It is closed with wire-gauze, whose meshes, two millimetres wide (.078 inch .-- Translator's Note.), do not permit of the Fly's passage.

The Bluebottle comes to my apparatus, guided by scent rather than sight.

She hastens to the test-tube, whose contents are veiled under an opaque cover, with the same alacrity as to the open tube.

The invisible attracts her quite as much as the visible.
She stays awhile on the lattice of the mouth, inspects it attentively; but, whether because circumstances failed to serve me, or because the wire network inspired her with distrust, I never saw her dab her eggs upon it for certain.

As her evidence was doubtful, I had recourse to the Flesh-fly (Sarcophaga carnaria).
This Fly is less finicking in her preparations, she has more faith in the strength of her worms, which are born ready-formed and vigorous, and easily shows me what I wish to see.