Ask An Entomologist: Eye of the Dragon(fly)
April 18th, 2007 by Kelley
I am preparing slides for an exhibit and was hoping to do a dragonfly eye can you tell me the process of dissecting the eye?
Hmm, can’t say that I have ever removed a dragonfly eye before. I have, however, removed a blow fly’s eye before and it is definitely a little messy.
First of all, you have to know a little something about an insect’s eye. Insects have compound eyes that are made up of units called ommatidia. I haven’t personally counted, but it is said that some dragonflies can have as many as 10,000 ommatidia. An ommatidium is a bundle of cells that contains the corneal lens and crystalline cone. Next to the cone are the retinular cells, which are in turn arranged around the rhabdom (the axial structure)…together, the retinular cells and rhabdom are called the retinula. Why is dissecting the eye messy? Because surrounding the retinula is a sheath of pigment cells. Each cell is full of red, yellow or brown granules of pigment. Rupturing these pigment cells while trying to remove the eye is what causes it to be messy. Next, axons of the retinula enter into the optic lobes of the brain.
I think in order to get the cleanest dissection, or removal of the eye, you would want to pick open (with forceps) and remove the cuticle of the head capsule away from the compound eye…and ideally, you would want to expose the brain, gentlly pull apart the optic lobe from the brain, and then use a forcep to gently grasp the entire compound eye + optic lobe. You may even want to just try pulling the eye away from the head capsule just by the optic lobe. Don’t try to pull at the compound eye from directly on with the forcep because you will more than likely rupture those pigment cells.
So with all of that, some of you may be wondering how they actually see? All those ommatidia…are they seeing thousands of the same picture? No. Johannes Muller in 1826 and Sigmund Exner in 1891 developed the classic theory on how insects form images, called the ‘mosaic theory of insect vision’. Basically, each ommatidium is sensitive to light that enters at a small angle. As light enters the ommatidia a dot-like pattern is formed. The entire image of what the insect sees is contructed from a ‘mosaic’ of information that is relayed from each ommatidia.
Let me know how the dissection goes 
Well I had a pretty successful first run. I might try another attempt later this summer. I first took the head off and soaked it in a 10% solution of drano and that got most of the gooey parts off and I was able to take off the eyes and mouthparts in one piece. If you use a pin then retrieving the head is a lot easier then fishing for it (took a couple of tries to figure this out) Well I wanted to see if I could get the pigments flushed out as well so back into the drano it went and would you know it worked the eyes were pigment free. So I took a knife and sliced off the lens and got an ok slide it was a little thick in a couple of places but it had great results.