Fishes nourish on the surface of the skin a great number of these, which helminthologists have thought proper to classify under the name of _Ectoparasites_. Among many crustaceans and insects, only one of the sexes is parasitical. The males remain entirely free, and preserve all their attributes, while the females seek for assistance, and require food and lodging. The female alone sacrifices her 256 liberty, and changes her form entirely in order to secure the preservation of her posterity. The insects called _Strepsiptera_, which live as parasites on wasps, furnish a curious example of this (Fig. 77). These insects, the _Polistes_, the _Andrenæ_, and the _Halicti_, do not kill the larvæ of the Hymenoptera on which they feed; they suck the blood of their victim slowly, and leave him just enough strength to go through his metamorphoses. The females are condemned to remain almost completely immovable on their prey, while the males are winged. [Illustration: Fig. 77.--Stylops. Male, natural size, and magnified.] Naturalists have paid great attention to these latter insects, as much on account of their mode of life as of the difficulties which they have suggested to entomologists in the appreciation of their natural affinities. Are they coleoptera, as was for a long time, and perhaps correctly, supposed, or do they form a distinct order by themselves? However this may be, these are the facts known concerning them, according to the recent observations of Mons. Chapmann, a conscientious naturalist. The females do not lay their eggs in the nests of wasps, but the larvæ, under the form of meloë, penetrate into the cells, by the assistance of the larvæ of the wasps, which carry them hidden between the second and third ring. The larvæ of the 257 Rhipiptera are developed at the expense of the larvæ of the wasp, suck their blood, swell, and their skin remains adhering to the fourth segment. [Illustration: Fig. 78.--Black Stylops, female, showing the embryos in the abdomen.] [Illustration: Fig. 79.--Black Stylops, larva at its birth (from Blanchard).] When the rhipipterous insect is six millimètres in length, it changes its skin the second time, and this splits on the back, so that the 258 skin remains fixed between the larva of the parasite and that of the wasp. It then sucks the rest of the juices of the young wasp, and becomes a nymph in the prison which it has formed for itself. This evolution lasts from twelve to twenty-four hours.