Morphology of the eyes in adult Hyalomma truncatum ticks (Acari: Ixodidae).

The eyes of Hyalomma truncatum ticks are morphologically similar in both sexes. They appear as paired hemispherical structures situated dorsally between the first and second pair of legs on the lateral scutal margin. Each eye consists of a lens and photoreceptor cells, which are separated by a fine fibrillar layer and the hypodermis. The lens contains numerous channels which open beneath the epicuticle, converge uninterruptedly to the interior of the lens and end below the fine fibrillar layer. The inner closure of the lens is formed as an oval plateau with a cone-like projection situated caudolaterally and eccentrically to the longitudinal axis of the lens. The hypodermis is a single layer of cells, situated immediately below the fine fibrillar layer. Beneath the hypodermis, directly below the cone-like projection of the inner lens are the photoreceptor cells localized in a rosette-like arrangement. Facing the hypodermis, each photoreceptor cell is provided with numerous microvilli. The microvilli consistently border the microvilli regions of other cells at different angles but are always oriented at a right angle to the lenticular channels. The photoreceptor cells are unipolar neurons, whose axons arise from the basal portion of the cell and join to nerve fibre bundles forming the optic nerve. It is concluded that the eyes in adult H. truncatum ticks possess all the structures necessary to perceive and conduct light stimuli and visual signals.