Matthew sent this tw**t by entomologist Gil Wizen:
One of the most amazing bilateral gynandromorphs I have ever seen, a Dorcus stag beetle. Right – male side; Left – female side https://t.co/WIF7gygpws
— Gil Wizen (@wizentrop) September 8, 2017
Which highlighted the picture below (I can’t read the Japanese; if you can, please translate).
Look carefully:
メスの維持が感じられる pic.twitter.com/SKKaAtuzuN
— 虫キチ (@insect_crazy) September 7, 2017
“Gynandromorphs” are animals that are part male and part female, with the parts strongly demarcated; they’re not “interesexes”. Both Matthew and I have written about these anomalies before (see here and here), and, as we and this longer piece explain, such mosaic animals usually result from loss of a sex chromosome during embryonic development, which results in some parts bit being male and others female. How it works depends on the system of sex determination.
In the fruit fly Drosophila, and many insects, secondary sex characteristics (those traits that make you appear either “male” or “female”) depend on the ratio of X chromosomes to autosomes (non-sex chromosomes). If you’re XX and have the normal set of paired autosomes, then the ratio of Xs to autosomes is 1, and you look female. If you’re XY, that ratio is 0.5 and you look male. But if an XX female embryo loses one X at some stage of development, the descendant cells will be XO (“O” means “no Y’) and your ratio will become 0.5—male. Thus, the cells losing the X, and their descendants, will produce male body parts, while the original XX tissue will produce female body parts. (XO males, however, are sterile, as they lack the Y chromosome genes necessary for male fertility.)
If the X is lost in one of the first two cells, then exactly half the body will be male and half female: split right down the middle. this produces the “bilateral gynandromorphs” that I used to see about once a year while pushing flies. This is what they look like:
Males are smaller than females, so that parts of the right side of the body above, which has lost an X, are smaller than those on the left (note the shorter wings). Also, the fly has male-like pigmentation on the right (dark pigment on the posterior abdomen) and typical female striping on the left. The “sex comb” (a tuft of stiff bristles on the foreleg, probably used to grasp females during mating) is present on the shorter right foreleg but not on the left.
Here’s a photo of a real fly gynandromorph (source here):
The male half has white eyes because the original female was heterozygous for a recessive allele on the X chromosome that eliminates eye pigmentation: it was Ww, where “w” is the recessive “white” allele. The X chromosome containing the W allele was lost on the right side of the body, producing an XO male half that expresses the w allele. Note the shorter wings and darker pigmentation on the right side. If you turned the fly over, you’d see that half of its genitalia was female and the other half male.
You can use this technique to investigate where in the body male pheromones and behaviors are “produced”. I won’t get into the details of that technique (developed by Seymour Benzer), but an example of such work, done by myself and my undergraduate student Ryan Oyama, can be seen here. (I have to say, it was a clever experiment but the idea of studying sex-specific chemicals and behaviors came from Benzer).
Now as for that stag beetle: it’s was probably produced the same way the flies above were: loss of an X chromosome in a female embryo. Most beetles have the same kind of X/autosome ratio sex determination as do Drosophila, so that loss of an X chromosome in part of a beetle produces male secondary sexual traits. In the stag beetle above, an X was probably lost on the left side at the two-cell embryonic stage, and so the left side became male while the right remained female.
The clearest difference in their morphology is seen in the mandibles: the right side has the typical large and fearsome mandibles of a male, while the left bears the smaller mandibles of a female. There are probably other differences between the sides that you can’t see, as in the shape of the genitalia. Such “split” animals are a very clear demonstration of sex differences, and, in this case, the mandible difference probably resulted from sexual selection.
Dorcus is a genus of stag beetle (family family Lucanidae), and the name “stag beetle” comes from the antler-like mandibles of the males. They use these to fight over females or good mating sites (a form of sexual selection called “male/male competition”), and sometimes over food, though the sexual dimorphism was undoubtedly produced by competition for females. Here’s what those battles look like (I don’t think the beetles actually damage each other):
There will be a quiz later.


























