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Ectoparasites - Life Cycles

Life cycles of arthropods involve a series of structural changes know as metamorphoses, the actual sequence of which varies with different parasite groups. Complete metamorphosis begins when adults lay eggs from which larvae hatch. The larval forms grow and shed their skins (moult) several times, each time to accommodate their increases in size. Larvae either may live freely or may be dependent on their hosts for obtaining nourishment. Eventually a hard-cased structure called a pupa is formed, which may have the capacity to survive winter. The pupa hatches into the adult parasite, the final stage of metamorphosis. Thus, there are four distinct stages in the life cycle: egg, larva, pupa, and adult. Incomplete metamorphosis involves a larva that grows and moults one or more times to become an adult-like form known as a nymph, which in turn grows and moults one or more times to become an adult. In this case there are only three distinct stages, namely eggs, larvae, and immature adults (nymphs) that grow to maturity without further change in body type.

Arachnids

Ticks

Ticks have a life cycle incorporating incomplete metamorphosis. Adult ticks feed and mate on mammals. Engorged females drop to the ground and lay eggs. The eggs hatch, producing six-legged larvae, called seed ticks. The seed tick moults twice, passing through an eight-legged nymphal stage before reaching maturity. A blood meal must be taken before each moult can occur. Ticks are classified as one-, two-, or three-host ticks, depending on how many times they drop off, moult, and seek a new animal. A one-host tick remains on the animal from the seed-tick stage to maturity. A two-host tick drops off the initial host to moult from nymph to adult. The adult seeks a second animal for the final blood meal before copulation. The three-host tick drops to the ground for each moult, after which a new host is sought.

A knowledge of the life cycles of ticks is of great practical importance in planning a control program. In some areas control measures must be practiced at regular intervals for cattle to be reared profitably.

Mites

In contrast to ticks, mites complete their entire life cycle, which incorporates incomplete metamorphoses, on the host. This is generally accomplished in 3 to 5 weeks. Mites cannot live for more than several days away from animals and are transmitted by direct body contact between cattle.

Insects

Lice

All stages of the louse life cycle, which takes about 3 to 4 weeks, occur on the host. Lice can live for only about a week in the absence of the host. Louse eggs, cemented to animal hair, hatch as nymphs, which are small, immature adults. The nymph moults three times before becoming an adult.



Life cycle of 1-host tick

 


Life cycle of 2-host tick
Note: In any stage of the multi-host tick life cycle, the host may be the same animal, another animal in the same species, or a member of another species.

 


Life cycle of 3-host tick
Note: In any stage of the multi-host tick life cycle, the host may be the same animal, another animal in the same species, or a member of another species.

 


Life cycle of mites

 


Life cycle of cattle grubs

Warble Flies of Grubs

Adult warble flies live freely in the environment and contact cattle only to deposit eggs on the cattle hair. Hypoderma larvae hatch, climb down hairs, penetrate the skin, and migrate extensively under the skin. The larvae grow and mature before leaving the animal to pupate in the ground. Adults eventually emerge from the pupae.

Hornflies and Buffalo Flies

These flies lay eggs in manure. The eggs hatch into larvae that feed on the manure, grow, and moult into pupae, which rest in the dried manure or in the ground. They may overwinter as pupae, or emerge as adult flies in a few days. The adults mate and lay eggs to commence to next generation.

Blowflies & Screwworms

Adult flies lay eggs in any open sores on cattle. The eggs hatch into larvae which feed on the flesh of the host for 3 to 6 days and then drop to the ground where the transformation to the pupal stage occurs. Adults hatch from the pupal stage, and the life cycle is repeated.


Life cycle of screwworm or blowfly

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