Skip Ribbon Commands Skip to main content
Share This

Ectoparasites - Life Cycles

Life cycles of arthropods involve a series of structural changes known 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 larva to nymph. The adult seeks a second animal for the final blood meal before final moult to adult. 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 sheep to be reared profitably.

Mites

In contrast to ticks, mites complete their entire life cycle, which incorporates incomplete metamorphosis, 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 sheep.

  

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 larvae, which are small, immature adults. Louse eggs are also called nits. The larvae moult three times before becoming adults.

Sheep Keds

Adult female melophagus produce pupae that undergo a cycle of complete metamorphosis. Pupae are 3 to 4 mm long and require 19 to 36 days to become adults. Maximum amounts of time are required in cold weather, although ked populations are heaviest in autumn and winter months. Completion of the life cycle requires a minimum of 32 days. Engorged females may live up to 8 days off sheep. The life span is 3 to 4 months.

Blowflies

Adult female blowflies deposit their eggs on sheep in areas of moist or rotting flesh. Species of Chrysomyia deposit live larvae. Larvae hatch, feed on skin or other tissues, grow, and moult twice before becoming mature larvae or maggots, some 2 to 19 days after hatching. Maggots drop off sheep and pupate in the ground for about 3 to 7 days before reaching adult stage. Thus, blowflies can complete their life cycle in as short a time as 6 days in hot, moist conditions.


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 blowfly

           

Back to Sheep Disease Information 

 

©2017