REVERTING to the recent correspondence under this heading between Mr. Davenport Hill and Prof. Hickson (NATURE, August 24 and 31), I recall that a few years back many house-flies with Chelifers attached were sent to me at the Natural History Museum for determination of the species and explanation of the phenomenon. The first task was as easy as the second was difficult. The Chelifer was in most, nay in all, cases, so far as my memory serves, Chernes nodosus. But those who suggest that the explanation is to be sought and found in the value of the habit as a means of securing dispersal hardly realise, I think, the difficulties in the way of its acceptance. Chelifers are minute, active, and, for arthropods, not exceptionally prolific. Hence the sufficiency of “elbow-room” for the survivors of a family of, say, forty, on the site chosen by the female for her progeny does not coincide with the view that they have special need of transportation. Moreover, when we remember that a Chelifer attached to a fly is exposed to the danger of being killed by the enemies of that insect, and also to the great chance of being landed in a wholly unsuitable environment, it can hardly be maintained that the advantage derived from this method of dispersal has been a sufficiently important factor in survival to preserve and foster an initial instinct to grab and hang on to the legs of flies. That the aerial porterage thus secured, whether fortuitously or “intentionally,” must be a means of dispersal is too obvious to dispute; but I do not think more than that can be claimed for it, since it is as likely to end in failure as in success.