Parasitoids are a type of parasite that requires their victim's death, thereby reducing host fitness to zero. Entomologists use the term parasitoid to describe wasps and flies that lay eggs in or on insect hosts. The larvae then consume the host from the inside. When the carcass is consumed, the parasitoid wasps or flies metamorphose into free-living adults. For this reason, parasitoids are relatively large parasites. Other taxonomic groups (tur-bellarians, nematodes, crustaceans) also use a parasitoid life-history strategy. For instance, an intertidal turbellar-ian infects young crabs, grows to a large size, and, on adulthood, bursts through the crabs' exoskeleton to become a free-living adult worm. Because a diversity of taxonomic groups must kill their hosts as a normal aspect of their development, the term parasitoid is best used as a description of a parasite life-history strategy instead of a taxonomic category for certain flies and wasps.
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