On the 21st September, 1891, on opening a small box of neglected insects, etc., I caught a glimpse of several minute black files as they flew out. At the time I could not imagine what they were. The next day I collected a lot of pupæ of Tachina flies and enclosed them in tin boxes to transform to imagines, but instead of the perfect fly appearing, the boxes were filled with the same small flies as I had observed issue from the opened box on the day previous, and then, as I remembered having had some of those Tachina puparia in the box, I readily understood whence the flies came.