Visual acuity comparison in developmental stages of the praying mantis (tenodera sinenesis)

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Ille, Lisa A., McGown, Taylor, Mitchell, Haley A.
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The praying mantis is a predatory insect that catches live prey with extreme accuracy. In this study we want to determine possible changes in visual acuity throughout the various develomental stages of Mantis, from the first instar to the adult stage. Insects have compound eyes composed of many single eyes. In order to optimize vision, these eyes have to balance their sensitivity to light intensity and their strength of resolving power. The larger the surface of the single eye, the more light it can capture. However, the larger the single eyes, the fewer eyes that can be fit onto the whole compound eye. In turn,this decreases the resolution. From microscopic images we measured the visual angle between the single eyes and along the long axis of the eye, and also the number and size of the eyes making up the compound eye. After these measurements, we conclude that the visual acuity increases within the sequential instars up to the adult stage.
Method: The Praying Mantis were raised from egg packages in the lab. At each instar stage a mantis was chosen and anesthetized. The head-thorax length (HT) was measured under a dissecting scope. We determined three parameters: a) the total number of ommatidia, b) the interom-matidial angle (Φ) within 20° intervals and c) the diameter of the ommatidial lenses (D). a) The eyes of the various stages were dissected under a dissecting scope. We then removed the sensory structures until the separated cuticle could be flattened on a microscopic slide and photographed using a digital camera. The total number of lens facets were counted Development of Visual Acuity in the Praying Mantis Lisa Ille, Haley Mitchell, Taylor McGown (Dr. Ursula Jander– Mentor) Fig 4: Two superimposed pictures of the pseudopupil at 40° and 60° rotation. The red bracket represents the number of lens rows between the two rotational positions. The lens diameter is measured over 5 eye facets. In this example the diameter of one facet is 208.21/5 = ~ 40μm at that pseudopupil location. Introduction: Unlike a human’s single eye, insects have compound eyes that are made up of numerous single eyes called ommatidia. The image that the insect sees is the sum of all single pixels produced by each ommatidium. The more ommatidia the insect has, the better the resolu-tion of the image. However, due to the optical proper-ties of lenses, the most acute pixel point is produced when the lens diameter (D) of the ommatidium is large and the angle between the ommatidia (Φ) is small (For the Praying Mantis, (Tendora ardifolia sinensis), visual acuity is of great importance because they have to aim and jump at their prey with extreme accuracy (Fig 1). The purpose of this study is to determine how the visual resolution of the Mantis changes over its lifetime, par-ticularly through the various distinct immature instar stages. To accomplish this, the lens diameters and interommatidial angles of the smallest first instar up to the adult were measured. The measurements were made every 20° by rotating the eye along it’s long perimeter. The rotation was from the top to the bottom of the eye (Fig 2 and 3). We then compared the relation of lens diameter and interommatidial angles between the immature instars and the adult. Our hypothesis proposed that the visual acuity would increase between developmental stages and would vary over the different regions of the individual eyes. Results: After flattening the eye cuticles we counted the number of ommatidia: ~ 3,500 for the first instar and ~7,500 for the adult. Fig 5 shows the overall increase of the ommatidial lens diameters in the various developmental stages. The developmental stages were deter-mined by the measurement of the head-thorax length from the first instar to the adult. The meas-urements were taken for each stage at 20° rota-tions along the perimeter from the top to bottom of the eye. In addition, we found larger eye lenses in the area where the mantis focuses toward the prey, which is between the rotational angle of 120° - 180°. For the correlation of lens diameter and interom-matidial angles, we only present the results for the 1st instar and the adult, for comparison. Both are inversely related, showing that for the first instar and the adult the interommatidial angle decreases (Fig 6a) and the lens size increases (Fig 6b) over the perimeter of the eye. In particular, the adult in Fig 6b shows the largest lens diameter in the focal area of 140°. Fig 6c combines Fig 6a and b. The re-gression lines indicate that large lenses are corre-lated with small interommatidial angles regardless of the body size. Conclusion: We show that the lens diameter increases with each instar and that the 120° to 180° area has the highest visual acuity (Fig 5). For the interommatidial angles we present only the comparison of the first instar and the adult. The intermediate stages were not distinct enough for comparison. We conclude that the adult’s eye has about 50% greater resolution power than the first instar. This is indicated by the count of the individ-ual ommatidia, the overall lens diameters and the in-terommatidial angle. Our data shows that both, in the 1st instar and the adult, there is an inverse correlation between the interommatidial angle and lens diameter (Fig 6c). This inverse correlation is only possible geometrically and structurally, if the lengths of the ommatidia vary over different regions of the eye. The ommatidia with a larger lens and a smaller angle are therefore longer. For this reason they intersect at a more distant point. These differences in ommatidial lengths can clearly be seen in the actual microscopic section of an insect eye (Fig 1). 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). Fig 2: Turn table for the rotation of the eye under the microscope. The eye is mounted on a needle inserted into the center of the turn table. 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. b ) After removing the head, a fine needle was placed through the base of one eye, perpendicular to the top-to-bottom long axis of the oval shaped eye. The needle was then mounted onto a turn table (Fig 2) in order to rotate the eye’s perimeter over successive 20° angles. 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 location 20° apart (Fig 3). The interommatidial angles (Φ) were deter-mined by superimposing two consecutive images taken before and af-ter a 20° rotation (Fig 4). We calculated the interommatidial angle by counting the number of lens rows between the two pseudopupils and divided these by the 20° angle of rotation. c) The lens diameter (D) was measured directly on the digital microscope images on the computer screen (Fig 4). Fig 5 Lens diameters for seven developmental stages measured at 20° angles along the top-to-bottom perimeter of the eye. Fig 6a Fig 6b Fig 6c Fig 6a and b show the relationship of the interom-matidial angle and the lens diameter in relation to the rotational angle, respectively. Fig 6c combines Fig 6 a and b, showing the correlation of the lens diameters and the interommatidial angles. Reference: Horridge, G.A., 1977. The compound eyes of insects. Scientific American 237, 108-120 Land, M.F., 1997. Visual acuity in insects. 20° A B