70 INSECTS ABROAD. 



sideways, it is seen to bear some resemblance in outline to a 

 tortoise. The under surface is nearly flat, while the upper rises 

 with a bold ridge in the middle, and thence flattens down to the 

 tips of the elytra and the end of the snout. 



The genus Porrorhynchus is a very large one, and has repre- 

 sentatives in many parts of the world. In the British Museum 

 there are specimens from North and South Africa, Madagascar, 

 North and South America, India, and the Philippine Islands. 



In the whole of the Water Beetles, however diverse theii 

 size, form, colour, or habitat, one characteristic is common to 

 all, — namely, the polished smoothness of the entire surface, the 

 manner in which all angular projections are avoided, and the 

 absolute closeness with which the elytra are fitted together, so 

 as to be net only water-tight but air-tight also. This structure 

 is absolutely needed, because, although finding their food in the 

 water, and passing the greater part of their time beneath its 

 surface, they are really denizens of air, and not of water. 



In fact, they play much the same part among the Insects as 

 do the whales, seals, and dolphins among the Mammalia, living 

 in the water though they do not breathe it, and imitating the 

 fishes in mode of life though differing from them in mode of 

 respiration. During their imperfect or larval life, they were 

 actually inhabitants of the water, and capable of extracting the 

 oxygen from it by means of gills, just as the fish do ; but when 

 they attained the perfect state, the gills, or " branchiae," as they 

 are scientifically termed, were lost, and another system of respi- 

 ration was developed. Like all other insects, they then begin 

 to breathe the same air as ourselves, but, instead of having the 

 respiratory apparatus confined to the lungs, as is the case with 

 us, they have it extending over the entire body, the tubes 

 through which the air passes running even to the ends of 

 the antenna?, and terminating in a series of apertures called 

 "spiracles" along the sides. 



Now, it is evident that if an insect has to fulfil two appa- 

 rently opposite conditions — i.e. living beneath the water and yet 

 breathing atmospheric air — it must possess some peculiar modi- 

 fication of structure whereby the air is ensured admission into 

 the Bpiracles and the water is kept out of them. These condi- 

 tions are fulfilled by the structure of the elytra, which are wide 



