Posted by: arrogantscientist | March 6, 2009

The Differences Between the Sexes

An updated version of this post can now be found here:

Being able to tell male Drosophila apart from female Drosophila is about the most basic requirement for Drosophila genetics. There’s nothing dumber than trying to mate two females together, or more frustrating than putting a male in with a vial full of virgins (for you that is, I’m sure the male would disagree).

So, here are three ways to tell them apart.

The Bad Way

Male (left) and Female (right) wild-type Drosophila (OregonR)

Male (left) and Female (right) wild-type Drosophila (OregonR).

The above picture shows a clear difference in size between the male and female flies, but the place to look here is the lower abdomen – the tergites here are black on the male and not in the female. This is simple, but in practice it is not a good way to tell them apart. The different body shapes and colours between stocks and individuals can vary significantly, and the distinction is rarely as clear as above. Often, inexperienced people use this as the way to tell them apart, but if you do, you will at some point get stuck, and are very likely to make a mistake.

The Right Way 

Sex combs on a male fly.

Sex combs on a male fly.

Male Drosophila have a patch of bristles (the black bits above) on their forelegs, used during courtship, that females do not. If it has sex combs, it’s a male. This method is probably the most accurate, but rarely used to sex drosophila in practice. It would take far too long to look at the forelegs of every fly you need, when it is possible to sex the fly from a distance (especially since you have to know what you’re looking for to even be able to notice the sex combs).

The Best Way

Ideally, you need to be able to sex flies accurately, but quickly. The best method is to simply look at their genitals.

Drosophila genitals, front view.

Drosophila genitals, front view. (Male left, female, right).

These rarely differ in appearance between individuals, and can be seen from a distance and from various angles.

Drosophila genitals, side view. (Male left, female, right).

Drosophila genitals, side view. (Male left, female, right).

In practice, someone experienced in sexing flies will use a combination of these three methods. With a little practice, it is easy to become proficient at separating males from females, no matter what phenotypes the flies may have.

Things to remember

Depending on the genotype, age, conditions and other factors, individuals can vary significantly. If in doubt, try verifying using the sex combs, but if you can’t, just don’t use the fly.

If it has has an egg sticking out of it… it’s a female.

Males are not always smaller than females (especially if you are dealing with tubby mutants – often found on TM balancers).

Hermaphrodites can occur, but they’re very rare. I might have a picture of one I found somwhere…



  1. Helpful and fun to read i love the pics
    Thanks = )

  2. Hi,
    Your photos about sexing flies are really nice! Would you object if we were to reproduce these with attribution for the manual for the laboratory course we run at Cornell?
    Mike Goldberg

  3. Hi Mike,

    I’d be more than happy for you to use my pictures. If you drop me an email (, I can give you better versions with the lighting corrected and no text.

  4. Does the female in the top photo have an egg sticking partway out of her? I can see something small, round and translucent at the very tip of her abdomen, and I don’t know if it’s an egg or just part of her.

    These are really wonderful photos!

  5. That little “knob” is not an egg, it’s part of the female, although I confess I don’t know what it’s called. The eggs are actually somewhat larger and are pearly white.

  6. The little “knob” as you call it is the gonopod of segment 8 or in layman terms the vaginal plate.

  7. […] More fly sexing resources […]

  8. 財布 レザー メンズ

  9. This is exact same pattern I followed as I learned, and mostly the same advice I would give. However, I would note that the bristles can be difficult to see in some types, especially if you are doing virgin collection (I.e., the males are young and therefore paler.) I’ve been confident in many a separation, only to come back a couple of days later and find a male slipped his way into my stock!

  10. […] like part of a beehive or maybe a fancy hair care tool, but the “combs” in this case refer to sex combs.  Sex combs are patches of bristles found on the forearms of male fruit flies (so they can hold on […]

  11. thankyou sir, now i can determine D.M. sex in whole new level 😀

  12. Sir,
    Presently I’m studying about the experiments carried out by Thomas Hunt Morgan on Drosophila to verify the ‘Chromosomal Theory of Inheritence’.
    It would be helpful if you post pictures of ‘yellow bodied, white eyed’ females and ‘brown bodied, red eyed’ males. This will help me to understand the topic better.
    Thank you.

    Dhruv Nimbalkar

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  14. Good job sharing this content

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