The Nematoda which are parasitic during their whole life may similarly be divided into two classes - those which undergo their development in a single host, and those which undergo their development in the bodies of two distinct hosts.
Many of the adults are bloodsuckers (Tabanidae, Culicidae, &c.); others are parasitic in their larval stage (Oestridae, &c.).
In modern countries it takes myriads of forms, from the sweating of parasitic trades to the organization of scientific research.
Pear scab is caused by a parasitic fungus, Fusicladium pyrinum, very closely allied and perhaps merely a form of the apple scab fungus, F.
Certain extremely aberrant Diptera, which, in consequence of the adoption of a parasitic mode of life, have undergone great structural modification, are further remarkable for their peculiar mode of reproduction, on account of which the families composing the group are often termed Pupipara.