424 The Great Water-Beetle. 



aquatic life, this circumstance must strike the mind of the 

 most heedless observer. The limbs used in swimming exhibit 

 the same parts, the same number of joints, and almost the 

 same shape as those employed for creeping*, climbing-, leaping, 

 and numerous other purposes ; yet how different is the function 

 assigned to them. In a common water-beetle, the Dyticus 

 marginalis, the two anterior pairs of legs, that could be of 

 small service as instruments of propulsion, are so small as to 

 appear quite disproportionate to the size of the insect, while 

 the hinder pair are of great size and strength ; the last-men- 

 tioned limbs are, moreover, removed as far as possible by the 

 development of the hinder section of the thorax, in order to 

 approximate their origins to the centre of the body, and the 

 individual segments composing them are broad and compressed, 

 so as to present an extended surface to the water, which is still 

 further enlarged by the presence of flat spines appended to 

 the end of the tibia, as well as of a broad fringe of stiff hairs 

 inserted all round the tarsus. The powerful oars thus formed 

 can open until they form right angles with the axis of the 

 body, and from the strength of their stroke are well adapted 

 to the piratical habits of their possessors, who wage successful 

 war, not only with other aquatic insects and worms, but even 

 with small fishes, the co-inhabitants of the ponds wherein they 

 live." 



The eggs, which are cylindrical in form, are deposited at 

 different times in the summer and autumn. Like the eggs of 

 the Ephemera, they are dropped in packets into the water, and 

 left to take care of themselves. It is said that the larva is 

 hatched in about the course of a fortnight, and that it casts its 

 skin when four or five days old, at which time it is about five 

 bines long. As the growth of the larva proceeds, the skin, 

 which, as in the Crustacea, becomes too tight for the body, is 

 repeatedly cast, and it is very common to see these cast-off 

 exuviae floating amongst the duckweed and conferva? of ponds. 

 A full-grown larva is about two inches in length, and is said to 

 attain its full size in about fifteen days. It then quits the 

 water, and forms itself a round cavity in the adjacent bank, 

 changing in about a week's time to a pupa. (See Figure.) 

 In this state it continues for two or three weeks, and then 

 changes to the perfect insect, which at first is soft and yellow- 

 ish, but gradually hardens, and acquires its proper consistency 

 in about the space of eight days. 



Let us now take a view of some parts of the internal struc- 

 ture of the beetle, and more particularly the intestinal canal, 

 or digestive apparatus. Wo will place this specimen (which 

 we know from the furrows on the elytra to be a female) in a. 

 small gutta-percha trough, with the back uppermost, fastening 



