^iTig'J Searle, Gleanings of a City Naturalist. yy 



end a small hemispherical knob is attached. They are pro- 

 vided with muscles at the base, and can, like the wings, execute 

 most rapid vibrations. The squama covers the halter like a 

 hood. A typical insect has three pairs of legs, which are 

 attached to the thorax. Each leg consists of five parts — the 

 coxa, trochanter, femur, tibia, and tarsus or foot. In the 

 case of the fly this foot is subdivided into five joints. It is 

 furnished with a pair of formidable claws, and between them 

 a pair of membranous pads or pul villi. The pulvilli are 

 covered on their ventral surfaces with innumerable closely-set 

 secretory hairs, from which a sticky fluid is given out, and 

 this enables the foot to adhere to any slippery surface over 

 which the fly is walking. By means of the claws the insect is 

 enabled to cling to the httle irregularities of the ceihng when 

 walking upside down. As may be expected from the at- 

 tachment of the wings and legs, we, of course, find within 

 the thorax a highly-developed set of rapidly contracting 

 muscles. The resulting movements have this further sig- 

 nificance : that they help in the respiratory exchange of gases 

 and in the circulation of the blood. 



The respiratory or tracheal system of the fly is very highly 

 developed. Altogether, it occupies more space in the body of 

 the fly than any other set of organs. It consists of three parts : 

 the spiracles, or breathing pores, situated at the sides of the 

 body ; air-sacs, and air-tubes (or tracheae). A large pair of 

 spiracles is situated on the bases of the first pair of legs. 

 Above and behind the bases of the last pair of legs is another 

 pair of spiracles, and, in addition to these thoracic there are 

 a number of pairs of spiracles at the sides of the abdominal 

 segments. All these spiracles communicate with tracheae 

 which ramify among the various organs of the fly's anatomy. 

 The abdomen or hindermost division of the body is composed 

 of several segments — eight in the male and nine in the 

 female. The segments succeeding the fifth are greatly reduced 

 in the male, and in the female form the tubular ovipositor, 

 which, in repose, is telescoped within the abdomen. The blood 

 system of the fly is simple. The body cavity forms a blood 

 cavity, so that all the organs and muscles are bathed in the 

 blood fluid, which is colourless, and contains fatty corpuscles. 

 There is a muscular tube, a heart, lying in a cavity 

 immediately under the dorsal side of the abdomen. It extends 

 from the posterior end to the anterior end of the abdomen, 

 and is divided into four chambers, each having a pair of 

 openings into which the blood is sucked, so to speak, from the 

 pericardial cavity. If it is in the warmer months of the 

 year that we are making our dissection, and the fly happens 

 to be a female one, the abdomen will be found practically filled 



