Visually controlled head movements in perched anisopteran dragonflies
Odonatologica , Volume 24 - Issue 3 p. 301- 310
Small, moving, visual targets commonly excite fast saccadic head movements (head-cocks) in perched dragonflies. Head-cocking in response to natural stimuli has been examined by video-recording perched individuals of 7 libellulid and 1 gomphid species in the field during periods of feeding or sexual activity. Head-cocks varied in duration from 40 to > 1000 ms (mean 350 ms) and they usually occurred many times per minute. Typically a head-cock consisted of 3 components: a fast saccadic movement in the rolling and/or pitching plane; a brief period of stasis or of slow, smooth head movement, often involving a yaw component and interpreted as visual tracking; and finally a fast return to the rest position. A minority of head-cocks was followed by take-off, normally in the direction to which the head had been aimed. In species which kept their forelegs lifted off the ground when perched (e.g. Orthetrum), yawing was sometimes amplified by a turning of the prothorax on the synthorax, and also sometimes by rotations of the whole body on the legs. Head-cocks could be evoked by throwing or swinging stones 10-20 cm in front of a perched dragonfly, but their mean duration was significantly less than that of spontaneous head-cocks. No head-cocking has been seen in perched aeshnids or in the zygopterans examined. Head-cocks are believed to aim the acute zones of the eyes at a target, and they may also allow its distance to be estimated by stereopsis since the axes of some ommatidia in a forward-looking acute zone either side of the irons converge a few cm in front of the insect. Head cocks may also determine the direction of take-off through the action of proprioceptors in the neck.
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