INSECTS. 



293 



fig. 130. 



Highly magnified portions of the 

 trachea of the Hydrophilus, 

 showing the spiral tubes, and 

 their arrangement. 



ments ; enclosing within them sets of powerful muscles, whereby their 

 movements are effected. The first is 

 called the hip ; it is short, and is con- 

 nected by a ball-and-socket joint to the 

 thorax ; the second, called the trochanter, 

 is connected with the former by a tough 

 membrane, fitting to a corresponding 

 socket ; the third, the femur, or thigh, is 

 the largest and strongest, serving as a 

 fulcrum, upon which the leg depends for 

 strength and motion; the fourth is named 

 the tibia, or shank ; this is connected 

 to the thigh by a beautiful hinge-joint, 

 which permits of both bending and ex- 

 tension ; to the end of this is attached 

 the tarsus, or foot, generally terminated 

 by two horny hooks or claws, whereby 

 the insect holds to the object upon which it moves ; between these 

 hooks, in most species, is situated a cushion, sucker,, or broad flaps 

 (jpulvilli), wherewith to take hold of a 

 smooth surface. In fig. 132 the lower 

 joints, with the feet of flies, are beau- 

 tifully represented, marking peculiari- 

 ties of structure in each ; and in fig. 

 131, we have a sucker somewhat re- 

 sembling the admirably- constructed 

 Slicker attached to the under-surface 

 of the feet of house-flies, by means of 

 which " they tread the ceiling or in- 

 verted floor, and from its precipice de- 

 pend secure.'' It was formerly sup- 

 posed, from the experiments of Sir 

 Everard Home, that flies were enabled 



to walk against glass, and with the _ ,„, 



.... fig. 131. 



back downwards m various situations, Sneker on the foot of Water-beetle. 



The circle encloses- the object of 

 the natural size. 



solely by the formation of a vacuum 



under the soles of their feet, if they 



may be so termed ; as it was observed that the margins of the feet 



were closely applied to the glass, while the central part was drawn up. 



It has, however, now been discovered that this hypothesis was not cor- 



