12 ME. D. J. SCOUBFIELD ON ENTOMOSTEACA 



ScapTioleberis, at least in so far as they relate to the surface-film, 

 and many authors have long since recorded that animals be- 

 longing to this genus often swim ahout just under the surface. 

 I have myself seen groups of N. monacTia, in a quiet stream, 

 moving about close under the surface, much in the same way as 

 groups of "Whirligig-beetles move about on the surface, though 

 more leisurely. But observations of this sort are not sufficient to 

 reveal much about the exact relation of this species to the sur- 

 face-film and the specific action of its modified ventral area with 

 the curious ridges. For this purpose the same methods must be 

 used as in the previous case. Close attention to the movements 

 of an isolated specimen will show that although it swims nearly 

 vertically, the moment it touches the surface it assumes a hori- 

 zontal position, back downwards, thus bringing its straight 

 ventral margin into close contact with the surface-film. This 

 action is obviously precisely similar to that already noticed 

 in ScapJioleheris. In this position the animal may continue to 

 move about for an indefinite period, usually rather briskly, but 

 sometimes so leisurely that no doubt is left in the observer's 

 mind that the weight of its body is actually supported by the 

 surface-film. To make as sure as possible of this point, the 

 surface can be examined with the reflected beam of light as 

 already described, and then it will be found that little irregu- 

 larities are formed whenever the animal comes to the surface, 

 and that these last as long as contact is maintained. There are 

 usually three such to be seen — two lateral ones anteriorly, and a 

 median one some distance farther back. Here, again, there can 

 be DO reasonable doubt that these little irregularities are really 

 capillary depressions, and that they also must owe their origin 

 to the piercing of the surface-film by the ventral ridges, or 

 rather perhaps only the anterior parts of them, and by the 

 extremities of a pair of feet or the caudal rami. The two assump- 

 tions that these parts are water-repellent and that they can be 

 pushed through the surface-film are as necessary here to complete 

 the argument as in the case of ScapJioleheris, although in regard 

 to the first it may be noted that the general surface of the shell 

 of Notodromas can be easily show^n to be water-repellent, and 

 this of course greatly increases the probability that the same is 

 true of the ridges. 



It is not to be imagined that the explanation just given of the 

 means by which the surface-film is utilised, clears up all the 



