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. * * * * * ONE of the achievements of modern Architecture is the Self-closing Door, especially where it must of necessity close by its own weight, and when the fitting is so exact, that even the most experienced eye can scarcely detect it. Such a door is to be found guarding the nest of the Trap-door Spiders, several species of which are found scattered over all the warm parts of the earth. A side view of one of these extraordinary nests is given in the accompanying illustration, and on the other side is the common trap-door of our cellars. [Illustration: DOOR OF TRAP-DOOR SPIDER.] [Illustration: TRAP-DOOR OF COAL-CELLAR.]