Visual Adaptation of a Herbivore and a Carnivore Insect
Chui, Teresa H.
Insect eyes are very special and different from camera eyes of most animals, in that they are composed of many single eyes. Each of the single eyes (ommatidia) produces one pixel of a composite image. Therefore, the more single eyes an insect has, the greater the resolution the whole eye provides. We are interested in comparing the eyes from a predatory praying mantis (Tenodera sinensis) and a herbivore stick insect (Anisomorpha buprestoides) and determine their respective resolution. Our prediction is that the predatory insect will have a higher resolution because predators need more accuracy in their vision. In order to determine the resolution we have to count the number and size of the ommatidia, as well as their distribution over the entire eye. In addition, we have to determine the visual direction of each single eye which represents the visual angle. To achieve this goal we dissected and flattened the eye surface. Pictures were obtained from microscopic images. The number and diameter of the ommatidia was calculated from the printed pictures. To measure the angle we rotated an intact eye under the microscope and observed the directional angles over the periphery in consecutive intervals of 20 degrees. The eye parameter (product of diameter and interommatidial angle) for both insects indicates the herbivore and carnivore adaptations of the stick insects (herbivore) and praying mantis (carnivore).
Method: We anesthetized female adults from Tenodera sinensis and Anisomorpha buprestoides. We determined three parameters: a) the body size , b) the interommatidial angle (Φ) within 20° intervals and c) the average diam-eter of the ommatidial lenses (D), and d) the total number of ommatidia. After removing the head, a fine needle was placed un-derneath the base of the eye and perpendicular to the vertical axis. The needle was then mounted onto a turn table (Fig 2). In the area where the microscope focuses parallel to the axis of the ommatidia, a black spot will be seen. This spot is called the pseudopupil (see title picture). With the rotation of the eye, the pseudopupil moves in the same rotational direction to a new loca-tion 20° apart (Fig 3). The interommatidial angles (Φ) were determined by dividing the degree intervals (20°) by the number of ommatidia between two pseu-dopupils. The lens diameter (D) was measured directly on the digital microscope images on the computer screen. The eyes of both species were dissected under a dis-secting scope. We then removed the sensory structures until the separated eye cuticle could be flattened on a microscopic slide and photographed using a digital camera. The total number of lens facets were counted. Visual Adaptation of a Herbivore and a Carnivore Insect Teresa Chui (Dr. Ursula Jander– Mentor) Introduction: Insect eyes are very special and different from the camera eyes of most animals, in that they are com-posed of many single eyes (Fig. 1). Each of a single eyes (ommatidia) produces one pixel of the whole image. Therefore, the more single eyes an insect has, the greater the resolution the whole eye provides. We are interested in comparing the eyes of the preda-tory praying mantis (Tenodera sinensis) and the herbi-vore stick insect (Anisomorpha buprestoides) to deter-mine their respective resolution. Our hypothesis is that the predatory insect will have a higher visual acui-ty because predators need more accuracy in their vi-sion. In order to determine the resolution we have to count the number and size of the ommatidia, as well as their distribution over the entire eye. In addition, we have to determine the visual direction of each sin-gle eye which represents the visual angle. Results: The size of the mantis had a whole body length of 10 cm and an eye surface area of 166 mm2. The stick insect had a whole body length of 6.5 cm and an eye surface area of 11.5 mm2. The average interommatidial angle (Φ) is much larger in the stick insect compared to the praying mantis (Fig. 4). The average lens size is greater in the praying mantis than the stick bug (Fig. 5). The combined results form Figure 4 and 5 support the known fact that in insects the greater the lens and the smaller the interommatidial angle, the better resolution of the ommatidium (see reference). Figure 6 shows the comparison of the distribution of the ommatidial lens sizes over the periphery of the whole eye from the posterior to anterior. In the mantis we find that the lens sizes within the focal area are larger than the om-matidia in the posterior and anterior rim. This indicates a better resolution in the focal area. In contrast, the stick in-sect’s ommatidial lenses do not show any distinct size dif-ferences and no focal area. We found extreme differences between the number of ommatidia per eye (Fig 7). This is an indication for a much greater resolution of the whole eye for the mantis com-pared to the stick insect. Fig 2: Turn table for the rotation of the eye un-der the microscope. The eye is mounted on a needle inserted into the center of the turn ta-ble. Fig.1 Cross section through an insect eye, showing the lens (D) and the interommatidial angle(Φ). The longer the ommatidium the smaller the angle and the larger the lens can be. Reference: Horridge, G.A., 1977. The compound eyes of insects. Scientific American 237, 108-120 Land, M.F., 1997. Visual acuity in insects. Annual Reviews of Entomology 42, 147-177. Fig 3: Microscope viewing into the ommatidum along its axis, showing the dark pseudopupil spot (A). Rotation of the eye will move the pseudopupil to a new location (B). 20° A B - 10.00 20.00 30.00 40.00 50.00 60.00 Average Lens Sizes (μm) Praying Mantis Stick Bug Fig 5 Average ommaatidial lens size between praying mantis and the Stick insect. Fig 6 Lens size distribution over the periphery of the eye from the back to the front. Rotating the eye at twenty degree angles. Praying mantis and stick insect. - 1.00 2.00 3.00 4.00 5.00 6.00 Average Interommatidial Angle (º) Praying Mantis Stick Bug Fig 4. The average interommatidial angle over the periphery of the eye comparing praying mantis and stick insect . 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Number of Ommatidia per Eye Praying Mantis Stick Bug Fig. 7 The number of ommatidia for the whole eye of the praying mantis and stick bug . Discussion: Since the mantis is a day active predator insect, it needs a much greater visual acuity than the stick insect which is a night feeding herbivore. We found that the resolution of the single ommatidium is greater in the mantis due to a relative large lens diameter especially in the focal area and the interommatidial angle is smaller. The stick insect, be-ing night active, is more chemically oriented and has a smaller lens size and a larger interommatidial angle, which does not provide a good resolution for the ommatidium. In regards to the whole eye, the mantis has a much larger eye and many more ommatidia (8000 vs. 1300) providing a much greater resolution. This study supports our hypothesis that the mantis eye and the stick insect eye are adapted to their specific living environment and behavior. Data: - 10.00 20.00 30.00 40.00 50.00 60.00 Ommatidia Lens Size Over the Perephrial of the Whole Eye (μm)