Because tongue worms are endoparasites of the respiratory tracts of tetrapods, what little is known about behavior has been inferred mostly from findings at autopsy. In the case of direct life cycles, eggs containing primary larvae gain entry to the definitive host through the alimentary tract as contaminants of food or water. When there are intermediate hosts in the life cycle, larvae acquired by the same means escape from the egg but invade the viscera, where they molt several times to form infective nymphs. The nymphs excyst when intermediate hosts are eaten, and larvae penetrate the stomach or intestinal wall of the final host. This stage is followed by a period of growth in the body cavity before larvae penetrate the lung through the pleura. It is possible to culture in vitro the lung-stage worms of certain species in a blood-based medium under sterile conditions. For example, developing nymphs of Porocephalus crotali, normally resident in the lung of their rattlesnake host, ingest ad libitum the medium in which they are suspended and molt normally through several instars to the adult stage. Thus lung-dwelling species appear not to require specific cues for successful development.
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