DIAQRAM AND TWENTY-SIX FIGURES (PLATE) To the student of chromosomes and chromosome behavior a cytological study of the Diptera presents many features of interest. Some of these have been described or mentioned by Miss N. M. Stevens in connection with her work on the sex chromosomes,' but aside from this I know of no serious attempts at such a study. When compared with the detailed and critical works on the chromosomes of the Hemiptera, Orthoptera, and Coleoptera, this lack of knowledge of the

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... iptera is surprising, and cqn be explained only by the fact that the latter are in many ways unsatisfactory objects for cytological study. The difEculties in such a study are admittedly numerous, but notwithstanding, much can be accomplished, and the interest of the questions involved seems to me to more than justify the extra effort expended in their investigation. For this reason a series of such studies has been undertaken, the extent of which will depend upon the time and facilities available. Some are under way at present and others will be taken up subsequently. The results I hope to present in a series of papers, to which the present is introductory. 1 So far as known to the writer, the papers of Dr. Stevens are the only ones dealing with the chromosomes of the Diptera. They are four in number: The chromosomes of Drosophila ampelophila. '190'7. Proc. VII Internat. Zool. Cong. A study of the germ cells of certain Diptera, with reference to the heterochromosomes and the phenomena of synapsis. 1908. Jour.