Appearance

Hard tick or soft tick?

Ixodids (hard ticks) differ from argasids (soft ticks) in several ways. They have a plate-like shield on their backs (a scutum), which does not stretch when they engorge. In females, larvae, and nymphs, the scutum is confined to the anterior part of the body, while in males it covers the whole dorsal surface.

Argasids lack the scutum but instead have a leathery or granular dorsal surface, which extends like a hood over the capitulum (head and mouth parts). This means that the mouth parts in argasids are only visible from below, while in ixodids they are clearly seen from both above and below.

Ixodid (Female)

Argasid

Mouth parts

The mouth parts of all tick species comprise 2 chelicerae (cutting tools), 2 palps (limbs with sensory organs) and a hypostome, which is a barbed tube that anchors into the flesh of the host. The mouth parts can vary in size and shape depending on the species of tick.

Limbs

The first pair of the tick's limbs forms the chelicerae, the second pair forms the palps, which accurately position the capitulum when feeding, and the third to sixth pairs are the legs that are used for walking about. These can be retracted against the body for protection.

Shape

Ticks are generally flat and oval in shape in their unfed state, although some species can be more hemispherical or cylindrical. Once engorged, they become more coffee-bean shaped or almost spherical.

Ornithodoros

Dermacentor Variabilis (Female)

Colour

The colouration of ticks varies between species and sexes, but generally they are reddish, brown or black. Some species are plain in colour, while others have markings. Once engorged, they can be fleshy-pink, purple, dark-red, or a blue-grey colour.

Dermacentor reticulatus

Engorged ticks various colours

Size

Tick sizes vary considerably between their stages, sexes and species. One of the most common species to bite and infect people is Ixodes ricinus (also known as the sheep tick, wood tick, deer tick and castor bean tick), which in its larval stage is a tiny 0.5mm (as small as a poppy seed). At this size they can still transmit disease. Nymphs are slightly larger at 1.5mm, adult males are approximately 2.5mm, while adult females are about 3mm. Once fed, each stage will grow considerably. Adult females can expand to approximately 11mm.