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| Ectoparasites - Life Cycles |
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Life cycles of arthropods involve a series of structural changes known as metamorphoses, the actual sequence 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 may either live freely or be dependent on their hosts for obtaining nourishment. Eventually a hardcased structure called a pupa is formed, which may enable the capacity to overwinter. The pupa hatches into the adult parasite, the final stage of metamorphosis. Thus, there are four distinct stages in the life cycle: egg, larvae, pupa, and adult. Incomplete metamorphosis involves a larva that 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) which grow to maturity without further change in body type. 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, or 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, based 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. The only tick in New Zealand is the three host cattle tick Haemaphysalis longicornis. Dogs can become infested when walking through pasture grazed by cattle. Mites The life cycle of the common canine mite take about three weeks and are spent entirely on the bodies of dogs. Mites cannot live for long off their hosts, as a slight reduction of temperature below that of the dog for a relatively short time is fatal for the mites. Thus, mites are usually transmitted by direct body contact. For example, demodectic mange on a nursing bitch will often spread to the pups. Mites undergo incomplete metamorphosis.
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 into nymphs, which are small, immature adults. The nymph moults three times before becoming an adult. Fleas The flea cycle incorporates complete metamorphosis and can take between three weeks to 1 year. Larval fleas develop in the environment, feeding on skin and debris. Parasitic adult fleas hatch from the pupae.
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