Insects in Relation to Helminths (complete information)

Wormlike animals are called helminths. We are concerned here with the phyla Platyhelminthes, Aschelminthes, and Acanthocephala.

You may wish to renew your knowledge of these phyla by consulting a recent text in zoology (Storer, et a/., 1972) or animal parasitology (Olsen, 1974; Dunn, 1969).

Among the flatworms, or Platyhelminthes, are two classes that involve insects as intermediate hosts. The digenean flukes of the class Trematoda usually depend on snails as the first intermediate host, then enter a second intermediate host (snails, crustaceans, or fish) or encyst on aquatic plants before being ingested by the definitive vertebrate host.

The fluke Prosthogonimus spp., however, seeks drogonfly naiads as the second intermediate host after parasitizing snails. The parasites enter the anus while the naiad takes in water during respiration. Birds eat the infested naiads or adult dragonflies and become the definitive hosts.

One of the most remarkable effects of a parasite on its vector has been observed in the case of the fluke Dicrocoeiium spp..Eggs are released by the adult flukes in the definitive host and exerted.

The eggs are then ingested by land snails and the parasites later expelled from the snails’ respiratory pore in a ball of slime. Hundreds of parasites may be protected in each ball. The balls are collected by several species of ants in the genera Formica and Proformica. In the nest, the slime balls are eaten by the colony, resulting in a high rate of infection.

Most parasites encyst in the ants’ abdomen, but one or two often migrate to the brain, where a lesion develops. In the evening, ants usually descend from vegetation, but those with brain lesions remain on the vegetation until warmed by the next day’s sun. Ruminants grazing early in the day ingest the infected ants, thus completing the cycle.

The tapeworms, or Cestodea, depend primarily on arthropods as intermediate hosts. Crustaceans are the usual hosts because most tapeworms are marine. The adults occupy the intestines of vertebrates. Some tapeworms in the terrestrial environment utilize insects as intermediate hosts. Eggs of Dipylidium caninum, the dog tapeworm, are ingested by larval dog fleas, Ctenocephalides canis (Pulicidae); cat fleas, C. felis; human fleas; Pulex irritans (Pulicidae); and the dog louse, Trichodectes canis Crichodectidae). The larval fleas and the biting lice have chewing mouthparts. They are able to ingest eggs, whereas the adult fleas cannot. Dogs and cats and their wild relatives acquire tapeworms by eating adult fleas or lice that were infected in earlier instars. Humans are occasionally hosts.

Various species of Hymenolepis infect aquatic crustaceans, earwigs, dung and scavenger beetles, millipedes, meal moths, and fleas. Hymenolepis nana, the dwarf tapeworm of humans, mice, and rats, and H. diminuta, another tapeworm of mice and rats, both depend on insects as intermediate hosts. Coprophagic scavengers, including larval fleas and the mealworm beetle, Tenebrio molitor, ingest infected rodent feces. The insect is eaten in turn by the definitive vertebrate hosts. Humans may be parasitized also. A number of other genera of tapeworms involve beetles, grasshoppers, larval and adult flies, and ants in their life cycles (Dunn, 1969).

The spiny-headed worms, or Acanthocephala, also live as endoparasitic adults in the intestines of terrestrial and aquatic vertebrates. On land, the eggs are ingested by insects and in water probably by both insects and crustaceans. The intermediate hosts are later eaten by the vertebrate.

Macracanthor-hynchus hirudinaceus is a parasite of pigs and their wild relatives, and occasionally humans. The insect hosts are coprophagic beetle larvae of the Families Scarabaeidae, Tenebrionidae and Hydrophilidae. They acquire eggs of the parasite be eating pig feces. The pigs then eat the infected larvae in the course of rooting in the soil.

The Aschelminthes include two classes of worms that infect insects: roundworm or Nematoda, and hairworms or Nematomorpha. The immature stages of the latter are endoparasitic in insects and crustaceans.

Adult hairworms are free-living in saltwater or freshwater. The complete cycle in terrestrial and freshwater habitats is not well known. The adult worms copulate, lay eggs, and die in water. Larval worms emerge and may either penetrate soft-bodied aquatic invertebrates or be ingested by aquatic and possibly by terrestrial insects. Some hosts are not suitable for further development.

In these the larvae encyst. Development proceeds, however, in larger terrestrial insects that acquire infection directly from water or by eating in intermediate hosts infested with encysted larvae. The parasite enters the hemocoel, feeds, and matures. When the host is near or in water, the adult worm emerges from the host quickly and becomes free-living. Heavily parasitized hosts may have the reproductive organs reduced in size. Emergence of the worm is fatal to the host.

