A flight-stabilization device based on ocelli

responsibility for wording of article: Akira Takashima (OIST)

Structure and function of ocelli

 Compared with manmade machinery, one characteristic of insects is their very fast flight capability. Mimicking the acrobatic flying ability of flies and dragonflies is cutting-edge research in aeroengineering; however, this has not been achieved till date. Insects use various sensory organs to control flight; these include the ocelli. There are 3 ocelli between the paired compound eyes: 1 facing straight ahead and the other 2 facing left and right in a slightly anterior direction. Because the lenses of the ocelli are focused far behind the photoreceptor cell layer, they can transmit only an out-of-focus image to the photoreceptor cell layer. However, the incident light comes from a wide field of view; therefore, their light collection efficiency is high. The ocelli are packed with several thousand photoreceptor cells; however, the signal converges on only a few dozen secondary neurons. This mechanism indicates that ocelli are not suitable for image perception but suitable for the detection of small light–dark variations. The ocelli sacrifice resolution for extreme sensitivity to light–dark changes. Many secondary neurons of the ocelli have thick axons and are directly projected to the thoracic ganglia so that information of light–dark changes can be quickly transmitted to the flight motor circuits in the thoracic ganglia. 

For flying straight and consistently, insects need to detect disruptions in the flight attitude on 3 axes, namely roll, pitch, and yaw axes, and then control them. When an insect is flying straight, its ocelli are facing the horizon. The top part of the visual field is the sky and it is bright, while the bottom part is dark. The insect controls its flight attitude by detecting the light–dark contrast. For example, during a left roll, the brightness in the central ocellus will not change; however, the left one will become darker and the right one will become lighter. The wing beat will be controlled to maintain the horizontal position so that the light from the left and right ocelli will be almost the same. If the body pitches so that it points downward, all 3 ocelli will simultaneously become darker and the position will be adjusted until the light in the ocelli is the same as that when the body was horizontal in order to maintain a horizontal position. Under natural light conditions, the insect can maintain horizontal flight by monitoring the brightness of the 3 ocelli.

The ocelli are not the only means in which the insect controls its flight attitude. The compound eyes, wind sensillae on the head, and mechanoreceptors that sense the movement of the antennae also play important roles. The compound eyes are not suitable for instant control of the flight attitude owing to their complex information processing (refer to the optic lobe section); rather, they can detect the disturbances from rolling, pitching, or yawing. The wind sensillae are not suitable for complex flight attitude control; however, they play a role in roughly controlling disturbances in the flight attitude caused by yawing and pitching. Therefore, if the insect is buffeted by a wind gust, the information from the ocelli and the wind sensillae are first used for loose control of the flight attitude. Following this, the flight attitude is controlled more precisely on the basis of information from the compound eyes. Among the neurons descending from the brain to the thoracic ganglia, some integrate information from all sensory organs and control the function of the flight attitude control circuit in the thoracic ganglia. In this manner, sophisticated motor control that exploits the characteristics of each sensory organ is achieved.

Application of the ocelli function

A “flight stabilizer” equipped with 3 light sensors that mimics the mechanism of the ocelli in controlling the flight attitude is being sold as an industrial product. It is used in radio-controlled airplanes. The stabilizer functions to correct the attitude if the radio-controlled airplane falls in an abnormal manner, such as in a spin.

Further reading:

昆虫ミメティクス Insect Mimetics(2008), 鉢山孝彦,下澤楯夫 pp.669-671

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