(50,000) Windows to the soul

One of the things I find most interesting about all insects, and not just this highly cooperative grasshopper that I happened upon while walking a tomato field a bit ago, is their compound eyes. We say that these are compound eyes because each is composed of thousands of photosensitive units called ommatidia (the diagram shows 15 of these forming part of the surface of the eye). Light enters each ommatidium through a cornea (labeled lens in the figure) and this is focused by the crystalline cone into a beam which enters a narrow tube called the rhabdom. The rhabdom is surrounded by retinula cells that generate electrochemical signals which pass along axons which are bundled to form the optic nerve, and this forms a direct connection to the brain. Contrary to popular belief, insects do not see a myriad of repeated images, akin to the wall of flat-screen televisions one encounters upon entering any well-supplied electronics store. Instead, each ommatidium forms an image of one tiny region of the animal’s visual field; all of these are then processed to represent the immediate visual environment. Because the cornea is fixed and cannot be focused, the resulting image isn’t highly resolved (the image of an insect’s view-of-the-world included below shows a stream, a near shore and a distant treeline). This arrangement is best suited for discerning movements as they occur across areas of contrast. The eyes of a horsefly, however, each with 50,000 ommatidia, can form a much more detailed picture of the world. Notice also that the grasshopper has a dark spot in the shallow depression between the antennae and also a spot between the right antenna and the right eye (there is a corresponding one on the animal’s left which is obscured by the antenna there). These are also organs of photoreception, ocelli, which are not as complex as the compound eyes and the function of which is limited to the discernment of light and dark. Finally, the title to this post begs the question of whether insects have a soul. I’ll get back to you on that one.

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