Fig. 66.—Olives attacked by Dacus oleæ.
Fig. 66, taken from the Memoir published by M. Guérin-Méneville, in the "Revue Nouvelle" of the 15th July, 1847, shows the Dacus laying its eggs on the olive, and the larvæ that are already hatched in another of the same fruit. The larvæ which succeed these eggs ([Fig. 67]) are whitish, soft, and without limbs. They pass fifteen or sixteen days in boring a gallery in the pulp of the olive, at first vertically, until they reach the stone, then on one side, and along the side of the stone. When they have reached the term of their development, they approach the surface, enlarging the first channel and leaving between it and the exterior air only a thin pellicle, in the middle of which may be perceived the first small opening by which the mother had introduced her egg in the commencement.
| Fig. 67. Larvæ of Dacus oleæ (magnified and natural size). | Fig. 68. Gallery formed by larva of Dacus oleæ. |
Fig. 68, copied from a drawing in the Memoirs of M. Guérin-Méneville, shows the gallery bored round the olive by the larva of the Dacus. The larva thus prepares an easy issue for the perfect insect. Its skin then contracts, its body diminishes in length, and is transformed into an oval cocoon, which soon gets brown, and is the chrysalis of the insect. At the side of the head it shows a curved line, a thin suture which marks a sort of cap or door, which, at the time of its hatching, the insect will be easily able to force open with its head. The fly is hatched twelve days after its metamorphosis from the larva to the pupa. It has thus taken the Dacus twenty-seven to twenty-eight days to arrive at this state, from the time the egg was laid; besides which, this species, in the warm climates of Provence and Italy, can reproduce itself several times from the beginning of July, the period at which the first flies begin to lay, till the end of autumn.
In order to save a considerable portion of the olive crop of these countries, M. Guérin-Méneville has advised hastening the harvest sufficiently for all the olives to be pressed at a time when the larvæ of the last generation, which would be preserved in the olives that are left, or in the earth, according to the climate, are still in the fruit. If a first operation were not sufficient to destroy them all, it should be repeated the following year. The sacrifice entailed by this practice would be amply compensated by a succession of good crops and the certainty of a sure and permanent profit. In fact, by an early gathering at least half a crop of oil is still obtained; whereas, by waiting for the usual period of gathering the olives, sufficient time is left for the larvæ of the Dacus to devour their parenchyma, which deprives them of the little oil that they might have yielded if their destruction had been accomplished earlier. This early gathering has the advantage of causing the destruction of a great number of larvæ, which will be so much towards diminishing the means of reproduction of the fly.
HEMIPTERA.
The Hemiptera are particularly distinguished from other kinds of insects by the form of their mouth, which consists of a beak, more or less long, composed of six parts: that is, of a lower lip, or sheath; four internal threads, representing the mandibles and jaws of the grinding insects, and which are the perforating parts of the beaks; and, lastly, of the upper lip or labrum. Owing to this apparatus, these insects are essentially sucking ones, and chiefly nourish themselves with the juices of vegetables, which they draw up with their beak. The wings of the Hemiptera are usually four in number; in some species they are membranous and similar to each other, and in others the upper are of rather harder consistency than the lower ones. In general, the former are quite different from the lower wings, and are only membranous at the tip, whereas the other part is thick, tough, and coriaceous.