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, which latter are said to possess each four thousand facets. The compound eyes of the male fly are proportionately larger than those of the female; it is quite observable that they approach each other more closely at the top of the head, a feature of sex differentiation which is shared with bees, wasps, and many other insect creatures. It is thought that a single brain image arises from the combined views of the four thousand facets of the compound eyes blending with the view conveyed through the "ocelli." However, it is a most curious fact that it is the inconspicuous ocelli which are of supreme importance visually. The compound eyes have doubtless some special function, but throughout the insect world the size of compound eyes is not a certain indication of keenness of sight. The vision of the fly is good for distinguishing the movement of any broad mass, but it is rather ineffective for observing a thin line, as may be proved by slowly lowering a knife blade, with a steady hand, when its body may be severed before the fly takes alarm. It is a remarkable fact that the family of _Tabanida_ (blood-sucking breeze flies), which are destitute of "ocelli," are the dullest sighted of all flies; in fact, at least semi-blind. Moses Harris observed that a blue-bottle became practically blindfolded when its ocelli were covered with an opaque pigment. Probably this is the case with other insects. Bees, which require long distance sight for home returning, are well provided with ocelli. Butterflies, however, without the use of ocelli have a distinct faculty of daylight vision for a moderate distance. The investigation of sight by blindfolding is very difficult in flies. There are two cephalic ganglions, which are regarded as the brain; these are situated in the upper part of the head close to the neck. There are also ganglions in the thorax with connections extending into the abdomen. 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.