INTRODUCTION. xix 



digging, fossorial; or swimming, natatorial; in the latter form, 

 the hind legs assume the form of oars in Dytiscida3 and some 

 Hydrophilidae ; or the middle and hind legs become broad, short, 

 and flat, as in Gyrinidae. At the extremity of the tibiae are two 

 movable spines, called tibial spurs; rarely, one of these is absent, 

 but frequently both are so small as to be indistinct. 



Attached to the tibiae is a series of from three to five pieces, 

 constituting the foot, or tarsus; the last joint bears two claws, 

 which, by a very rare exception, are sometimes wanting. The 

 genus Phanseus of Sarabagidte and the family of Stylopidse are 

 the only examples in our fauna in which this is the case. Between 

 the claws is seen in many species a small appendage, called 

 onychium, which usually bears at its extremity two bristles. 



ABDOMEN. 



The portion of body behind the metathorax is called the abdo- 

 men, and consists of a series of rings, the normal number of 

 which is nine, though, by coalescence and disappearance, this num- 

 ber is not visible ; these rings are divided into two portions ; the 

 dorsal segments, more or less covered by the elytra, and the ven- 

 tral segments, visible on the under surface. The union between 

 these takes place on the dorsal surface, and is by membrane, ex- 

 cept in the last two, which are frequently very closely united. 



The breathing pores, or spiracles, are situated in the connect- 

 ing membranes, or in the upper inflexed portions of the ventral 

 segments. 



The ventral segments are not always opposed to and connected 

 with the corresponding dorsal segments, but are situated differ- 

 ently in different families, though no use is made of these differ- 

 ences for systematic arrangement. 



The anal aperture is between the last dorsal and ventral seg- 

 ments, and above it, in the same fissure, is situated the genital 

 opening ; each side of this are horny valves, rarely visible exter- 

 nally, but sometimes of very complex structure constituting the 

 genital armature. 



The last dorsal segment is called the pygidium, and the penul- 

 timate the propygidium, when they are exposed behind the elytra. 



