Compound eye of the firefly Photinus pyralis has approximately 3,036 facets. SEM Copyright © 2011 by Dennis Kunkel Microscopy, Inc. Specimen collected, preserved, provided, identified, described, and digitally enhanced by Terry Lynch.
Did you ever really wonder how many facets (or ommatidia) were in the compound eye of an insect? And how would you go about making an educated guess or estimate in this regard?
Using the above scanning electron micrograph (SEM) let me show you how this can be easily done.
First draw a line the length of ten facets and measure the length of that line. In the above case I got 28.5 mm using the size of the image that was upon my computer screen. You may get something different if you print out the image or have a different size for the image on your computer screen. However, that does not matter as all measurements are in proportion for whatever size you make the image.
Now take a mean measure across the diameter of the firefly's eye. As in the above example I measured across the firefly's eye and got 105 mm. Remember, this calculation is NOT exact because actually a firefly's compound eye is a hemisphere, and even then it may not be an exact hemisphere. In any cast this method will give a good approximation because all we need to know is how many facets are in the radius of the hemisphere..
Now a simple ratio and proportion calculations says that if there are 10 facets in 28.5 mm there are X facets in 105 mm. Therefore
Therefore there are 36.84 facets in the diameter of the hemisphere; in one-half the diameter or the radius of the hemisphere there are 36.84/2=18.42 facets.
Now the surface area of a hemisphere is one-half the surface area of a sphere or 4pr²/2 or 2pr², where p=pi or 3.14. In this case the radius of the circle, r, formed by a plane through the base of the hemisphere as measured in facets is r=18.42.
Thus the surface area in facets would be 2(3.14)(18.42)(18.42) = 2130.78 facets or approximately 2131 facets!
This estimate is good if facets were round, but they are hexagons; hence more can fit into an area. In this case it is approximately 1.425 more, given the factor one uses to determine this amount is the radius of the facets or 2.85/2=1.425. Hence (2130.78)1.425 = 3,036.36 facets or approximately 3,036 facets. That's a pretty good estimate of the number of facets in a firefly's eye.
So the next time you wonder how many facets there really are in a firefly's compound eye, you know it is
Of course this is just an approximation as the compound eye of a firefly is not an exact hemisphere and the SEM image being used to make this calculation is a projection of the hemispherical compound eye of a firefly upon a two-dimentional surface.
Of course there are other ways you can get a much more accurate calculation, but this is a very easy method and gives you a good estimate. And yes, I know we are making a big jump from a straight line measures across a firefly's eye to make a calculation based upon the formula for a perfect hemisphere. But still, this will give you a good estimate given we are not doing rocket science here.
You may want to try this with other insects. For example, try calculating the number of facets in the compound eye of a house fly or a dragonfly. What type of insect has compound eyes with the most facets? You can browse the Internet for images of compound eyes of different insects and then use the above method to estimate the number of facets in their compound eyes.
Given this, can you devise a more accurate way to calculate the number of facets in an insect's compound eye? Perhaps if you can devise a very accurate way to calculate this number you may then study fireflies or other insects to see how this number may vary.
One method might be to make clear impressions or images of a compound eye and then take very careful measurements using a microscope. Or perhaps computer simulation or modeling could take an image of an insect's compound eye and scribe it in 3-D such that you might then make very accurate calculations of the number of facets in an insect's compound eye.
In any case, I just thought I would share this bit of firefly muse with other firefly enthusiasts. Now don't forget, the next time someone asks you how many facets are in a firefly's compound eye you can tell them
It may be of interest to note that that dragonflies have the largest number of facets in their compound eyes. "The migratory, fast-flying aeschnids have the largest eyes and most impressive acute zones. Exactly 28,672 ommatidia have been counted in one eye of Anax Junius, which has the smallest interommatidial angles of any insect (0.24° in the dorsal acute zone) and facets of corresponding size (62 |im)." Source: Eyes and vision (Insects).
This report is an excellent review on the structure and function of insect eyes and notes that, "Insect eyes are of two basic types: compound (or multifaceted) and simple (or single chambered)." However, what many people are not aware of is that there are actually two different types of compound eyes, i.e, "Compound eyes are of two distinct and optically different kinds: apposition eyes, in which each receptor cluster has its own lens, and superposition eyes, in which the image at any point on the retina is the product of many lenses." This is important as in the case of fireflies they have superposition eyes and instead of imaging hundreds of inverted images, a firefly actually perceives an erect image! This was explained and demonstrated by Sigmund Exner in 1891 who actually took a photograph of himself through the cleared eye of Photuris sp.
These photographs taken through the cleared eye of insects show what they see. On the left is the image of a candle through the eye of a horse fly showing many inverted images. On the right is the image of Sigmund Exner taken through the eye of Photuris sp. Exner published this photograph of himself in 1891 and explained that the eyes of a firefly actually work much like those of a telescope which has two lenses.
Enjoy reading these original articles and reports about fireflies by Terry Lynch who has long enjoyed spending his summer evenings watching the dance of fireflies in fields, meadows and forests. Lynch has been studying fireflies since he was a teenager and lived at the edge of Talladega National Forest in Jacksonville, Alabama, where Photinus pyralis occurred in such great numbers one could easily net hundreds in the course of five or ten minutes. Today such large numbers of firefly have decreased in many areas due to the impact of urbanization, over spraying for mosquitoes, light pollution and other factors. Learning about fireflies is the first step in preserving them that our children's children will still be able to enjoy watching fireflies twinkle and flash during their twilight mating dance each summer.
The Firefly Gallery Help save the firefly! The proceeds from these charming design items will be used to support firefly studies and research and advocate preservation of the environment and fireflies.
Firefly Toxins, Lucibufagins, Bitter Tasting Agents May Serve As Deterrent To Substance Abuse General safety rules with respect to handling arthropods and providing a safe environment for children. By Terry Lynch
Credits: Specimens of fireflies collected, preserved, identified, described, and provided were imaged by Dennis Kunkel of Dennis Kunkel Microscopy, Inc. Any usage of these images requires prior and written permission of the owner/s.