THE MICROSCOPE IN ZOOLOGY. 



77 



insoluble in water, and solidifies under that fluid. 

 The whole contents of the tarsus becomes solid very 

 rapidly as soon as the insect is dead, or the part is 

 removed. 



" 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, in- 

 stead 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 \ery 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 simpler than the arrangement by 

 which the foot is liberated, and in the healthy insect 

 the secretion probably never becomes solid as long as 



