Reproduced courtesy of David Spears © Clouds Hill Imaging Ltd.

DEFINITION:1 any of an order (Siphonaptera) of small, flattened, wingless insects with large legs adapted for jumping: as adults they are bloodsucking parasites on mammals and birds.

The flea is one of the most troublesome of insects and one of the most dangerous. Rat fleas carry the germs of bubonic plague from rats to man. They also spread the germs of a type of typhus fever.

Fleas are tiny insects with bodies thin and flattened from side to side much as a fish is flattened. This makes it easy for them to slip quickly about among the hairs of animals upon which they live, for all fleas are parasitic.

Fleas have no wings, but their long, frog like hind legs make them wonderful jumpers. The head has a long, sharp sucking beak that the insect uses for puncturing skin and sucking blood.

The eggs of the female flea become scattered in places where animals sleep and in rugs and carpets. The larvae, or young, are worm like and have biting mouth parts They live in animal tissues and waste.

Fleas infest rats, dogs, cats, hogs, rabbits, pigeons, and poultry. Animal fleas can be destroyed by rubbing an insecticidal dust into the host animal’s fur and by applying the dust to the animal’s sleeping place, in floor cracks, behind baseboards, and in ventilator openings. Since some insecticides are poisonous to some animals that harbour fleas, the label should always be read to ensure that the preparation is safe for a given animal.

There is a kind of flea that prefers to live upon human beings. This species does not occur in the United States to any great extent.

The scientific name of the dog flea is Ctenocephalides canis; of the cat flea, C. felis; of the man flea, Pulex irritans. There are about 500 known species of fleas. All fleas constitute the order Siphonaptera.

DEFINITION. any of a large family (Aphididae) of small, soft-bodied homopteran insects that suck the juice from plants.

On a stem or on the underside of a leaf sometimes a crowded colony of plant lice, or aphids, may be visible. They are parasites that have sharp sucking beaks and live on the sap of plants. There are many kinds. Most feed exclusively on a particular crop, weed, or tree.

The smallest aphids measure about 1/20 of an inch (0.13 centimetre) in length and the largest about 1/14 of an inch (0.64 centimetre). Most species are green, but some are pink, white, brown, or black. Those that migrate are born with wings. Most generations are made up of wingless females. During the feeding season these females, without mating, produce living young that are all females and that themselves produce several generations of young during the summer. In the fall a generation is born that includes both males and females. After mating, the females of this generation lay eggs that will hatch in the spring to start new colonies.

Aphids secrete from the alimentary canal a sweet watery liquid that is called honeydew. Ants relish this as food. Some species of ants care for whole herds of aphids (so-called “ants’ cows”). The ants build mud shelters for them at the roots of plants and move them often to new pastures as the old ones wither. To induce the flow of honeydew the ants milk the “cows” by stroking them with the antennae. Most aphids, particularly the woolly aphids, spread a white, waxy secretion over themselves for protection.

Many aphids suck plant sap or inject poisonous saliva into plants, causing the plants’ leaves to curl and sometimes drop off. Some aphids produce gall-like swellings on roots and bark. One of the most destructive aphids is the green bug, which infests oats, wheat, and other small grains. Fields of corn are often destroyed by the corn-root aphis, which is dependent on the cornfield ant for survival.

Aphids reproduce so rapidly that if unchecked they can destroy entire fields of crops. Their numbers may be controlled by such natural enemies as ladybird beetles (ladybugs), aphid lions, and lacewings. Farmers frequently control the insects by spraying with pest-control agents. Aphids belong to the order Homoptera and the family Aphididae.

Bed bug

An organism that lives on or within another organism, called the host, and that gains its sustenance from the host organism is known as a parasite. Parasites occur among all the major groups of living things. There are parasitic fishes for example, the lamprey, which attaches itself to other fishes and sucks their body fluids. There are many parasitic arthropods, including fleas, lice, biting flies, and mosquitoes.

Many worms are parasitic. Some live in their host’s digestive tract and feed on the food that passes through. Some attach to the intestinal wall and suck the host’s blood. Some, such as those that cause trichinosis, enter the host through the digestive tract and then burrow into the tissues of the entire body. Some also parasitism plants.

Many fungi are parasitic. The rusts are fungi that are responsible for many diseases of major food plants. Parasitic bacteria are responsible for diseases ranging in severity from acne and tooth decay to such major plagues as the Black Death.

The viruses are unique in that they are all parasitic. They are the smallest of the parasites and may enter the host through the respiratory system or may be spread through sexual contact.

Characteristics

 As originally defined, parasites included any organisms that live by drawing food from a host organism. Defined in this broad way, parasitism included relationships that ranged from benign to harmful and even fatal to the host. The term parasitosis was later developed to describe those forms of parasitism that injure the host, and today the term symbiosis describes benign or even mutually beneficial associations between organisms.

Effects on the host. A parasite’s effect on its host is determined by various factors. Many parasites, for example, do not reproduce in their hosts, or reproduce only to a limited degree. Such parasites, including many parasitic worms, produce eggs that enter another host before they develop. The damage done by such parasites depends in part on the number of parasites in the host, known as the host’s parasite burden. Many hosts can carry a light parasite burden that is, they can support a small number of parasites and suffer no ill effects. A heavy parasite burden, however, may cause severe injury to the host.

In the case of parasites that may undergo unlimited reproduction in their hosts for example, the protozoans, bacteria, and viruses the factors determining the final effect on the host can be quite complicated. The ability of the hosts’ natural defences to destroy the parasites often plays a major role. Very young, old, or weak hosts that have limited defences may be severely harmed by large parasite populations that are able to develop unchecked.

