GEEAT WATER BEETLE. 239 



GREAT WATER BEETLE (Dyticus marginalis) . 



It is impossible to point to a more perfect example 

 of design than this creature, which is made for two 

 elements only, namely, water and air, and is quite 

 helpless when on land. The whole of the insect is 

 covered with a very hard shelly skin, composed of the 

 horny substance called " chitine/' which is, I believe, 

 peculiar to the insect tribes. The surface of the 

 insect has such a polish, and is so hard, that to hold 

 one of these beetles in a living state is not at all an 

 easy matter ; and even when they are dead and dry, 

 they are very apt to slip through the fingers. They 

 are absolutely impervious to water, which has no hold 

 on their surface ; and, if the beetle be captured, as 

 it comes up after a dive, it will be found that not a 

 drop of water has adhered to its polished surface. 



The wing-cases, or elytra, as they are technically 

 named, are very large, somewhat convex, and come 

 well over the sides, to which they cling so tightly, that 

 no water can penetrate the junction. Under the elytra 

 are the ample and powerful wings, which, in spite of 

 their size, are packed so closely as to leave space for 

 the curious structure which enables the insect to 

 remain under water for a considerable time. 



The reader is probably aware that insects have no 

 lungs, and do not breathe through nostrils as do the 

 higher animals ; but that their whole body is perme- 

 ated with air tubes opening into certain apertures 

 along the sides, called spiracles. 



It is evident, therefore, that the body of the insect 



