170 A NATURALIST IN HIMALAYA 



wounded or dying. Water-boatmen, when handled, 

 sometimes give a sharp bite. I am sure they inject 

 a small dose of poison, as the numbness remains for 

 twenty-four hours and resembles that of a bee-sting. 

 I suspect that the boatmen, in addition to drowning 

 their prey, possess also the power of injecting poison 

 into the insect, especially if its struggles make it 

 difficult to control. 



The manner In which these insects swim on their 

 backs is their most interesting feature. The long 

 hind legs make a most perfect pair of paddles, and the 

 plan on which they work may be easily observed 

 when the boatman is placed in a glass of water. From 

 the posterior margin of the tibia and tarsus there is 

 directed backwards a double fringe of delicate hairs. 

 At each thrust of the limbs these two fringes separate; 

 the hairs spread out so as to form a broad resisting 

 blade through the medium of which the pressure is 

 exerted on the water. As the limb is again brought 

 forward the fringes collapse, and a narrow edge, 

 offering the minimum of resistance, is now presented 

 to the water. We know that in flight the point of 

 the wing of a bird or insect not only moves to and fro 

 but traces in the air a succession of ellipses. I think 

 it is possible to detect a similar motion in the 

 swimming-paddles of the water-boatmen. They not 

 only oscillate but also rotate. It is difficult to follow 

 this in the rapidity of their motion, but, when I placed 

 the insects in a glass of water coloured with methylene 

 blue, I felt certain I could see the particles of pigment 

 being whirled round in a spiral at each rotation of 

 the limbs ; but it is difficult to be confident of this 

 observation. 



