438 THE BIOLOGY OF INSECTS 



Insects which are continually in contact with mankind 

 may act as disease-carriers even if they do not suck blood, 

 and this section of our subject is abundantly illustrated by 

 the importance, now well recognised, of Miisca domestica, 

 the " common house-fly." Every one knows the abund- 

 ance of these insects in our dwellings during the late summer 

 and autumn months, and the manner in which they are 

 attracted by various food-substances so that they walk over 

 bread, butter, and meat, and frequently fall into jugs and 

 other receptacles containing milk. Like all members of its 

 family, the house-fly has a pair of delicate adhesive pads 

 on each foot, so that it readily takes up material from 

 whatever objects it may rest on. The danger of the house- 

 fly to human health is due to its breeding habits, for the 

 female seeks all kinds of unclean substances wherein to lay 

 her eggs. Garden rubbish, horse- dung, and many other 

 kinds of animal waste-matter, including human excrement 

 if available, heaps of old rags and similar refuse — such are 

 the objects from which house-flies come through the open 

 window for an exploration of the breakfast- table. The 

 house-flies' breeding haunts swarm with countless micro- 

 organisms, and it has been reckoned that a single insect 

 may carry a million and a quarter bacteria for dissemination 

 among human food. It has been abundantly proved that 

 house-flies are thus concerned in the spread of typhoid, 

 infantile diarrhoea, and other diseases of the human digestive 

 tract. Their harmful activity in this respect has been well 

 demonstrated by L. O. Howard (1900), C. G. Hewitt (1914), 

 and R. Newstead (1907), and the sanitary importance of 

 the knowledge of the house-fly's habits and the conse- 

 quences thereof has been proved by the comparative im- 

 munity from typhoid during the recent *' Great War " of 

 large bodies of troops whose medical officers acted on the 

 ascertained facts so as to minimise the risk of infection. 

 By screening carefully from approach of flies all refuse in 

 which they seek to lay their eggs the insects are prevented 

 from carrying infection to food, and it is also possible to 

 catch and kill large numbers of flies by means of suitable 



