A gadfly found at Cayenne is distinguished by the name of the Macaco Worm; it belongs to the genus _Cuterebra_, and usually attacks the skin of oxen and dogs in South America. It is accidentally found sometimes on man. This is the _Cuterebra noxialis_. We here give the representation of it. There is also a gadfly on the ox. Professor Joly has devoted himself to zoological researches on OEstridæ in general. Professor Schroeder Vander Kolken, in Holland, and Mons. Brauer, in Austria, have studied them with great success. The _Hippoboscus_ is a fly which is very greedy of blood, and attaches itself to horses and oxen, especially under the tail, in the parts where there is less hair. It sometimes also attacks man. [Illustration: Fig. 37.--Macaco Worm.] 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 176 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 lepidoptera 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 177 the _Lipoptena_ of the stag. We give figures of these two curious insects. [Illustration: Fig. 38.--Melophagus ovis.]