INSECTA. 



245 



however, select but one other illustration before leaving this part 

 of our subject, namely, the conversion of these organs into instru- 

 ments for swimming, whereby, in aquatic insects, they become adapt- 

 ed 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 examining 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 marginalis (Jig- 109, c), 



Fig-. 109. 



the two anterior pairs of legs, that could be of small service as in- 

 struments 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 developement of the hinder seg- 

 ment of the thorax, in order to approximate their origins to the 

 centre of the body, and the individual segments composing them 



