14 ME, D. J. SCOUEFIELD ON ENTOMOSTEACA 



practically invisible to the naked eye. That such minute par- 

 ticles also occur upon the surface of ponds and ditches &c. is, 

 however, rendered fairly certain by the fact that in all 

 situations there is known to be a continuous rain of fine dust, 

 a proportion of which is always organic in origin. There is 

 another source of surface-food particles which must be men- 

 tioned, although its value in this case is unknown. Some 

 Bacteria in the zoogloea state often form extensive patches on 

 still surfaces, and these frequently afford a sort of rendezvous 

 for numerous small forms of life such as Ehizopods and Infu- 

 soria. On the whole, it seems reasonable to suppose that the 

 surface-film does supply these animals with abundant and varied 

 food-material, for which, too (and this, after all, is the crucial 

 point), there is, so far as is yet known, very little competition. 

 It has occurred to me that this peculiar power of obtaining food 

 from the surface may largely explain why these animals are 

 never seen far from the shore. Prom their structure it might 

 be supposed that they could live equally well at any part of the 

 surface of a piece of water, no matter how large, and so probably 

 they could if food were equally abundant in all parts. But, 

 owing to the surface-drift produced by movements of the air, 

 the middle portions of the area of even small ponds, if not too 

 much sheltered, are always much cleaner than the marginal 

 portions. The third advantage mentioned, namely the compara- 

 tive easiness of respiration, following as it does directly from 

 the perfect aeration of the surface-water, demands no special 

 comment. 



As a contrast to the foregoing advantages, it should be re- 

 marked that there seems to be one very probable disadvantage 

 attaching to this mode of life. It can scarcely be doubted that 

 the animals using the surface-film for the purpose of support 

 are much exposed to the attacks of predaceous insects living 

 upon the surface, such as the "Whirligig-beetles {Oyrinus). Un- 

 fortunately no positive proof of this has yet been obtained. If, 

 however, this view is subsequently substantiated, as seems 

 probable enough, the remarkable darkening of these creatures 

 on and near the ventral surface could then be interpreted as an 

 example of protective coloration, for it can be readily observed 

 that their dark colour renders them very inconspicuous in their 

 normal habitats, when seen from above. 



