This large diving beetle bounced off a lighted wall right next to me at about 11:00 p.m. It was not injured, and I captured it, keeping it cool for a photo session later. Length was measured by photograping adjacent to a scale. It was released into a flooded ditch after the session.
This series of photos, I believe, illustrates the difference between Cybister
and the very similar Dytiscus
. In the southeastern United States, the only two very large (circa 30 mm) members of the family Dytiscidae
are these two genera. Cybister fimbriolatus
is apparently the only species of that genus in my area--see guide page for species.
This photo shows the beetle swimming in a dish. The yellow margin to elytra and pronotum is found in both Cybister
. Looking at the illustrations in Dillon, (1)
it seems that Dytiscus
has a much more prominent fringe on hind legs (metatarsi). Looking at the illustrations in White, (2)
it appears that male Dytiscus
has more prominent rows of punctures on the elytra than does Cybister
. You can just see the punctures here. I don't know if this character is consistent.