28 Guide to Insects and Ticks 



is spread by the insect ; in large towns, where the arrange- 

 ments for the disposal of sewage are good, the greatest harm 

 done by the house-fly is in the dissemination of infantile summer 

 diarrhoea, with a large death rate. In camps and other places 

 away from towns, where it is difficult to secure adequate disposal 

 of human excreta, the danger of the spread of gastro-intestinal 

 diseases by flies becomes very considerable. The organisms 

 causing these diseases are mostly of a bacterial nature ; enteric or 

 typhoid fever is due to a motile bacillus (Bacilliix iiiphosus Eberth- 

 Gaffky), which in certain conditions exhibits from eight to twelve 

 delicate threads or cilia projecting from the body (fig. 7). 



In a table-case that stands on the East side of the Hall, 

 between the statue of Sir Richard Owen and that of Prof. Huxley, 

 are models of the common house-fly, Musca domestica Linn., 

 adult female (fig. 8), and a group of eggs, a larva and a pupa, all 



FIG. 7. ORGANISMS OF TYPHOID FEVKK, 

 Bacillus typhosus ; X 1,500. 



enlarged 28 diameters. Actual specimens are also shown of the 

 common house-fly and of three other species of house-fly, the 

 most abundant of which in houses is the lesser house-fly, Fann'nt 

 run irul aris Linn. 



The lesser house-fly is smaller than Musca domes lira, and is 

 often taken by the ignorant to be a common house-fly that has 

 not yet grown up. As a matter of fact, a fly that has once 

 emerged from its pupa-case does not increase in size. Occasionally 

 small specimens of Musca dmm'xlit'a are met with, but these are 

 undersized flies whose small dimensions are due to insufficiency 

 or excessive dryness of food during the larval or maggot stage, 

 lesser house-fly differs in its hahits Irom the common house- 

 fly, and rarely n wiih the latter. While Ihr common 

 house-fly is found largely in ihe kitchen and dining-room, and 

 settles indiscriminately on the wall, the tahle, exposed lo:,d, and 



