36 



INSECTS. 



is the hind leg or oar of the common water-boat- 

 man. This leg is a true oar, and little more. It is 

 elongated and strongly fringed with hairs, only two joints 

 Fig. 10. Fig. 11. Fig. 12. 



Hind leg of 

 Notonecta. 



Hind leg of (jryrinus extended, 

 greatly magnified. 



The same contracted. 



are developed in the tarsus, and the claws are generally 

 wanting. 



Fig. 11 is the hind leg of the little shiny black "whirli- 

 gig beetle," also a denizen of the waters, where it is ren- 

 dered conspicuous by the marvellous rapidity of its evolu- 

 tions. The wonderful little living paddles by which these 

 motions are made deserve close attention, and far exceed 

 in beauty the oars of the Notonecta. They are quite flat, 

 the femur, tibia, and joints of the tarsus being composed 

 of horny plates beautifully articulated together. The femur 

 and tibia are triangular. The three upper joints of 

 the tarsi are excessively dilated on the inner side, so 

 that when expanded they form, with the crescent-shaped 

 fourth and fifth joints, a thin semicircular disk. The 

 limb in this state opposes to the water these broad flat 

 horny plates. When contracted (fig. 12), the tarsal joints 

 fold over each other like the vanes of a fan, and may 

 consequently be drawn through the water with little 



