Sexual Dimorphism in Moths

Sometimes I'll observe two distinct forms of moth that turn out to be the same species. Initially this was puzzling to me, but I have learned a bit about sexual dimorphism (each gender looking different) in moths, so now when this happens I usually suspect this is what's going on.

Sexual dimorphism is not rare in the animal kingdom. Many people are familiar with sexual dimorphism in birds. Generally males are more colorful, and females are drabber so they are better camouflaged while sitting still on a nest. The Northern Cardinal is a great example of a bird everyone in Oklahoma is familiar with - a bright red male and a tan female. And if you've been lucky enough to see a male Painted Bunting you'll know the male looks like a rainbow. Meanwhile the female is light green all over.

Some birds have size differences between male and female. This is most common with the raptors, where the females are larger than the males. There is an article describing the theories for this size difference here.

This is also common in insects like ants and bees. Most people are familiar with the Queen Bee and how she can usually be spotted in a hive due to her much larger size.

Just like with birds and many insects, if there is a size difference between male and female moths, the female is usually larger. Here is an example:

Larger and lighter female Waterlily Leafcutter Moth (Elophila obliteralis)

Smaller and darker male Waterlily Leafcutter Moth (Elophila obliteralis)

Curiously the theories for larger female birds doesn't have any relevance with moths. There must be a different reason. Biologists believe the females are larger to allow them to lay more eggs. There was a study done about a decade ago at the University of Arizona to determine how the females become larger. They determined they become larger by eating longer as larva before pupating. So now (we think) we know the how and the why.

I don't think there is a general rule on color differences between the sexes. For instance, the Promethea moth (Callosamia promethea) has a very dark male and a lighter and more colorful female, while the Wedgling Moth (Galgula partita) has a light brown male and a darker brown/maroon female.

Dark male (above) and colorful female (below) Promethea silkmoth (Callosamia promethea)

male Wedgling Moth (Galgula partita)

female Wedgling Moth (Galgula partita)

Here is a mating pair of Fall Webworm Moths (Hyphantria cunea). The one with darker and larger spots is the male. I know because a little bit later the lighter one started laying eggs. The bugguide page for this species does not mention sexual dimorphism. Instead it says that the difference in extent of spots is regional. I'm not so sure after having seen this mating pair...

male (left) and female (right) Fall Webworm Moths (Hyphantria cunea)

Other common differences are antenna size. Many males have large, bushy antenna that allow them to smell the pheromones released by females of their species. For example, check out the size of the antenna on the male and female Chickweed Geometers (Haematopis grataria) below:

male Chickweed Geometer (Haematopis grataria)

female Chickweed Geometer (Haematopis grataria)

Now, all this being said, variation in coloration and size and other factors is not always attributed to gender. As mentioned earlier, sometimes these variations are due to location, others are a mystery. A great example is the One-spotted Variant (Hypagyrtis unipunctata), which as the name suggests, is quite variable. Bugguide says that the female is usually larger with a more scalloped hindwing, but there is a lot of variation in coloration based on geography and seasonality, as well. Here are three I have observed which look quite different.

Probably a male One-spotted Variant (Hypagyrtis unipunctata)