i. _Exclusion._ The exclusion or extrusion of the impregnated eggs takes place, when, passing from the ovary into the oviduct, they are conducted by means of the ovipositor, in which it terminates, to their proper situation. By far the greater number of insects extrude them _singly_, a longer interval elapsing between the passage of each egg in some than in others. In those tribes which place their eggs in groups, such as most butterflies and moths, and many beetles, they pass from the ovaries usually with great rapidity; while in the _Ichneumonidæ_, _Sphegidæ_, _Oestri_, and other parasitic genera, which usually deposit their eggs singly, an interval of some minutes, hours, or perhaps even days, intervenes between the extrusion of each egg. One remarkable instance of the former mode I noticed in my letter on the _Perfect Societies_ of Insects[87]; another may be cited, to which you may yourself be a witness--I allude to that common moth, vulgarly called the _Ghost (Hepialus Humuli)_, which lays a large number of minute black eggs, resembling grains of gunpowder, and ejects them so fast that, according to De Geer, they may be said to _run_ from the oviduct, and are sometimes expelled with the force of a popgun[88]. A Tetrapterous insect, the genus of which is uncertain, is said, when it is taken, to discharge its eggs like shot from a gun[89]. And a friend of mine, who had observed with attention the proceedings of a common crane-fly (_Tipula_), assured me that several females which he caught projected their eggs to the distance of more than ten inches.
A few _Diptera_ extrude them in a sort of chain or necklace, each egg being connected by a glutinous matter with that which precedes and follows it. In a small species of a genus allied to _Psychoda_ (a kind of midge), which one season was abundant in a window of my house, this necklace is composed of eggs joined by their sides, not unlike those strung by children of the seeds of the mallow[90]. Other _Tipulidæ_ on the contrary extrude their eggs joined end to end, so as to resemble a necklace of oval beads. _Beris clavipes_ and _Sciura Thomæ_, two other flies, produce a chain about an inch long, consisting of oval eggs connected, in an oblique position, side by side; an arrangement very similar prevails in the ribband of eggs which drop from some of the _Ephemeræ_[91].