In the summer of 1889, while engaged in an investigation of the habits and life-history of the horn fly of cattle (Haematobia serrata), the writer at various times brought to Washington from different points in Virginia, large quantities of cow-manure collected in the field, and eventually succeeded in working out the complete life-history of the horn fly, as displayed in Insect Life, Vol. II., No.4, October, 1889. In this article the statement is made, in concluding, that the observations were greatly hindered and rendered difficult by the fact that fresh cow-dung is the nidus for a number of species of Diptera, some about the same size and general appearance as the horn fly, and that no less than twenty distinct species of flies had been reared from horse- and cow-dung, mainly the latter, and six species of parasitic insects as well.