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What's the diameter of a fly's eye?

  1. Apr 23, 2008 #1
    [SOLVED] what's the diameter of a fly's eye?

    I'm a maths teacher and in a topic on units and 'big and small numbers', I've set my class various items to estimate, including the diameter of a fly's eye...

    Now it's driving me mad, I've spent hours googling to try to find out a reasonable range of answers, I can't find anything, I'd be glad to find ANY answer!

    Please help me!
  2. jcsd
  3. Apr 23, 2008 #2


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    I googled entomology anatomy, and got lots of good hits.

    The 3rd hit in the list has a nice fly anatomy zoom-in page, where you could figure out the size of the eye if you know the size of the fly:

  4. Apr 23, 2008 #3
    aha, as a mathematician, I didn't know the word entomology, thanks

    The link you gave has wonderful photos of flies. Like you say, I still need to know the size of the fly, so I'll get back to google

    thank you
  5. Apr 23, 2008 #4


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    What species of fly? Flies come in quite a variety of sizes.
  6. Apr 23, 2008 #5
    What I really want is a range of reasonable answers, (for mass of an orange, I have put 100-300g), but I'm still struggling to find anything. I've just started googling on "electron micrograph fly eye" and hoping for an enlargement factor!
  7. Apr 23, 2008 #6


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    Well, as a guesstimate (which is probably all you're expecting of your students), I'd say fruit fly eyes are probably about 0.5 mm diameter, while some of those bigger flies are closer to 1 mm, maybe 2 mm in those big horseflies. There are smaller species of flies than fruit flies, so they'd be reasonable to guess smaller than that. I've never seen those HUGE horseflies in person (thankfully)...there's more than one type of horsefly...so am not sure how big their eyes look.
  8. Apr 23, 2008 #7
    Thank you! I'll go with that.
  9. Apr 24, 2008 #8

    Andy Resnick

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  10. Apr 24, 2008 #9

    jim mcnamara

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    Andy -

    I think the point you might want to make is that compound eyes are made of a range of ommatidia -

    Ranging from a few to the head being completely covered.

    The OP was probably thinking of a housefly when he posed the question.
  11. Apr 24, 2008 #10


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    Given the level of the question, I'm assuming this is an assignment for rather young students, so don't expect they'd have learned about compound eyes yet or consider that in their answers.

    I do wonder how fair it is to ask students a question on an assignment that the teacher cannot even answer. Afterall, if the teacher's experience with observing flies isn't sufficient to estimate the size of their eyes, how are children supposed to come up with an estimate? If the answer can't be determined without searching online for actual numbers, then it entirely defeats the purpose of the exercise of learning to make estimates when exact numbers are not available.

    Perhaps this time the teacher has learned the most important lesson...don't give an assignment to students if you don't already know the answer to the problem you're asking. If you don't know the answer, there is no way they should be expected to know it. It's not even fly season yet, so there aren't even any readily available specimens for students to observe to attempt to come up with an answer.
  12. Apr 24, 2008 #11
    Thank you everybody for your help.

    I'd like to defend my right to ask difficult questions. I think it's important to work and learn with the students; to encourage students to ask questions that occur to them, and where appropriate they/I/we work at solving them. I have an impressive academic record, but I'd never pretend to students that I know all the answers.
    I'll happily estimate the size of a fly's eye, the mass of a lorry, capacity of a bath, the temperature of a glacier. I am confident that I would give answers of the right order, in appropriate units, but I wanted to have some real data for comparison with my answers and my students' answers.

    For me the surprising aspect of all this, was how hard I found it to look up the answer, I'd imagined it would be a 5 minute google, like the height of a giraffe.

    Anyway, thank you
  13. Apr 24, 2008 #12
    I think it is important to note that he has only asked the class to estimate the size of a flies eye. That does not mean that they have been asked to go home and look it up for hours on end, as far as I am aware he is just trying to expose his students to a variety of quantities. The focus isn't on the exact size, as I doubt that is the point of the excercise, in which case it doesn't matter that the teacher is not sure himself.

    If you are female, I apologise! :smile:

    Goodluck, _Mayday_
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