The cradle of these Diptera is the interior of leaves. They work as the miners of the vegetable world, in the parenchyma or cellular tissue of the leaf, between the two epidermal membranes. The henbane, the sorrel, and the thistle, especially nourish them. If one holds a leaf in which one of these miners has established itself against the light, one sees the workman boring the vegetable membrane. Its head is armed with a hook, formed of two horny pieces, and with this hook it digs into the parenchyma of the leaf. The effect of this digging is visible, as those places become by degrees transparent. Each blow detaches a small portion of the substance of the leaf. It is thus that these miners hollow out galleries for themselves, in which they find shelter, food, and security. Some are changed into pupæ in the gallery which they have hollowed out, others go out of the leaves when they are near their final transformation. _Section of Acalyptera._--The _Acalyptera_, which are the last of the great tribe of _Muscidæ_, comprehend the greater number of these insects. Their constitution appears to be peculiar and slow. They live principally in the thickest part of woods, on grasses, and aquatic plants. Fearing the lustre and warmth of the sun, they never draw the nectar from flowers. Their flight is feeble, and they never indulge in those joyous ethereal dances which we have mentioned when speaking of the preceding groups. Their life is generally melancholy, obscure, and hidden. Some of them seek decomposed animal and vegetable substances, others living vegetables. We shall only be able in this immense group of _Muscidæ_ to mention a few types which are interesting from various reasons, such as the _Helomyzæ_, the _Scatophagæ_, the _Ortalides_, the _Daci_, and the _Thyreophoræ_. [Illustration: Fig. 64.--A species of Helomyza.] The _Helomyzæ_ (Fig. 64) live in the woods. Their larvæ are developed in the interior of fungi. Réaumur studied the larvæ of the Truffle Helomyza. The head of this fly is ferruginous, its thorax is of a brownish grey, its shoulders of a brownish yellow, its wings brownish, the abdomen yellow and brown, and the feet red. The larvæ of these insects commit depredations for which gourmands will never forgive them, destroying, as they do, their truffles. When one presses between one's fingers a truffle that is in a too advanced state, one feels certain soft parts, which yield under pressure. On opening the truffle, the larvæ of the insect of which we are speaking will be found inside. These larvæ are white and very transparent. Their mouth is armed with two black hooks, by means of which they dig into the truffle in the same way as other larvæ dig into meat. The excretions of these little parasites cause the truffle to become decomposed and rotten. In a few days the larvæ become full-grown. They then leave their abode and go into the ground, there to change into pupæ.