48 THE INTERNAL ANATOMY OF THE HOUSE-FLY 



the systems are described very briefly, since they are not con- 

 cerned in the transmission of bacteria. The alimentary system^ 

 which is of the greatest importance in this connection is more 

 fully described. The chief external features have already been 

 described (pp. 8 — 12) and the structure and function of the pro- 

 boscis is discussed in Chapter V, and the function of the crop 

 and proventriculus in Chapter VL 



Externally the integument (skin) of the fly consists of a hard 

 chitinous layer, with softer portions at the joints, which acts as 

 a skeleton or supporting framework, for the attachment of muscles 

 and other structures. 



In these insects the wiiscu/ar system is particularly well 

 developed, the thoracic muscles being enormous and almost 

 filling the thorax. They are arranged in two series. The 

 dorsales (Fig. 19, G) arranged in six pairs of muscle bands, on each 

 side of the median line, run longitudinally, and the sterno-dorsaleSy 

 which are arranged vertically and external to the dorsales, are 

 arranged in three bundles on each side. The former depress 

 and the latter lift the wing. There are also muscles controlling 

 the proboscis, roots of the wings, legs, halteres, etc. 



The nervous system is remarkable for its concentration. In 

 the head is placed a large mass of nervous tissue, the cephalic 

 ganglion or brain (Fig. 19, E), perforated by a small opening for 

 the passage of the oesophagus, and in the thorax a very large 

 mass the compound thoracic ganglion. These two masses are 

 connected by a median ventral nerve cord. Froin the main 

 ganglia nerves run to the various organs and limbs. 



The only definite organ belonging to the vascnlar system is 

 the heart, a long vessel which lies immediately below the dorsal 

 surface of the abdomen, and extends from its posterior to its 

 anterior end. It has four large chambers corresponding to the 

 four visible abdominal segments, and is continued anteriorly as 

 a narrow tube along the dorsal side of the ventriculus. By the 

 rhythmic contractions of the heart the colourless blood is circu- 

 lated through the body-cavity, which forms a closed chamber, 

 so that all the organs are bathed in blood. "Associated with 

 the blood system is a diffuse structure known as the fat-body 

 which consists of a large number of very large cells. The size 



