The Insect World. Figuier Louis

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Fig. 64.—A species of Helomyza.

      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.

      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.

      The Ortalidæ form a tribe which is remarkable for the upright carriage of the wings, which are generally speckled, by the vibratory movement of these organs, and especially for the cradle chosen by them for their progeny in fruits and grains. Nature seems to have assigned to each species its own particular vegetable.

      We will only mention here the Cherry-tree Ortalis, whose larva lives on the pulp of that fruit. This fly is about a line and a half long. It is of rather a metallic black colour, its head light yellow, the edges of its eyes white, and the tarsi red. The wings have four broad black stripes.

Fig. 65.—Dacus oleæ.

      The Olive Dacus (Dacus oleæ, Fig. 65) is a little fly, about half the size of the house fly, of ashy grey colour on the back, its head orange-yellow, its eyes green, and its forehead yellow, marked with two large black spots. The thorax is adorned with four lightish yellow spots, and its hind part, as well as its antennæ and wings, are of the same colour. The wings are transparent, reflecting green, gold, pink, and blue, according as the rays of light fall upon them, and are remarkable for having a small black spot at their extremity. The abdomen is of a fawn colour or orange-yellow, spotted with black on each side. This fly performs sudden and jerking movements; it keeps its wings extended, and rather jumps than flies. It is a destructive insect, a perfect scourge, which causes every two or three years a loss of five or six millions of francs to French agriculture.

      M. Guérin-Méneville has made some valuable observations on the Olive Dacus, and at the request of the Imperial Society of Agriculture of Paris, has indicated the way to preserve the olive from these ruinous larvæ, which generally destroy two crops out of three. We will borrow the following details from this learned entomologist: "At the time when the olives are formed the Dacus proceeds to place an egg under the skin of each of the fruits. By means of a little horny instrument, with which the female is provided, and which contains a small lancet, she pierces the skin of the olive; she moves her wings and lays her egg. She afterwards cleans and rests herself, by passing her feet over her head, wings, and other parts of her body. She then flies away, and seeks another olive, to deposit in it another egg; she repeats this operation until she has placed on as many olives the three or four hundred eggs which she bears."

       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.