

THE ANIMALS OF THE FRESHWATER AQUARIUM 119 



swiftly, and their methods of breathing. As to the first, note 

 that the animals are peculiarly helpless on land, where they can- 

 not walk but only execute ungainly scrambling movements. 

 Put them back into the water and notice the power of swift 

 swimming, and the adaptations which make it possible. A few 

 points only can be noticed here. First of all, note that the last 

 pair of legs is inserted very far back. You will of course not omit 

 to draw the attention of the class to the fact that this also occurs 

 in the ducks, and makes them at once clumsy on land and grace- 

 ful in water. If a museum be available it will serve to demon- 

 strate the further point that in the penguins the legs are inserted 

 even farther back than in ducks, and the result, as in the water- 

 beetles, is an extraordinary clumsiness on land ; the true seals 

 afford another example of the same thing. Again, the water- 

 beetle really rows itself along by means of its flattened legs. 

 You will note that the power of movement of the leg, like that 

 of an oar, is limited to one plane ; ordinary insects can move 

 their legs vertically as well as horizontally, but the water-beetle 

 only possesses the latter power. Again, a skilful rower feathers 

 his oar to diminish the resistance in his preparation for the next 

 stroke ; do not omit to notice that the beetle's legs are furnished 

 with a series of bristles which can be raised or lowered so as 

 to increase or diminish the resistance, the lowering of which is 

 equivalent to the feathering of the oar. If a freshwater cray- 

 fish can be kept for a day or two in the aquarium an adaptation 

 on similar lines may be observed in its tail-fan, which can be 

 spread out or folded into very small compass, according as the 

 creature is giving the effective stroke or merely getting into 

 position for the next stroke. 



In regard to breathing, we have already noticed how the 

 beetle breathes at the surface. The wing-covers are somewhat 

 curved, while the body beneath is flattened. There is thus a 

 space, an air-chamber, left between the tightly fitting wing-covers 

 and the surface of the body. Into this space the breathing 

 tubes open by pores or stigmata, and it thus forms a reservoir of 

 air which the beetle may take with it during its excursions through 

 the water. 



Under exceptionally favourable conditions the female Dyticus 



