Funnel-web Hadronyche sp Click to enlarge image
Female Hadronyche sp from Clarence, Blue Mountains infested with parasitic mites. Close up of side of the head view Image: Carl Bento
Courtesy Australian Museum, Public Domain, Photographer: Carl Bento, Date: [year] (This image is in the Australian Museum collections)

A parasite is an organism that lives at the expense of another organism - the host.

In general, parasites share the following features:

  • Parasites are usually smaller than their host.
  • Parasites use both invertebrate and vertebrate hosts.
  • Adult parasites may live on the host (e.g. lice), in the host (e.g. tapeworms) or feed on a host occasionally (e.g. mosquitoes).
  • Parasites generally do not kill the host but may harm the host indirectly by spreading pathogens. This may affect the host's behaviour, metabolism or its reproductive activity.
  • Many parasites have hooks, claws or suckers to attach to their host.
  • Generally parasites have either a sucker (e.g. leeches) or piercing and sucking type mouthparts (e.g. fleas) for feeding.
  • Both adults and young can be parasitic. In some cases the young are parasites but the adult is not.

Which invertebrates are parasites?

Many invertebrate groups have parasitic members. Some well known parasites are fleas, ticks, parasitic mites, leeches, worms (e.g. round worms) and some parasitic flies (e.g. mosquitoes). Stylops are parasites of wasps, bees and bugs. The female is larvae-like and spends its entire lifecycle within the host. The winged, free-living males locate females by scent and mate with the small portion of the female protruding from the host.

In nature, when two individuals of different species often live in close association with each other, this leads to a phenomenon called symbiosis.

There are three types of symbiosis:

  • Mutualism is a win-win situation for both organisms because both benefit from the relationship.
  • With commensalism, one organism benefits while the other is unaffected.
  • Then there's parasitism, where one organism (the parasite) benefits at the expense of the other (the host).

Parasites can be protists, bacteria, viruses, fungi, plants or animals. An estimated 40 per cent of animal species are parasites. Some parasites live on their hosts (ectoparasites) while others live inside them (endoparasites).

Here are some of the most common parasites

Fleas and tapeworms

Siphonaptera, flea


Image: Cate Lowe
Courtesy Australian Museum, Public Domain, Photographer: Cate Lowe, Date: [year] (This image is in the Australian Museum collection, but copyright has expired. This licence allows sharing, remixing and use for commercial purposes)

Fleas and tapeworms

Well known parasite examples of parasites to those who have domestic pets fleas and tapeworms.

Fleas are ectoparasites since they live on other animals' skin and suck their blood. In contrast, tape worms are endoparasites, as they attach to other animals' guts to absorb nutrients from the food they eat.


Fleas are small, wingless insects ranging in size from approximately 1-10 millimetres in length depending on the species. Almost everybody, especially those with cats or dogs will be familiar with these small biting insects and will have either seen them or the effects of their nuisance bites. Fleas can be recognised by the following features:

  • Laterally compressed bodies
  • Piercing-sucking mouth parts
  • Enlarged hind legs adapted for jumping
  • Strong tarsal claws adapted for holding onto their hosts
  • Backward pointing hairs and bristles for ease of movement through the hair of a host
  • Small antennae which tuck away into special groves in the head

The larvae of all fleas appear grub-like and are usually found in the nests of their host or other areas where they commonly rest.

Life Cycle
Fleas mate on their host animal and lay their eggs either onto the animal where they fall to the nest or directly in the nest. The small larvae hatch from the eggs and do not begin to feed on blood like that of their parents but eat the dead skin and other dirt and dust from the host animal. The larvae develop through 3 instars and when fully grown spin a silken cocoon and pupate in the nest of the host. The vibrations caused by scratching by a host often trigger the emergence of the adult flea from the pupal case, enabling it to immediately find a host and begin feeding. The complete life cycle may take from several weeks to many months depending on the species.




Image: Centres for Disease Control and Prevention
Courtesy Australian Museum, Public Domain, Photographer: Centres for Disease Control and Prevention, Date: [year] (This image is in the Australian Museum collection, but copyright has expired. This licence allows sharing, remixing and use for commercial purposes)

In contrast to fleas, tape worms are endoparasites, as they attach to other animals' guts to absorb nutrients from the food they eat. They live in the small intestines of many different species of animals, including humans.

There are three main groups of tapeworms, each containing one or more species, that are a concern for most domestic animals and humans. Each group poses a different level of risk to people, and may be spread between animals and people in a different way.

The genus Dipylidium is most commonly found in domestic dogs, cats and is transmitted by swallowing an infected adult flea.

Flea control is essential for preventing infection in pets, because without adult fleas the parasite cannot be transmitted.

Remember that pets can be exposed to fleas from other animals if they go outside, even if they don’t become infested themselves. Do not allow pets to hunt or scavenge other animals. Keep cats indoors and prevent rodent infestations in the house. Keep dogs on a leash or at least in sight when outdoors.