96 MODERN CLASSIFICATION OF INSECTS. 



deeply immersed in the protlioracic cavity ; the eyes large and but 

 slightly prominent ; the upper lip is transverse {fig- 5. 2. head of 

 Dyticus marginalis) ; the upper jaws short and robust, with a strong 

 internal tooth (Jig. 5. 3. mandibles of Dyt. marg.) ; the lower jaws short, 

 flat, and internally ciliated, with the tip acute ; the outer lobe is palpi- 

 form, and is of nearly equal length with the jaw itself; the true max- 

 illary palpi are about twice as long as the jaws (Jig. 5. 4. maxilla of 

 Dyt. marg.j ; the mentum is transverse, with the sides produced into 

 two lobes ; the under lip is short and square, and the labial palpi about 

 twice as long as the lip and 3-jointed; when unemployed they are 

 generally folded backwards {Jig. 5. 5. labium, &c. of Dyt. marg.). 

 The general structure of the mouth is more compact, and it is capable 

 of being more closely shut than in the predaceous Land-beetles, a 

 peculiarity beautifully dependent upon the opposing nature of the 

 element in which these insects reside. The thorax is broader than 

 long, and of equal breadth at its base with the elytra, its sides being 

 rugose in the females of some species ; the elytra are generally de- 

 pressed, with deep longitudinal and pilose furrows in the females of 

 some of the larger species; the presternum is terminated by an acute 

 point received into the emargination of the anteriorly produced and 

 furcate metasternum (^g. 5. 7. meso- and meta-thorax of Dyticus, 

 beneath). The anterior legs are lodged in a deep cavity on each side 

 of the base of the prosternum ; the legs are of moderate length, the 

 anterior being the shortest and the posterior the longest; the tai'si are 

 5-jointed, the two posterior pairs being flattened and strongly ciliated, 

 so as to be employed in swimming, being capable only of a horizontal 

 motion (^g. 5. 7.). In many species the basal joints of the four 

 anterior tarsi are dilated, whilst in some of the larger species the two 

 anterior male tarsi have the three basal joints enlarged into a broad and 

 nearly circular shield, convex above, fringed with fine hairs and 

 cushioned beneath, or rather covered with a multitude of minute in- 

 verted caps, with several larger portions resembling suckers, varying in 

 number and size in the various species (Jfff. 5. 6. anterior tarsus, 

 Dyticus marginalis S' , seen beneath ; Jig. 5. 8. ditto, Cybister <J ; 

 ^g. 5. 9. ditto, Hydaticus ^ ). This structure enables the male to 

 retain his situation upon the back of the female during copulation, 

 the rugosities upon the thorax and elytra of the latter being also 

 similarly serviceable. The elytra are furnished at the base, internally, 

 with a pair of remarkable small circular membranous and ciliated 



