M. Guérin-Méneville is of opinion that these larvæ, which exist in great numbers in certain districts, sometimes devour all the nutritive substances contained in the ground. After having done so, they are obliged to come out of it, in order to seek at a distance a place where they will find food, or perhaps only a suitable place to undergo their metamorphosis. It is then that this singular journey commences. As regards the uniting of these myriads of individuals into columns, M. Guérin-Méneville thinks that it can be explained by the necessity these insects feel for mutual protection against the drying effect of the atmosphere when they are forced to leave the ground. United into masses, and moistened by the glutinous matter which connects them, they can leave their former place of abode without danger; if each were by itself, they would soon perish. Here, as in other cases, union is strength; and the strength of these larvæ lies in this protecting moisture. However it may be explained, the migration of these troops of insects are among the most astonishing phenomena of Nature. BRACHYCERA. The Brachycera, from [Greek: brachus], "short;" and [Greek: keras], "a horn"--these Diptera having short antennæ--are divided into four groups. In this subdivision the sucker is composed of six bristles. Amongst other families it includes that of the _Tabanidæ_; the insects belonging to which family are of remarkable strength, and possessed of daring and courage in the highest degree. Their wings are provided with powerful muscles, their feet are very strong, and their trunk is provided with six flat, sharp lancets. Distributed over the entire world, their instinct is everywhere the same, it is the desire for blood, at least in the females; for the males are not so warlike, they do no harm, but live on the juices of flowers. They are chiefly found in woods and pastures, and during the hottest part of the day in summer may be seen flying about seeking for their prey. M. de Saint-Fargeau has described the manner in which the males fly. They may be seen flying hither and thither in the glades of woods, remaining for some time suspended in the air, then darting quickly and suddenly away a yard or two, again taking up the same immovable position, and in each of these movements turning the head to the opposite way from that in which they are going. This naturalist is certain that on these occasions they are watching for the females, which they dart upon. When they have succeeded in doing so, they rise so high as to be out of sight. To this group belongs the genus _Tabanus_. The first species we shall mention, _Tabanus autumnalis_ (Fig. 33), a common species, is eight or nine lines in length, and of blackish colour. The palpi, the face, and the forehead are grey; the antennæ black; the thorax grey, striped with brown; the abdomen spotted with yellow; the legs of a yellowish white; and the outer edge of the wings brown.