170 PHYSIOLOGY OF INSECTS. 



first, second, third, fourth, and fifth joints of the tarsus, 

 or planta, metaplanta, allux, arthrium and ungula ; the 

 planta being nearest to the tibia, the others in regular 

 succession. In pedunculated insects the five joints of 

 the tarsus are almost invariably present ; in other insects 

 one, two, or three of them, are frequently missing : when 

 an insect has four joints to the tarsus, the arthrium is 

 wanting ; when only three, the arthrium and allux have 

 disappeared. The ungula is armed at its extremity with one, 

 two, or four sharp curved claws or unguicula, a 5 : in the 

 stag-beetle a sixth joint of the tarsus called the plantula 

 arises from between the claws, and this joint is furnished 

 with two other claws. The under surface of the joints of 

 the tarsus is very frequently covered with soft cushions 

 or pulvilli ; these are composed of an almost infinite 

 number of minute hairs, which, at their ends, are slightly 

 dilated, and constantly exude an adhesive gummy matter. 

 By means of these gummy pulvilli, insects possessing 

 them are enabled to walk freely on smooth surfaces, even 

 with their backs downwards, against the power of gravity : 

 the power of overcoming the laws of gravity seems to have 

 sorely perplexed our philosophers, ancient and modern ; 

 some thought the act perfectly supernatural, and the effect 

 of a charm or enchantment ; others, with equal gravity, 

 attributed it to suction, and imagined an insect carried a 

 kind of air-pump in each foot, to exhaust the air between 

 the foot and the surface to which it adhered ; and, until 

 lately discovered by Mr. Lister, the true cause of the 

 phenomenon was unknown. We often thus content our- 

 selves with any theoretical mode of accounting for a phe- 

 nomenon, and fondly cling to a fiction savouring of the 

 wonderful, rather than take the pains to seek a rational 

 and intelligible explanation. There are other insects whose 

 pulvilli exude an oily or greasy matter, and these not only 



