Walcher, F., and K. Kral. 1994. Visual deprivation and distance estimation in the praying mantis larva. Physiological Entomology 19: 230-240.
When observing a praying mantis, one often observes the peering or side-to-side movements of the head. These head movements are called "saccades". There must be an advantage to performing this action since it is noted in all mantids. Why does the praying mantis (Tenodera aridifolia sinensis) perform saccades?
To increase the success of prey capture, a praying mantid must be able to determine the distance between the prey and itself. Other insects that also have binocular vision, such as locusts, have demonstrated that peering is used to measure distance.
First and second larval instars of praying mantis were used in the experiment. Mantid eyes were covered with black paint on various locations of the eye, and injected with a dye to cause degeneration in the foveal region of the eye. Peering before a jump resulted in the mantid successfully reaching its target.
Walcher and Kral concluded that every time a jump was preceded by a saccade, mantids were able to reach their target successfully. They also concluded that "Šthe peering mechanism is influenced not only briefly by the momentary visual input, but also over the long term by visual experienceŠ". The study also discovered that finding the nearest object is possible with only one intact eye, but jumps are only made when both eyes are intact. Therefore, peering movements of the head are indeed used to measure distances.
(230 words)
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