40 



this mixture I killed and dissected a number of them, and found the small intestines- 

 intensely coloured, of a deep reddish orange shade, representing the digested spores of 

 the tricholoma. I observed in the contents a few well defined orange spores, but none 

 of them ai:)peared to have germinated. Fastened between the hairs on the limbs of eaok 

 of the flies examined I found a number of the spores, and the efforts of the fly to get rid 

 of them only resulted in attaching them more firmly to it. They might, however, be 

 brushed off by objects with which they were brought in contact, while their germinating 

 powers would long outlast the life of the insect itself. It was evident from this experi- 

 ment that flies were capable of conveying such spores to plants and other bodies. On the 

 other hand, the fact that by far the greater part of the spores were consumed, in the one 

 case by the larvie of the fly, and in the other (i.e. when mixed with sugar), by the fly 

 itself, shows that this insect may destroy microscopic germs as well as disseminate them, 

 and indicates that in some cases its agency in keeping down their number may more 

 than counterbalance its action in contributing to their dissemination." 



Having thus briefly and imperfectly considered the organs of the head, we find, 

 on examining the second section of the body — the Thorax — that it bears the organs of 

 locomotion, which consist of three pairs of legs, a pair of wings, and a pair of halteres 

 or balancers. The most important of these are the wings, which are not large, when 

 compared with those of many other insects, yet suffice to enable the fly to sustain a swift 

 and prolonged flight. 



The vibrations of the wings during flight are exceedingly rapid, — about 330 strokes 

 each second being the usual rate, when the insect is flying swiftly. Thus the fly is able 

 to dart about with a swiftness surprising in so small a body, and which, if possessed by 

 a horse in ratio to its magnitude, would enable it to traverse the earth ■\yith inconceivable 

 velocity. 



The halteres, halancers or poisers, as they are indifferently called, are two small club- 

 shaped organs, which occupy the position of the second pair of wings in insects of other 

 orders, and which may be considered as merely modified or rudimentary wings." They 

 are kept in a constant vibration during the insect's flight, which they serve to regulate, 

 as has been proved by experiments. The possession of these little organs enables the 

 dij)tera to change the direction of their flight far more quickly than other insects, as may 

 be seen by watching the erratic movements of flies hovering in the air. These balancers 

 are more easily seen in some of the larger species, such as the tijmla or daddy-longlegs, 

 which sometimes enters our houses from the neighbouring fields. In the house-fly, as 

 in many species, they are partially hidden beneath a pair of protecting horny scales, 

 which project from the sides of the thorax. 



The legs are divided into five parts, or joints ; first, the coxa or hip, by which the leg 

 is articulated to the body, then a short, round joint called the trochanter, next ih.Q femur ox 

 thigh, then the tibia, and lastly the tarsus or foot. This latter merits particular atten- 

 tion, on account of its peculiar powers. It is subdivided into five joints, the last of which 

 bears the apparatus which permits a fly to crawl so easily on the most perfectly polished 

 surfaces, even when it is beneath them. On examining the last joint, that bearing the 

 claws, it is found to be expanded into two large pulrilli or pads, thickly set with minute 

 hairs or filaments, each of which terminates in a small fleshy bulb, kept moist by a viscid 

 exudation. Although very minute, they are so numerous that the foot adheres firmly 

 enough to sustain the fly in his pedestrian excursions overhead. 



The abdomen, or remaining section of the body, presents no external appendages for 

 present consideration, so that we will turn for a moment or two to the internal structure 

 of the fly, merely to mention the principal features. Directly under the dorsal surface 

 or back of the insect is attached the dorsal vessel, or heart, a tubular vessel, running the 

 full length of the abdomen and being constricted as it enters the thorax. This tube is 

 situated in a cavity in which the blood collects on completing the circulation of the body, 

 and is furnished with valvular orifices, which permit of the returned blood entering the. 

 heart, but which permit none to escape therefrom. The heart is also constricted at 

 several points, and divided into a series of chambers, separated by valves, through which 

 the blood is continually forced forward from one chamber to another, and finally dis- 



