Fig. 51.—Echinomyia grossa.
M. Macquart divides the Muscides into three sections—the Creophili, the Anthomyzides, and the Acalyptera.
The Creophili have the strongest organisation; their movements and their flight are rapid. The greater part feed on the juices of flowers, some on the blood or the humours of animals. Some deposit their eggs on different kinds of insects, others on bodies in a state of decomposition, some again are viviparous. The insects of the genus Echinomyia, for instance ([Fig. 51]), derive their nourishment from flowers. They deposit their eggs on caterpillars, and the young larvæ on hatching penetrate their bodies and feed on their viscera. How surprised, sometimes, is the naturalist, who, after carefully preserving a chrysalis, and awaiting day by day the appearance of the beautiful butterfly of which it is the coarse and mysterious envelope, sees a cloud of flies emerge in place of it!
But there is another singular manœuvre performed by some of the species of the Diptera with which we are at present occupied to prepare an abundant supply of provision for their larvæ as soon as they are hatched. The following are the means they employ. It is well known that certain fossorial Hymenoptera carry their prey—other insects which they have caught, weevils, flies, &c., and which they intend should serve as food for their own larvæ—into their subterranean abodes. These Diptera, spying a favourable moment, slip furtively into their retreats, and deposit their eggs on the very food which was intended for others. Their larvæ, which are soon hatched, make great havoc among the provisions gathered together in the cave, and cause the legitimate proprietors to die of starvation.
"This instinct," says M. Macquart, "is accompanied by the greatest agility, obstinacy, and audacity, which are necessary to carry on this brigandage; and, on the other hand, the Hymenoptera, seized with fear, or stupefied, offer no resistance to their enemies, and although they carry on a continual war against different insects, and particularly against different Muscides, they never seize those of whom they have so much to complain, and which, nevertheless, have no arms to oppose them with."
The Sarcophagæ are a very common family of Diptera, and are chiefly to be found on flowers, from which they steal the juice. The females do not lay eggs, but are viviparous.
Réaumur, with his usual care, observed this remarkable instance of viviparism proved in a fly, which seeks those parts of our houses where meat is kept to deposit its larvæ. This fly is grey, its legs are black, and its eyes red.
When one of them is taken and held between the fingers, there may often be seen a small, oblong, whitish, cylindrical worm come out of the posterior part of the body, and shake itself in order to disengage itself thoroughly. It has no sooner freed itself than the head of another begins to show. Thirty or forty sometimes come out in this manner, and, on pressing the abdomen of the fly slightly, more than eighty of these larvæ may sometimes be made to come out in a short space of time. If a piece of meat be put near these worms, they quickly get into it, and eat greedily. They grow rapidly, attaining their full size in a few days, and make a cocoon of their skin, from which in a certain time the imago issues. If the body of one of these ovo-viviparous flies (for the eggs hatch within the parent) be opened, a sort of thick ribbon of spiral form is soon seen. This ribbon appears at first sight to be nothing but an assemblage of worms, placed alongside of and parallel to one another.