Varieties. Parasites are commonly described in terms of their relationships to their hosts. Parasites that remain on the outer surfaces of their hosts are called ectoparasites. Parasitic arthropods are usually ectoparasites. Endoparasites are parasites that live within the bodies of their hosts. The various parasitic worms that live within the hosts’ digestive tracts are endoparasites. Many endoparasites actually dwell within the tissues of their hosts, not just in the cavities of the hollow organs. The bacterium Mycobacterium tuberculosis, the most common cause of human tuberculosis, lives within the cells of the lung tissues.

Bedbug, a small, flat, bloodsucking insect (Cimex lectularius), of reddish-brown colour, of order Hemiptera, family Cimicidae; is parasitic on humans.

Parasites may be permanent or temporary residents in or on their hosts. The bedbug is a temporary parasite. It crawls onto its host to feed and then returns to its hiding place, where it spends most of its life. The flatworm that causes a form of human schistosomiasis is a permanent parasite. Once it enters a host’s body, it remains there until it dies.

Some organisms can live either as parasites or as free-living forms; they are called facultative parasites. For example, the free-living protozoan Naegleria fowleri, which occurs in streams and lakes around the world, can cause infection of the brain after it enters the noses of swimmers. Other organisms, called obligate parasites, can live only a parasitic existence. Plasmodium falciparum, an organism responsible for a form of human malaria, is an obligate parasite.

Autoecious parasites are parasites that complete their life cycles within a single host. Many parasites, however, have quite complex life cycles and may require more than one host. In some cases the immature stages of the parasite develop in one host, and maturation and sexual development occur in a second host. Hosts in which the immature stages of the parasite develop are referred to as intermediate hosts. Parasites that require two or more hosts to complete their life cycles are referred to as heteroecious.

Malaria, disease consisting usually of successive chill, fever, and “intermission” or period of normality.

The pattern of having more than one host can sometimes provide parasites with a means of spreading. The protozoan that causes malaria has two hosts: humans and certain other animals, and anopheles mosquitoes. Asexual reproduction occurs in infected humans and animals, and sexual maturation, fertilization, and reproduction occur in infected mosquitoes. The protozoans depend on the mosquito to transmit them from one human host to another.

Methods of transmission. An organism that transmits a parasite, as the anopheles mosquito does, is called a vector. Vectors need not transmit parasites by biting, however. Some vectors transmit parasites when they are eaten by the hosts. Certain tapeworms that infect cats and dogs use fleas as vectors. When the cat or dog swallows a flea that is caught during grooming, the immature forms of the tapeworm emerge from the flea’s body and mature in the cat’s or dog’s intestine. The mature tapeworm produces numerous eggs that then pass out of the animal’s body with its faeces and contaminate the environment. If an immature, or larval, flea ingests the tapeworm’s eggs as it feeds on the infected faeces, it becomes infected in turn. The parasite’s life cycle is completed if the cat or dog catches and eats the mature infected flea. A situation such as this, in which a parasite (the tapeworm) is parasitic upon another parasite (the flea), is referred to as hyper-parasitism

Human Parasites

Parasitism in humans is widespread, but the type of parasite varies with geographic regions and social conditions. In areas where sanitation is poor, parasites that are spread by ingestion of faecal-contaminated food and water are common. In areas where housing is inadequate, parasitic insects may be common.

In parts of the world with adequate sanitation and housing, parasites transmitted by faecal contamination and biting insects are generally rare, but those transmitted by direct contact and through the respiratory system may still be common. The parasites that cause measles, mumps, and chicken pox, for example, can spread rapidly in crowded school environments.

Plant Parasites

Arthropod, animal of the phylum Arthropoda comprising invertebrates with external skeleton, segmented body, and jointed appendages.

In many respects the parasites of plants are similar to the parasites of animals. The arthropods, fungi, worms, bacteria, and viruses that parasitic plants may either grow on the plant’s surface or invade the plant’s tissues and, in the case of arthropods that suck plant fluids, may also transmit other parasites, particularly viruses.

Some plants have become parasites on other plants. The simplest form of plant parasitism is that in which the parasitic plant uses its host only for support. The strangler fig, a tropical tree that is grown as a common house-plant, slowly surrounds its host tree until the host dies. The fig then has access to the light above the forest canopy and can grow unhindered.

Other parasitic plants, such as the mistletoe, have a somewhat greater dependence on their plant hosts. Mistletoe grows on trees and uses them for support. In addition, though it makes some of its own food, the mistletoe sends modified roots into its host to draw out nutrients.

Dodder, a leafless parasitic plant introduced into U.S. from Europe with clover seeds; now a rapidly growing pest.

The most complete form of plant parasitism is that in which the parasite relies completely on the host for sustenance. Dodder, for example, is a parasitic vine that draws all its nutrients from its host.

Special Types of Parasitism

Entomologists, scientists who study insects, have described a type of parasitism in which one insect, usually a species of wasp, uses another insect to brood its young. This type of parasitism is called parasitoidism. The parasitoid wasp lays its eggs in or on the host insect, commonly a caterpillar. The wasp’s larvae develop inside the host, feeding on its body, and emerge as full-grown adults. Parasitoidism is being used by some farmers as a means of pest control. Various parasitic wasps, for example, are used to help control agricultural pests.

Another unusual form of parasitism is brood parasitism, which is common among certain birds, particularly the cow-bird and the cuckoo. In this form of parasitism, the parasitic bird lays its eggs in the nest of another species. The host bird then raises the intruder’s young as though they were its own.

A type of parasitism called social parasitism occurs among certain communal insects. Some species of ants, for example, kidnap and enslave the workers of other ant species.

Assisted by Julius P. Kreier, Professor of Microbiology, Ohio State University, and author of ‘Parasitic Protozoa’.