THE SWIMMING ICHNEUMON FLY. 261 



these advantages ; nor can much respiration take place 

 through the skin, which is thick and chitinous. 



(t Moreover, it has some difficulty in passing from 

 air to water, or vice versa : a bubble of air would 

 quite destroy its equilibrium when under water, and a 

 drop of water would equally prevent free motion in the 

 air. The difficulty is, however, mitigated by the fact 

 that the air in the tracheae requires changing only at 

 considerable intervals. A common house fly placed 

 under water ceased to move in half an hour. My 

 specimens, however, of Polynema natans lived under 

 water several hours without suffering any apparent 

 inconvenience. One, which I put in a bottle full of 

 water at 7 o'clock in the morning, was quite lively at 

 7 o' clock in the evening, after having therefore been 

 no less than twelve hours at least under water. I say 

 at least, because I had no means of knowing how long 

 it had been there before my experiment began. Pro- 

 bably, however, this was about the limit of its endur- 

 ance ; for four other specimens which I treated in the 

 same manner at about 6 o'clock in the evening, were 

 apparently dead at the same hour on the following 

 morning ; and the individual above-mentioned was 

 itself motionless at 9 o'clock, or after fourteen hours 

 of submersion. I then, however, put it in a dry bottle, 

 and next morning it was as lively as ever. Wishing 

 to see whether it retained any unpleasant recollections 

 of its drowning, I gave it the opportunity of again 

 entering the water, which it immediately proceeded 

 to do. 



" I was unfortunately unable to ascertain whether 

 they could fly : taking my opportunity when they 



