I'm a nature photographer and want to know how to identify the plant or animal within an image.
eg Common name: Housefly Scientific name: Musca domestica
Into which fields should the data in bold go?
If you're software supports it, technically I believe you would want to look at the Darwin Core Taxon tags.
For just general usage,
HierarchicalSubject would bet a good place. You can set up a hierarchy as simple as
Common Name|Housefly and
Scientific Name|Musca domestica or go full classification with
Animalia|Arthropoda|Insecta|Diptera|Schizophora|Muscidae|Musca|M. domestica (
Kingdom|Phylum|Class|Order|Section|Family|Genus|Species). You could probably find a Controlled Vocabulary that would have such things already set up for you.