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Old 01/14/2008, 05:35 PM   #1
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How stomatopods judge distance

Ever wonder how how a stomatopd knows how far away an object is that it is trying to spear? This image of a juvenile Lysiosquillina sulcata looking out of its burrow provides some of the answers. Each eye is made up of hundreds of indivdiual optical units called ommatidia. Each ommatidium is like a telescope looking out at a small piece of the world. When light enters an ommatidium "on axis" (straight down the barrel), the light is absorbed and is not reflected back. The ommatidium appears black. This is analogous to looking into the pupil of a human eye, No light is being reflected back and the pupil appears black. Hence, we call these black spots on the surface of a stomatopod eye "pseudopupils".

The pattern of pseudopupils in a stomatopod eye can tell us where the various ommatida are aimed. When I took this picture, the eye on the left was pointed in the direction of the camera lens. You can see a cluster of dark spots in the center of the upper and lower hemispheres of the eye as well as another group of dark spots in the center. This means that some of the ommatidia in the mid-band of the eye as well as as groups in the upper and lower part of the eye are pointing right at the camera.

The eye on the right is looking to the right. Only a few ommatidia on the extreme periphery are pointing towards the camera. Notice that the surface of the eye near the center is concave. The ommatidia (in spearers) generally point outward perpendicular to the surface. That means that objects located directly in front, but close to the eye will be seen by ommatidia close to the midband; objects further away but in the same direction will be seen by ommatidia further from the midband. Alternatively, if I moved my camera closer to the eye, the pseudopupils would be seen closer the the midband. The ommatida of the midband (made up of six rows of ommatidia) all look out on the same equatorial plane.

Just as you tell how far objects are away by using binocular vision analyzing how much your eyes are pointing toward the mid-line, the stomatopod can judge distanceby comparing which ommatidia are focused on the same spot. The difference is that when the stomatopod is looking intently at an object using both eyes, it has six regions of the eyes brought to bare on the target giving it hexnocular vision.


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