STRUCTURE OF THE LEGS. 21 



form, as is commonly the case with the latter, it is 

 most frequently more or less prismatic, and thickens 

 slightly from the base to the apex. Both the thighs 

 and tibiae, but especially the latter, are very commonly 

 armed with spines of various kinds ; at the apex of 

 the tibiae in particular we often find several very long 

 spines, which sometimes take the form of spurs, whilst 

 in other cases they constitute a sort of coronet around 

 the apex of the limb. In many insects these spines 

 are moveably attached to the tibiae. 



The legs are terminated by the feet or tarsi, which, 

 unlike the preceding parts, are usually composed of 

 from three to five joints, although in some cases there 

 are only two, or even one of these articulations. From 

 their structure they are of course flexible ; they vary 

 more or less in length, being sometimes longer than 

 any of the other parts of the leg, and sometimes so 

 small as to be capable of concealment in a small 

 groove at the tip of the tibiae. In the comparative 

 length of their component joints they also present 

 many varieties, and these likewise exhibit a great 

 dissimilarity of form. These peculiarities of the 

 tarsi furnish valuable characters in discriminating the 

 different groups of Insects. The last joint, which is 

 usually larger than the one which precedes it, is almost 

 always armed with a pair of hooked claws, and in 

 many insects these are accompanied by a pair of soft 

 membranous pads, called pulvilli, which, when applied 

 to the surface of any object, adhere to it with con- 

 siderable firmness. These organs are very distinctly 

 seen in the common House-fly, and it is by their 

 agency that those little creatures are enabled to climb 

 up smooth perpendicular surfaces, such as window- 

 glass, and even to walk upon ceilings with their backs 



