Ill FEET 105 



foot is extremely variable; it is very rarely absent, but may 

 consist of only one piece ^joint, as it is frequently called^ 

 or of any larger number up to five, which may be considered 

 the characteristic number in the higher Insect forms. The 

 terminal joint of the tarsus bears normally a pair of claws ; 

 between the claws there is frequently a lobe or process, 

 according to circumstances very varied in different Insects, 

 called empodium, arolium, palmula, plantula, pseudonychium, 

 or pulvillus. This latter name should only be used in those 

 cases in which the sole of the foot is covered with a dense 

 pubescence. The form of the individual tarsal joints and the 

 armature or vestiture of the lower surface are highly variable. 

 The most remarkable tarsus is that found on the front foot of the 

 male Dytiscus. 



It has been suggested that the claws and the terminal append- 

 age of the tarsus ought to be counted as forming a distinct joint ; 

 hence some authors state that the higher Insects have six joints 

 to the feet. These parts, however, are never counted as 

 separate joints by systematic entomologists, and it has recently 

 been stated that they are not such originally. 



The parts of the foot at the extremity of the last tarsal joint 

 proper are of great importance to the creature, and vary greatly 

 in different Insects. The most constant part of tliis apparatus 

 is a pair of claws, or a single claw. Between the two claws 

 there may exist the additional apparatus referred to above. This 

 in some Insects notably in the Diptera reaches a very complex 

 development. We figure these structures in Pelopaeus spinolae, 

 a fossorial Hymenopteron, remarking that our figures exhibit the 

 apparatus in a state of retraction (Fig. 59). According to the 

 nomenclature of Dahl and Ockler ^ the plate {h) on the dorsal 

 aspect is the pressure plate {Bruck-Platte), and acts as an agent 

 of pressure on the sole of the pad (C, e) ; c and d on the 

 underside are considered to be extension-agents; c, extension- 

 plate ; d, extension -sole (StrecJc- Platte, Streck-Sohle). These 

 agents are assisted in acting on the pad by means of an elastic 

 bow placed in the interior of the 'latter. The pad (e) is a very 

 remarkable structure, capable of much extension and retraction ; 



1 In entomological language the piece between each two joints of an appendage 

 is itself called a joint, thougli segment is doubtless a better term. 



2 Arch./. NaturgescMchte, Ivi. 1890, p. 221. 



