THE THORACIC EXO-SKELETON. 197 



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 {Musca corvina) 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 footprints 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 more simple 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 1,200 on each pad; 

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

 is detached separately, the resistance amounts practically to 

 nothing. 



' The direction and length of the hairs upon the pad are so 

 adapted to the oblique direction in which the strain is put 

 upon them when the tarsus is straight, that the insect has 

 a perfectly secure hold ; this is immediately released as soon as 

 the tarsus is curved, which is effected by the long slender 

 tendon of the flexor tarsi. In the small house-fly the pads 

 themselves are capable of being curved, for the tarsal tendon 

 branches, and is inserted into the distal extremity of each pad.' 



13—2 



