During investigations on the ecology of Aphids infesting the potato crop it became necessary to study the precise effect of the different meteorological factors upon migration. Traps, treated with adhesive material, which depended on a wind-vane to keep them at right angles to the direction of migration were first used but proved unreliable, since it was found that aphis migration attains a maximum when there is little or no wind. Kites, also, proved impracticable for the same reason. It was, therefore, essential to construct a trap which worked independently of meteorological conditions. I am indebted to Dr. C. B. Williams and Mr. P. S. Milne, of Rothamsted Experimental Station, for the opportunity of discussing and examining the construction of their electrical mechanical trap, described in the preceding paper, which formed the basis of one constructed at the College Farm, Aber, near Bangor. Since, however, the village of Aber has no electricity supply it was necessary to alter, materially, the mode of construction in order to drive the trap by water-power. For this reason and, since the modified form can be used in districts where there is no electricity supply, it was suggested that the present account of the water-power mechanical insect trap should be published along with the description of the Williams and Milne trap. I am, therefore, grateful to the authors of the preceding paper for the opportunity of seeing their manuscript, thus avoiding, by reference, repetition of the description of several parts.