1) The blue spots on the top of the wings extend much further on females than it does on males
2) The males have claspers that help them hold onto the female when pairing
When I get pictures of pinned specimens I’ll post them. I'm curious how extensive this extra amount of blue along the wing of females is in Papilio swallowtails.
I released almost all of these males a day after I confirmed their sex. Males are of little use to me since wild caught females are unlikely to hand-pair successfully and they are likely to lay eggs if you just sleeve them over their host plant anyway. So far I have been struggling to find females. I have caught over 30 Swallowtails, and only one was a female. To try and figure out what was going on here I turned to an amazing textbook that I picked up earlier this year:
Swallowtail Butterflies: Their Ecology and Evolutionary Biology by Mark Scriber, Yoshitaka Tsubaki and Robert Lederhouse (1992)
If you are studying Swallowtail biology buy this book. The massive male bias is explained by the breeding ecology of Papilio canadensis. In Ch. 13 of the text Lederhouse compares the mating behaviour and sexual selection of North American Swallowtail butterflies. In this chapter he explains that Papilio glaucus and P. canadensis have a very similar mating ecology; males patrol for females and may travel great distances searching. Apparently the operational sex ratio is strongly male-biased along flyways and in concentrations of host trees. Only at nectar sources will females be equally represented.
This is definitely consistent with what I have been seeing in the field. I will be shifting my search tactic to target habitats close to nectar sources. Fingers crossed.
Update: After changing my tactics I did end up catching 2 Females! There were both feeding at a yet to be ID'd nectar source. I transplanted some new host plants to sleeve these females in, but they looked pretty wilted by the end of the day. It was hot today (30°C), but I did give them lots of water. We’ll see what they look like tomorrow. For now I have them sleeved on the 2 live host plants that I had transplanted successfully earlier.