Can male Eastern Tiger Swallowtails be dark morphs?

I observed two adult Eastern Tiger Swallowtails - Papilio glaucus - today, one dark morph and one light morph. I believe both are females, but I cannot find any photos of male dark morphs to compare. Is this color morph only seen in females? If so, is there a known reason why it only occurs in females?

Oh, interesting question. AFAIK, dark morph only occurs in females. The genetics is interesting.

In butterflies, the sex chromosomes are the opposite of those in mammals. Female butterflies are the heterogametic sex (XY), and males are homogametic (XX). Yellow Papilio glaucus females give birth to yellow females, and dark females give birth to dark females indicating that the gene for color is on the Y chromosome (Scriber et al.1995).

Source: University of Florida–Featured Creatures (Eastern Tiger Swallowtail)

Edit. Poking around a bit more, I see that others use a different terminology for the chromosomes, and I believe that is more up-to-date:

In butterflies, sex is determined by chromosome differences. But unlike in humans with the familiar X and Y, in butterflies, it is the females that determine the sex of offspring. Males are ZZ, while females are ZW.

Source: When butterfly male sex-bias flaps its wings

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Very interesting! I had considered the possibility of a sex-linked gene as a possible explanation, but I totally forgot that sex is not coded the same way in all species.

indicating that the gene for color is on the [W] chromosome

This makes a lot of sense! Thanks for the information!

Out of my own curiosity, I looked a bit further and it seems that most Lepidoptera use the WZ/ZZ system, but a very few do not. I guess all butterflies use the WZ/ZZ system. See, for instance:

Remembering that the X and Y chromosomes got their names from their shapes – where the centromere is positioned – I’m trying to imagine what the W and Z look like in a karyotype.

Oh, interesting question. I searched for “ZW karyotype” and found a few images, such as this study of pigeons and, to my eye, they don’t look like the letters, at least not the “Z”. Maybe the “W”?. My guess is biologists just picked letters adjacent to X and Y to indicate the analogy, and it is not based on shape.