In terms of ecological processes, predation is one of the possible forms of energy transfer from living animal to living animal. In terms of behavior, it is the process through which an animal (the predator) captures and kills another animal (the prey) before eating the latter in part or completely. Following these broadly, although not universally, accepted definitions, the concept of predation does not apply to interspecific relationships where one of the partners is not an animal. In particular, it excludes insectivorous plants as well as plant-eating animals. Consistent with this restriction, the term should probably not be applied to interactions involving protozoans (no longer included in zoological classifications) or, still worse, bacteria. Nevertheless, it is commonplace to read descriptions of 'predation' of heterotrophic protozoans (ciliates, for example) on other protozoans or even on bacteria. The present article will anyway only deal with examples of predation of animals on other animals.

A further distinction is necessary, to differentiate predation from parasitism. In many instances, the contrast between the two types of interspecific relationships is clear. The association between a predator and any of its prey items is a short one and involves early killing of the prey, whereas the association between a parasite and its host is a long-lasting one, very often lifelong, and generally avoids killing of the host by its parasite. In addition, most predators are sensibly or conspicuously larger than their preys, whereas parasites are usually much smaller than their victims. There is clearly no problem allocating tigers and spiders to the predators, and tapeworms and trypanosomes to the parasites. However, there are also many animals exploiting other animals whose interspecific relationships do not fit clearly into either of the two classes. The most conspicuous example of prey-consuming animals whose feeding strategy differ from both the typical predator and the typical parasite are those insects (tens of thousands of species belonging to the hymenopterans and thousands of species of tachinid dipterans) whose females lay eggs in suitable hosts (very often the eggs or the larvae of other insects) without killing it. This behavior thus does not correspond to a predator's one. But the larva deriving from the hymenop-teran's or dipteran's egg will feed on the living tissues of its host and eventually bring it to death. This happens approximately at the time its killer has completed larval growth and is ready to metamorphose into a winged adult. These insects, whose association with the host is prolonged as in the true parasites (but eventually kill their victim, as predators do) are called parasitoids. These will not be discussed further in this article.

Finally, from the category of predators are generally excluded also the animals with microphagous feeding habits, that is, those aquatic animals which by use of filtering or mucus-trapping devices collect a diversity of small-size, and often, truly microscopic items, among which the animal fraction is sometimes more conspicuous than the bacterial or algal ones. Reasons for not treating them as predators are disputable, but we will follow here a common practice in excluding them from further consideration.

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