The Hippoboscus lives on the horse, and an allied species, of which a different genus has been formed, lives on bats (Strebla vespertilionis) in South America. Mons. Von Baër noticed hippobosci on the elan, during his residence in Königsberg.
Many other insects live and develop themselves at the expense of their nearest neighbours.
Travellers since Azara’s time assure us that Uruguay contains but few oxen and horses, because a fly exists in that country which lays its eggs in the navel of these animals at the moment of their birth. These animals, on the contrary, are abundant in Paraguay. In order to increase their number in Uruguay, it would be necessary to favour the multiplication of birds or insects which make war on these flies, either in the larval or the sexual state.
Diptera, known by the name of Conops, pass their first three changes in the soft parts of drone-bees. Dumeril had formerly suspected, from the curvature of the abdomen, that the Conops lays its eggs in the body of some other insect. Lachat and Victor Audouin have given an instance of this in the “Journal de Physique.”
Thus the Conops, in its larval state, inhabits the abdomen of drones or other hymenoptera; the Echinomyæ are developed within various epidoptera when in the state of caterpillars or chrysalids; there are even some which live on flesh, and prefer that which is in a state of incipient putrefaction.
We may also speak, in this category, of animals which seek assistance, while young, from neighbours of whom they take advantage during their life, and utilize them even after their death; these are insects of various orders. They are in general more cruel than beasts of prey, which often contend on equal terms with their victims. Here we have an enemy which furtively introduces itself into its neighbour, who is nearly sucked dry before he suspects the danger to which he is exposed. He harbours unawares the assassin who is about to murder him. This is the refinement of cruelty.
The Melophagus of the sheep is a wingless dipterous insect, like the Lipoptena of the stag. We give figures of these two curious insects.
Fig. 38.—Melophagus ovis.