n8 T/ie Common House Fly. [Sess. 



nerves run to the antennas, another to the ocelli and optic 

 lobes which supply the large compound eyes. In the nervous 

 substance we find uni-, bi- and tripolar ganglion cells. Each 

 compound eye contains about 4000 eye elements. Each 

 consists of a transparent cornea, a transparent rod known as 

 the crystalline cone, and a cluster of percipient elements 

 known as the retinula, and these in turn are continued back 

 to the cephalic ganglion by the optic nerve. Probably what 

 the fly sees is a mosaic image built up of the innumerable small 

 images furnished by the separate eye elements. 



The heart is a long vessel which lies just below the dorsal 

 surface of the abdomen, along almost its whole extent. It is 

 made up of four large chambers, and blood is carried from it 

 forwards by a long vessel. The heart contracts rhythmically 

 and drives the colourless blood throughout the body-cavity, 

 which forms a closed chamber, and thus all the organs are 

 bathed in blood. This blood is crowded with corpuscles, 

 mostly containing substances of a fatty nature. There is also 

 a body known as the ' fat body,' consisting of very large 

 cells and lying below the coils of the intestine. Before 

 hibernation it seems to fill almost the whole of the abdominal 

 cavity, and after hibernation it shrinks almost to nothing. 

 This fat body receives a very rich tracheal supply, and in it 

 are stored the products of digestion. 



The respiratory or tracheal system is most highly developed 

 and occupies more space than any other anatomical structure. 

 By means of the tracheae, which are thin-walled branching 

 tubes, supported by chitinous rings, air is distributed to every 

 organ of the body, in which minute ramifications run in all 

 directions. The system consists of tracheal sacs of varying 

 size with extremely thin walls, and from these run the tracheae, 

 but, in the case of the abdominal tracheae, they run independ- 

 ently from spiracles. The anterior thoracic spiracles are large 

 vertical openings in the thorax above the anterior legs, and 

 supply the head, legs, most of the thorax, and a large part 

 of the abdominal viscera. The posterior thoracic spiracles are 

 situate at the back part of the thorax, and supply only part of 

 the thorax. There are seven pairs of abdominal spiracles in the 

 male and five in the female. These communicate with tracheae, 

 which ramify among the abdominal viscera but are not connected 



