TJIC WINGS AND LEGS. 21 



There is no essential difference in the pads of flies and the 

 pulvilli of beetles, moths, and other insects ; the same fluid is 

 secreted in all. The only difference is that the pads of flies are 

 membranous and transparent, instead of hard and opaque. 



The feet of the smaller house-fly are the best to show the 

 manner in which the viscid fluid exudes from the extremities 

 of the trumpet-shaped hairs, as they are very large in this species, 

 and a glistening bead of fluid can be seen plainly at the extremity 

 of each hair by placing the living insect under the microscope. 

 The foot-prints left upon glass by flies consist of rows of dots 

 corresponding to these hairs ; this is best seen in those of the 

 lesser house-fly from their greater size. 



The whole appears precisely analogous to the manner in which 

 caterpillars and spiders suspend themselves by silken threads. In 

 both cases the fluid is exuded from minute pores and bears the 

 weight of the insect, the only difference being in the nature and 

 quantity of the fluid exuded. Much discussion has arisen as to 

 the manner in which flies liberate their feet, and it has even been 

 objected that they would become so firmly adherent after a time 

 that the insect would be glued to the spot. Nothing can be simpler 

 than the arrangement by which the foot is liberated, and in the 

 healthy insect the secretion probably never becomes solid as long 

 as it remains in contact with the foot. It is sufficiently glutinous, 

 even in the fluid, or rather semi-fluid, state it assumes as it exudes, 

 to sustain the weight of the insect, when the strain is put equally 

 upon all the hairs, of which there are about 1200 on each pad; 

 but when the pad is removed obliquely, so that each row is 

 detached separately, the resistance amounts practically to nothing. 

 A neat experiment will demonstrate this even to the most sceptical. 

 If a piece of adhesive label be cut for convenience into a pear-shaped 

 disc, an inch in diameter, and caused to adhere to the hand by 

 slightly damping it, a force of many pounds applied to the narrow 

 extremity in the axis of the paper will not stir it, whilst it is im- 

 mediately removed, with very little resistance, when the force is 

 applied so as to lift it gradually up. 



