These insects undergo four different moults before they become nymphs, and at each moult their appearance is completely changed. It may be easily understood that it was long before these little beings were recognized behind their masks. This is the manner in which they ravage our flowerbeds. While they still wear the dress of larvæ, they cling to certain female hymenoptera which they know very well; and being fully assured that the door would be shut in their face if they presented themselves openly, they enter, on their neighbour's back, the galleries where their housekeeping is carried on, and at the instant that the female host lays an egg in a cell of honey, the young Meloë glides in with it, and allows itself to be shut in. During this time it continues its metamorphosis, lying in a lake of honey; it devours it all at its ease, caring nothing for the provision laid up for the hymenoptera which introduced it. It is a brigand who, having secreted himself in the carriage of a rich neighbour, introduces himself on his shoulders into his children's bed-chamber, assassinates them, and grows fat on the provisions destined for his victims. "The _Sitaris_, the _Meloë_, and apparently other Meloedeæ, if not 172 all of them, are, when young, parasites of certain hymenoptera," says Mons. Fabri, who has watched with rare sagacity the obscure and interesting habits of these microscopic assassins. The _Sitaris humeralis_ has a progressive development at first, a recurrent one afterwards, and then again it becomes progressive. Aphides which are not yet full grown, and which arrest the exuberant vegetation of certain plants, are in their turn attacked by an insect which is by no means lukewarm in its proceedings. A small species of cynips (_Allotria victrix_) lays its eggs, like an ichneumon, in the body of a rose aphis, and multiplies rapidly at their expense. (Westwood). There are certain flies which are not more delicate in their mode of life than the preceding insects. We allude to the _OEstri_. We give the representation of the species which attacks the horse. [Illustration: Hinder part.] [Illustration: 36.--OEstrus of the Horse.] [Illustration: Anterior part.] Instead of making their attacks on those of their own class, the gadflies prefer to instal themselves on mammals and sometimes even on man. Fortunately their wants are not very great; they are contented with a little. Their presence can at most only cause some uneasiness, 173 or some trifling functional trouble. The oestri are dipterous like ordinary flies; but instead of passing their youth on some waste organic matter, they live in the nostrils or the stomach of some hairy animal, and undergo all their metamorphoses in the interior of its body. Thus they pass all their youth in a _crèche_; but when they have reached the adult state, they get their own living in freedom.