Question

Which one of the following can be categorized as a parasite in true sense?
A. The cuckoo (koel) lays its egg in the crow's nest.
B. The female Anopheles bites and sucks blood from humans.
C. Human foetus developing inside the uterus and draws nourishment from the mother.
D. Head louse living on the human scalp as well as laying eggs on human hair.

Answer 1

Hint: Parasitism is a relationship in which one organism obtains nutrients from another organism, i.e. one organism is enriched at the expense of the other. A parasite is smaller than its host in true parasitism and has a limited life period. The parasite does not kill the host.

Complete answer: A parasite is a species that can not live without a host and are fully dependent on another organism for their survival. The human foetus is not a parasite, since the parasite for its entire existence relies on the host, while the foetus is only the stage of development in humans.
The louse is a real parasite that lives in the human scalp and lays eggs and also reproduces in the scalp. For example; head louse (Pediculus humanus capitis). It triggers an infestation of head lice.
a) Koel leaving her young people in the crow's nest is a form of brood parasitism where other host individuals are used by certain species to raise their young ones. As they depend on others for their young ones to develop, cuckoos are brood parasites. It is not a real parasite.
b) Female anopheles is a vector and not parasites because it does not invade humans. The mosquito doesn't have to breed with blood, they lay their eggs in water. The female mosquito feeds only on blood.
Hence, the correct answer is option D.

Note: It's not easy to disperse lice. They live on a human scalp not for more than 24 hours. The lice can't jump out, so they crawl and as a result, most of the transmission is through direct touch. They can also invade animals.