376 



TEXT-BOOK OF ZOOLOGY 



(c) Ita long and movable legs adapt the fly for rapid running. Its 

 power of settling on and running up smooth window-panes or on the 

 ceiling, in apparent opposition to the law of gravity, is a well-known, but 

 nevertheless very wonderful, phenomenon. It is due to the presence of 

 two finely-haired adhesive pads or balls situated between the two claws 

 of the last tarsal joint, and constantly kept moist by a fluid perspiration. 

 According to some naturalists, this fluid is sticky,, so that the insect 

 is glued, as it were, to the supporting surface ; according to others, it 

 merely serves to attach the adhesive pads more firmly and securely, just 



as two glass plates adhere more firmly to each 

 other if a few drops of some liquid be intro- 

 duced between them. 



Flies, as we must have often observed, also 

 use their legs for cleaning their bodies, which, 

 owing to their habit of frequenting dusty 

 places, must get very dusty. This dust, how- 

 ever, would act very injuriously on the eyes. 

 (Why ? We wipe the glasses of our spectacles. 

 How are our eyes kept clean ?) Accordingly, 

 we frequently see flies brushing, as it were, 

 their movable heads, their large eyes, their 

 wings, thorax and abdomen, and finally clean- 

 ing the legs, i.e., the brushes themselves, by 

 rubbing them against one another. The legs, 

 being hairy and very movable, are excellently 

 adapted for these operations. 



Foot of House Fly (seen from 

 Above). (Strongly magnified.) D. Food. 



4.G. and 5.G., Fourth and fifth , T , . , , 



tarsal joints; K., claw; H., ^° sooner has a meal been placed on the 

 adhesive balls. table, or anything eatable, such as a dish of 



fruit, some sugar, etc., put out, than the flies also are on the spot. 



(a) We are ignorant as to the means by which the insect so quickly 

 discovers the presence of food. It is impossible to decide whether 

 this discovery is effected by the large hemispherical compound eyes, to 

 which three simple eyes, or ocelli, are further added, or by means of the 

 minute antennse. All we know is that the fly is there, and at once begins 

 its meal. 



(b) The proboscis is pestle-shaped, and used for sucking up fluids. It 

 is formed from several of the mouth parts, which are so disposed as to 

 form a tube. (In the house fly, the mandibles and maxillae are reduced 

 to rudiments ; the insect is therefore unable to sting like other Diptera ; 



