When the young is ready to take its place in the world, a new development occurs, which has been well related by Mr. Gosse:—
“All these phenomena have appeared in the egg we are now watching; and at this moment you see the crystalline little prisoner, writhing and turning impatiently within its prison, striving to burst forth into liberty.
“Now, a crack, like a line of light, shoots round one end of the egg, and in an instant, the anterior third of the egg is forced off, and the wheels of the infant Brachionus are seen rotating as perfectly as if the little creature had had a year’s practice.
“Away it glides, the very image of its mother, and swims to some distance before it casts anchor, beginning an independent life. At the moment of escape of the young, the pushed-off lid of the egg resumes its place, and the egg appears nearly whole again, but empty and perfectly hyaline (i.e. all but transparent), with no evidence of its fracture, except a slight interruption of its outline, and a very faint line running across it.”
To pass from the egg to a more advanced stage in life. All practical entomologists have been greatly annoyed, in their earlier years of collecting, to lose larva after larva, from the attacks of Ichneumon-flies. It is certainly rather beyond the limits of ordinary patience to discover, watch over, and secure successfully a rare caterpillar, and then to find that it has been “stung” by an Ichneumon-fly.
The veteran entomologist, however, troubles himself very little about such minor misfortunes, and, as a rule, more than compensates for them by preserving the intrusive Ichneumon-fly, and giving in his diary full details of the insect on which it was parasitic, of the plant on which the caterpillar lived, the date of its appearance, and its numbers.
Now, there are many of these parasitic insects, notably those belonging to the genus Microgaster, which invariably make doors in their cocoons. I have now before me groups of cocoons made of the two commonest British species, namely, Microgaster glomeratus and Microgaster alvearius, and in both of them each tiny cocoon is furnished with a hemispherical, hinged door. I have also some exquisitely beautiful groups of Microgaster cocoons found in the West Indies. They are the purest white, shine with a satiny lustre, and are arranged round a hollow centre, much as if they had been gummed to the outside of a very large thimble. There are many hundreds of them, and every one has its little door still open as it was when the fully developed insect first made its escape.
Another curious example of a natural door may be seen by those who will look for it.
On plants infested with aphides, or “green blight,” as the gardeners quaintly term them, may often be seen dead aphides much larger than the rest, globular, brown, and shining. These aphides have been “stung,” as it is called, by a little Ichneumon-fly belonging to the genus Ophion, and having, like all its congeners, a flat and sickle-shaped abdomen. The egg which has been laid in the aphis soon hatches, and the young Ophion absorbs into itself all the juices of the aphis. It remains within the body of its involuntary host until it is fully developed, when it cuts a tiny, but beautifully perfect circular door in the skin, and emerges, leaving the door open and still attached by its little hinge.
Considering the small size of the aphis, and that the diameter of the door is only one-eighth of the length of the insect, the perfection of its form is really remarkable.