One hot afternoon in the early part of August, 1893, while out collecting, I found a large web of the Fall Web-Worm (Hyphantriacunea, Drury), on a young apple tree. Reaching up to cut off the twigs to which the web was attached, my attention was attarcted by an unusual disturbance among the immates of the web. Closer insepection revealed the fact that a medium-sized Ichneumon fly had intruded within the family circle, and was proving herself a most unwelcome visitor. Tearing into the web with her feet she would force her way along until she arrived under the skeletonized leaves upon which the larvæ were resting and through which they were plainly visible.