Though it is easy to write down this heading, it is by no means so to define it. The simplest and best definition seems to be “A dipteron which spends a considerable portion of its existence in salt or brackish water,” but our present chaotic knowledge of the early life histories of shore diptera does not advance us far towards clearing up the doubtful period, and we are thrown back on the shore frequenting perfect insects, many of which are of wide distribution.