We spend so much time on predation because it is one of the major types of selective pressure members of a community exert on one another. Protection against predators always has a cost, whether it is a limitation on activity or the molecular and cellular resources spent on body armor. The widespread nature of such protections indicates that, evolutionarily speaking, the cost must be worth it. Parasitism could be considered a form of predation. We have already learned earlier in the course that animals can be both parasites or hosts.
Parasites evolve to avoid host immune responses, and naturally hosts evolve to avoid parasites.
A good example is provided by the cuckoo. The female cuckoo lays her eggs in the nests of other birds. She then leaves, and the other parents rear the baby cuckoo, usually at the expense of their own offspring. Parasitoids are a specialized form of parasite. They are insect larvae that develop inside the larval or pupal stages of other insects.