314 NATURAL HISTORY. 



of the body, and ending in a sharp point, is developed at the end, in some tropical African species, into 

 a very long and sharp stiletto, and serves, no doubt, as a defensive weapon. The allied genus (Tropis- 

 ternus). of which there ai-e many scores of smaller species in America, North and South, some of 

 metallic colour and others striped with yellow, also possesses this armature. In Hydrous caraboides, 

 a British species, next in siza to Hydropliilus piceus, the keel is very short, not passing beyond the 

 haunches of the hind legs. Besides these two large species, a considerable number of smaller Hydro- 

 philidse are found in our ponds and streams, some, like the Berosi and the rare Spercfieus etnarginatus, 

 wallowing in the soft mud at the bottom. 



FAMILIES GEOKYSSID^E, PARNID^E, AND HETEROCERID^E. 



Some authors associate with the PALPICORNIA, or Phihydrida, the three families, GEORYS- 

 SID/E, PARNID.E, and HETEROCERID.E, induced to this course probably by considerations of 

 convenience in reducing the number of detached families, rather than by any real similarity of 

 organisation. The only constant points of agreement between these families and the Palpicornia 

 are their aquatic mode of life and phytophagous habits. They vary much in the form of the 

 antennae, which are clavate only in some of the genera, and none of them have swimming 

 feet; but in all the maxillae and their lobes are, as in the Palpicornia, unarmed. As the only 

 alternative is to treat them as so many separate tribes, we may introduce the little we have 

 to say of them in this place. 



The GEORYSSID^E are small insects of short, convex, and solid build, inhabiting damp places near 

 water. Only seventeen species are known, chiefly from north temperate regions, but one or two have 

 l>een of late years detected in Australia and Ceylon. Their antenna? have nine joints, the last three 

 forming a club ; the chin (mentum) is large and horny ; the elytra are entire ; the anterior haunches 

 are cylindro-conic and exserted, and the posterior transversal ; and the abdomen is composed of five 

 segments only. The habits and early stages are unknown. One species (Georyssus pygmceus} 

 inhabits the British Islands. 



The PARNID.E are little Beetles of oblong, sometimes nearly cylindrical, form, and of strictly 

 aquatic habits. The principal species are clothed with a dense silky pile, which, on account of its 

 hydrofugal properties, has the important functional effect of aiding in the respiration of the insects, 

 by attaching to the body, at the moment of immersion, a globule of air sufficient for their needs 

 during long periods of submergence in the water. Thus, with more facility than a diver in his 

 diving-bell, the Parnus can move about the depths of the pool, carrying the needful supply with him 

 in his wanderings. According to Erichson, the globule is surrounded by a thin film of oily or viscous 

 matter, secreted by the hairs with which the body of the insect is clothed. The antenna) of the 

 Parnidre are variable in form in the different genera : in the sub-family Psepheninse being saw-shaped ; 

 in the typical Parninse clavate, sometimes lodged in repose for protection in a groove under the eyes, 

 and bearing a branch-like process from their second joint ; and finally, in the Elmime filiform and 

 simple. The mouth oi'gans are feebly developed. The legs are slender, and the last joint of their 

 feet is much elongated, with highly-developed claws. The latter peculiarity is of great importance 

 in the economy of the species, most of which inhabit swiftly-running waters, and require grappling 

 instruments, such as are afforded by these strong feet and long claws, to prevent their being swept 

 from the roots and stems of aquatic plants, or from the mossy stones amid which they find their 

 food. The longest feet and claws are seen in the sub-family Ehninse, these members reaching a truly 

 remarkable degree of development in Macronychus quadrituberculaius, a rare and local British 

 species, found in the river Trent. 



The HETEROCERID;E, like the GEORYSSID^, consist of one genus only. The family contains, how- 

 ever, upwards of seventy species, distributed over nearly the whole globe. They are not strictly 

 aquatic, preferring damp, sandy, or marly soil, on the margins of pools and ditches. They differ from 

 the Parnidse farther in the larger development of the parts of the mouth, and in the feet, which are 

 adapted for burrowing, being short and strong, with the shanks of the two anterior pair widened, 

 and furnished 011 their outer edge with a row of spines. The insects, in fact, have the habit of 

 burying themselves in the loose eai-th, in which labour their fossorial legs are aided by the robust 

 head and the great mobility of the prothorax. 



