LIFE HISTORY 27 



and he is obviously correct in supposing it to be a pro- 

 vision for the pushing away of the end of the puparium 

 when the pupa emerges from its case. This 'frontal 

 sac has been noticed by many observers, and was well 

 described as long ago as 1764 by Count von Gleichen. 



Structure of the Adult 



In the section on zoological position, a description 

 has been given of the characters which separate the 

 adult typhoid fly or house fly from other allied or sim- 

 ilar flies. The excellent illustrations given here (fig- 

 ures 12 and 13) show in more or less detail its exact 

 structure. Especial attention should be called, how- 

 ever, to the character of the mouth parts and of the 

 feet. The whole insect is more or less bristly and well 

 capable of carrying micro-organisms from putrescent 

 or semi-liquid substances, but the mouth parts and the 

 feet are especially adapted to this purpose. In addition 

 to two claws, each of the six feet is supplied with two 

 sticky pads of a light color. These are called pulvilli. 

 On the walking surface these pads are closely covered 

 with hairs which secrete a sticky fluid, and it is by 

 their help that flies are able to walk in any position 

 upon highly polished surfaces. 



The mouth parts are very complicated, but form in 

 the main a proboscis which is not fitted for piercing 

 but for sucking and is illustrated so well in figure 15 

 that detailed description will be unnecessary. This 

 organ can be retracted and expanded to a certain extent. 

 It is somewhat complicated in structure and consists 



