Although it has been ascertained that the Rhipiphorides exhibit instances of remarkable metamorphosis, their life-histories are still very imperfectly known. Dr. Chapman has ascertained some particulars as to _Metoecus paradoxus_, which has long been known to prey in the larval state on the larvae of the common social wasps.[137] The eggs are apparently not deposited in the nests of the wasps, but in old wood. The young larva is a triungulin, similar to that of the Cantharidae, we shall subsequently describe. It is not known how it makes its way to the wasps' nests, but it is possible that when a wasp visits some old wood haunted by these larvae, some of them may attach themselves to it and be carried to the wasps' nests. When {269}access is gained to the cells the little _Metoecus_ pierces the skin of one of the wasp-grubs, and entering in it feeds on the interior; after it has increased in size it emerges, changes its skin, and assumes a different form and habits; subsequently, as an external parasite, entirely devouring the wasp-larva, and then becoming a pupa, and finally a perfect _Metoecus_, in the cell of the wasp. The wasps, though they investigate the cells, do not apparently entertain any objection to the _Metoecus_, though there may be sometimes as many as twenty or thirty of the destroyers in a single nest. A few hours after the _Metoecus_ has become a winged Insect and has escaped from the cells, it appears however, from the observations of Erné[138] on nests of wasps in captivity, that the wasps become hostile to the foreigners, and it is probable that in a state of nature these leave the nest as quickly as possible. _Emenadia flabellata_, a genus allied to _Metoecus_, has been discovered by Chobaut to have a similar life-history, except that it attacks a solitary wasp of the genus _Odynerus_.[139] An old record to the effect that a second species of _Emenadia_, _E. bimaculata_, lives in the stalks of _Eryngium campestre_, on the pith, is now thought to be erroneous. Fabre has found the larvae and pupae of another Rhipiphorid in the cells of a bee, _Halictus sexcinctus_. The most remarkable of the Rhipiphorids, from the point of view of its habits, is certainly _Symbius blattarum_, which is now treated as the same as an Insect previously described by Thunberg from specimens found in amber and called _Ripidius pectinicornis_. This species is parasitic in cockroaches; the male and female are very different, the former being an active winged Insect, while the female is worm-like, differing but little from the larva, and never leaving the body of the cockroach. It is to be regretted that the life-history is not better known. The species has been found on board ship in vessels coming from India; the male has been met with in several European countries, but the female is excessively rare.