May (1919) found up to 20 per cent of tettigoniid grasshoppers near Urbana, Illinois, infected with Gordius robusts. Paragordius varius was found in gryllid crickets. May believed the parasites were obtained directly from water. Inoue (1962) traced part of the life cycle of Chordodes in Japan. Mayfly naiads ingest the larvae, and these encyst. The infected adult mayflies are eaten in turn by larger predators such as the praying mantis. The cyst is digested, freeing the larva to penetrate the host’s gut. Development is completed in the hemocoel.

The most numerous helminths associated with insects are nematodes. Much remains to be learned about the relationships between the largest group of animals, i.e., the insects, and what the second largest group is possibly, the nematode (Welch, 1965; Poinar, 1975). Nematodes have essentially five kinds of relationships to insects:

1. phoresy,

2. Facultative parasitism,

3. Obligatory parasitism that depends only on insects as hosts,

4. Parasitism involving both insects and plants as hosts, and

5. Obligatory parasitism with insects as intermediate hosts and vertebrates as definitive hosts.

Insects are vectors of important nematode diseases of humans and other animals, but they are curiously not vectors of nematodes causing plant diseases.

Insects regularly serve to transport nematodes in a phoretic relationship. Juvenile stages of rhabditoid nematodes in decaying matter or beetle galleries in wood attach themselves externally to insect scavengers or borers and are carried to new habitats. Some harmlessly enter the insect’s body, effectively avoiding desiccation.

The internal phoretic relationship becomes one of facultative parasitism when the nematode feeds on the host’s tissues; out is also able to complete its life cycle without an insect host (Poinar, 1972).

The effect on the host varies from no aparent harm to death. Species of Neoaplectana have an odd relationship to their insect hosts that defies classification. The juvenile worm penetrates the insect’s body like a true endoparasite. Specific bacteria are released by the worm after it enters the hemocoel.

The bacterial rapidly kill the host, and the nematode completes up to several generations, feeding on the dead host’s tissues and bacteria. Neoaplectana is probably obligatorily associated with insects in nature, yet can be reared on artificial media such as dog food. Is it a facultative parasite, predator, or saprophyte? Regardless of label, the nematodes are among the most promising organisms for use in the control of insect pests.

Obligate parasites feed only on insects. The juvenile stages of obligate nematode parasites oi insects develop in the host’s hemocoel, intestine, or reproductive organs. Adults may be free-living or remain in the host. One important family is the mermithids that infest aquatic insects and terrestrial insects in moist habitats.

The host is killed when the nematode emerges. Infection is sufficiently high in some areas to reduce significantly or to eradicate the host populations. Nematodes influence the host’s development by destroying organs, removing nutritional reserves, or possibly producing toxic effects on the corpora allata. Altered behaviour, reduced fecundity, sterlization, innersexes and intercastes of social insects may result from parasitism (Welch, 1965).

Juvenile nematodes of the genus Deladenus infest the hemocoel of siricid wood wasps. The adult parasites mature outside the host. Two types of females are produced. One type does not feed and after mating, penetrates a new host. The other type feeds on a fungal symbiote of the wood wasp.

Female wood wasps inoculate the fungus where they oviposit. Both sexes of the wood wasps are sterlized by the parasites, but the female continues to fly to new trees for opposition. The fungus-feeding nematodes are deposited during the futile opposition in new places where wood wasps are present (see Poinar, 1972). A related nematode is parasitic on weevils and mustard plants.

The last group of nematodes involves species whose definitive hosts are vertebrates. These are mainly the filarial worms, some of which are “host specific to humans. The adult worms feed on lymph and tissue fluids. The young or microfilariae are produced viviparously. Blood­sucking Diptera ingest the microfilariae.

These penetrate the insect’s gut and become intracellular parasites in fat, muscle, or Malpighian tubule cells. In 1 to 2 weeks, they metamorphose into an infective stage that migrates to the proboscis. When the insect feeds, the parasites migrate into the wound.

The grossly swollen legs, breasts, or genitalia characteristic of elephantiasis are caused by Wuchereria bancrofti. Culex pipiens quinquefasciatus (Culicidae) is the principal vector, but various species in other mosquito genera are also found infected.

Species of Simulium (Simuliidae) transmit Onchocera volvulus, a blinding disease of the Tropics. Another eye disease in Africa, caused by Loa loa, is carried by deerflies of the genus Chrysops (Tabanidae).

Other nematodes combining insects and vertebrates as hosts include the ascaridoid Subulura spp. that infects beetles and cockroaches, and then galliform birds. Many genera of spiruroid nematodes utilize coprophagous scrab beetles, cockroaches, termites, and anthomyiid flies as intermediate hosts. Definitive hosts are domestic animals and birds and their wild allies (Dunn, 1969).