the house-fly the proboscis is absolutely suctorial, and is not provided with the lancets used by the blood-sucking flies for piercing the skin. Two maxillary palpi are attached to the upper or basal part of the proboscis, which is called the rostrum (a snout); the lower part is called the haustellum (a pump), and it has at the end a pair of soft cushion-like lobes or lips, which, when spread apart, form a heart-shaped pad with an opening in the centre. The maxillary palpi are used for feeling and probably smelling. Each mouth-lobe has a main collecting central channel and thirty subsidiary cross channels of a wonderfully complex character. Imbibed fluids pass from the mouth-lobes to the gullet along a passage in the haustellum and the rostrum.