DYTISCUS MAKOINALIS. 



303 



select but one other illustration before leaving this part of our subject, 

 namely the conversion of these organs into instruments for swim- 

 ming, whereby, in aquatic insects, they become adapted to act as oars. 

 Nothing is, perhaps, better calculated to excite the admiration of the 

 student of animated nature than the amazing results obtained by the 

 slightest deviations from a common type of organization ; and in ex- 

 amining the changes required in order to metamorphose an organ which 

 we have already seen performing such a variety of offices into fins 

 adapted to an 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 already referred to, the Dytiscus marginalia 

 (fig. 152, c), the two anterior pairs of legs, that could be of small service 



Fig. 152. 



Metamorphoses of Dytiscus. 



as instruments of propulsion, are so small as to appear quite dispropor- 

 tionate to the size of the insect, while the hinder pair are of great size 

 and strength ; the last -mentioned limbs are, moreover, removed as far 

 backwards as possible, by the development of the hinder segment 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 to the water an extensive surface, which is still further 

 enlarged by the presence of flat spines appended to the end of the tibia, 



