30 THE BOOK OF NATURE STUDY 



The majority of beetles are terrestrial in their habits ; but 

 a fair number have become aquatic, and of these the largest 

 and one of the most abundant is Dyticus (often wrongly spelt 

 " Dytiscus ") marginalis. The mature insect is more than an 

 inch long ; its dorsal surface is of a deep brown-green colour, 

 excepting a yellow margin bounding the sides of the thorax 

 and outer edges of the elytra ; the ventral surface is of a lighter 

 brown tint. The surface is smooth and polished, and the outline 

 that of a long oval, the head being partly sunk into the thorax, 

 so that the " lines " of the animal suffer no interruption at the 

 neck. Along the mid- ventral line of the body, especially in 

 its anterior regions, is a ridge, or keel ; while the dorsal surface 

 is gently and evenly rounded. It will then be seen that the 

 form of the animal is admirably suited for swift passage through 

 the water. The success of the design of this living boat is testified 

 by the fact that Dyticus is carnivorous, feeding on soft-bodied 

 aquatic animals, including its own larvae, and even catching 

 as they swim such swift movers as fishes ; indeed, this beetle, 

 whose ancestors we cannot doubt to have been terrestrial, has 

 become so modified in many ways as now to be very perfectly 

 adapted to life in water. The rigidity of the " hull " (body), 

 notably deficient in most beetles by reason of the loose articula- 

 tion of the first (prothorax) to the second (mesothorax) segment 

 of the thorax, is here secured by the development of a strong 

 backwardly directed spine upon the ventral surface of the 

 prothorax, and a slot, into which the spine fits firmly, on 

 the anterior edge of the mesothorax. By this contrivance 

 pro- and mesothorax are locked together, and lateral bending 

 is prevented. Furthermore, the meso- and metathorax (third 

 segment) are intimately united to one another, and the 

 ventral plates of the first three abdominal segments are soldered 

 together so as to prevent all movement ; while the flexible 

 joint between thorax and abdomen is strengthened partly in 

 the mid-ventral line by a long and forked backward process 

 from the ventral plate of the third thoracic segment (meta- 

 sternum), and partly by the very large and flattened coxal 

 (first) joints of the hind pair of legs, being so placed as to 

 overflap the fore part of the abdomen, and, moreover, being 



