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Ectoparasites - Parasite Groups

Ectoparasites are divided into two main groups, arachnids and insects, classified by structural characteristics. The arachnid class includes ticks and mites. The insect class consists of flies, mosquitoes, fleas, and lice. In New Zealand only a very limited number of ectoparasites affect horses and mostly only in a minor way. The following ectoparasites have been reported in New Zealand to affect horses: Bot flies; NZ cattle tick; the mites, Demodex, Chorioptes and Psoroptes cuniculi and the stable fly Stomoxys calcitrans.

Ecto Drawing 


The arachnid class of ectoparasites includes ticks and mites. An arachnid has only two body segments: a fused head and thorax, and an abdomen. It has four pairs of legs as an adult, and no wings or antennae.

The life cycle of arachnids involves incomplete metamorphosis: the eggs hatch into nymphs which at least superficially resemble adults. On the other hand, complete metamorphosis is seen in some groups of insects, such as flies, in which the immature forms are totally unlike the adults.

Mating is also assisted by structures called spicules, used by the male to hold open the genital orifice of the female. The shape and arrangement of the male bursa and spicules vary from species to species and are frequently used to identify different nematodes.


Ticks thrive on blood obtained from the host. They are subdivided into hard and soft ticks according to structural characteristics.

Hard Ticks

The bodies of hard ticks are roughly oval in shape and pointed at the front. The anterior segment is a false head, or capitulum; the shape of its base is characteristic of the particular tick genus. Structures on the capitulum may also help to identify the tick genus. Palps are segmented structures used for probing the host. The hypostome anchors the tick to the host´s skin, and mouth parts on the capitulum are adapted for sucking blood.

Adult Ornate Tick   Larval Tick
Adult ornate tick   Larval tick

The abdomen, flattened top to bottom, can expand to several times its original size as a tick feeds on its host. This phenomenon, referred to as engorgement, is seen only in females. One part of the skin, the scutum, located on the back of the tick, does not expand during engorgement. In some species, the scutum may be decorated with coloured pigments. These patterns of pigmentation may help with identification. Male ticks are generally more colourful than females in ornate (coloured) species. The presence or absence of other structures may also help to identify ticks.

A further classification of hard ticks is made based on whether their life cycle involves one, two, or three hosts. This is described under the section on life cycles.

Soft Ticks

Soft ticks differ from hard ticks in many respects. They have a leathery outer skin rather than a hard cuticle, and both males and females engorge when feeding on the host. Their shapes vary among species. There is no scutum, and the capitulum is located on the bottom side of the tick near its front. Otobius megnini, the spinose ear tick, is an example of a soft tick. Only larvae and nymphs of this species are parasitic. Adults live in hidden areas in the environment, such as within cracks in the wood of barns.



Mites are arachnids which can be seen clearly only with a microscope. Their bodies are usually round and flattened. Mites eat normal skin debris, such as scales. An alternate feeding habit in some species is puncturing the skin to suck lymph fluid. Sarcoptes scabiei mites actually burrow tunnels into the skin where they live, feeding and depositing eggs. Larvae that hatch may create side tunnels, or leave the area and migrate to undamaged skin to burrow new tunnels.




Insects are characterised by having three distinct body parts: head, thorax, and abdomen. Wings may or may not be present. Insects have one pair of antennae on the head and three pairs of legs on the thorax. Many insect parasites -- including some species of flies, mosquitoes, and fleas -- spend little time on the host. In contrast, the larval stages of the botfly and all stages of equine lice remain on the bodies of horses for significant periods of time.


Lice are wingless, flattened insects with six legs adapted for clinging to hair. Two kinds of lice are common. The sucking louse (Haematopinus asini) has mouth parts adapted for puncturing skin and sucking tissue fluids and blood. The biting louse (Damalinia equi) feeds on skin debris and has mouth parts adapted for chewing.



Botflies cause great annoyance as the females dart in to lay their eggs on the legs and throats of horses. These parasites have been described more fully in the endoparasite section. Other species of flies (i.e. Musca) may serve as intermediate hosts for some endoparasites (i.e. Habronema - not in New Zealand).



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