i9i3" I 9 I 4-] The Common House Fly. 115 



or less covered with bristles, and these easily become con- 

 taminated with filth. The thorax consists of three segments, 

 firmly united, and carries the single pair of wings, the 

 halteres, and three pairs of legs. The dark lines running 

 through the wings and forming the supporting framework of 

 the wing membrane are termed veins or nervures, and the 

 intervening areas, cells. The differences in the arrangement 

 of the veins allow one to distinguish the common house flies 

 from others. Behind the wings the pair of halteres, balancers 

 or poisers, are placed. Each has a conical base, on which are 

 sense organs, and which supports a slender rod, with a round 

 knob at the end. Each haltere can, like the wings, be rapidly 

 vibrated. Each leg consists of five segments — coxa, tro- 

 chanter, femur, tibia, tarsus. Each of the six feet is pro- 

 vided with two claws and two sticky pad-like structures, 

 called pulvilli. The walking surface of these pads is thickly 

 covered with hairs, and secretes a sticky fluid which enables 

 the fly to adhere to polished surfaces. 



The abdomen is composed of eight segments 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 is usually retained within the ab- 

 domen. The skin of the fly consists of a hard chitinous 

 layer with softer portions at the joints, and acts as a skeleton 

 for the attachment of muscles and other structures. The 

 muscular system is exceedingly well developed ; the thoracic 

 muscles are enormous, and almost fill the thorax. They are 

 arranged in two series — six pairs running longitudinally, and 

 six pairs vertically, and these control the wing movements. 

 Numerous other muscles exist also in the head, abdomen, 

 and legs. 



Let us now look briefly at how the fly lives, moves, and 

 has its being. The mouth parts are specially interesting, not 

 only for their anatomy but from the facility which they have 

 in carrying micro-organisms obtained from semi -liquid or 

 putrescent substances. The mouth appendages form a very 

 complicated structure, but most prominently we note a pro- 

 boscis — not adapted for piercing but for sucking. It can 

 be retracted or protracted, and is made up of an upper and a 

 lower portion ; the upper carries two curved bristly lobes ; 



