Parasitism

Parasitism offers free lodging and food, so it has evolved in many groups, from plants to higher vertebrates.

Many parasites are host-specific, able to live on only one host species, so host and parasite co-evolve: each new host defence is countered by the parasite.

Adaptations of parasites: loss of unnecessary sense organs, adhesive organs or suckers to cling to the host, loss of the digestive system, and high reproductive capacity.

Life cycles are often complex. The human liver fluke (a trematode) needs two intermediate hosts, a snail and a fish; the malarial parasite needs a mosquito as vector.

Most parasites harm the host: they lower its survival, growth and reproduction, reduce its population density, and can weaken it enough to make it easier prey.

Ectoparasites feed on the host's outer surface: lice on humans, ticks on dogs, and copepods on many marine fish.

Cuscuta, a parasitic plant on hedge plants, has lost its chlorophyll and leaves and draws its food from the host.

The female mosquito needs human blood for reproduction but is not considered a parasite.

Endoparasites live inside the host (liver, kidney, lungs, red blood cells). Being extremely specialised, they have more complex life cycles and simplified body structure, with the emphasis on reproduction.

Brood parasitism: the parasitic bird lays eggs in the host's nest for the host to incubate. The cuckoo (koel) lays in the crow's nest, and its eggs have evolved to resemble the host's in size and colour.