Thanks for the comments, folks. These guys were swarming (~20 of them) on the parkway near my house in urban Toronto. The city cuts down trees in the parkway for various reasons (age, damage, disease) and then just buries the stump and roots. Apparently these beetles live in the rotting wood, and they dig out to mate at this time of year (at this latitude).
These stag beetles were in constant battles (apparently for the best mating grounds, which I think is weird for a bug... but then I'm not an entomologist). They'd wander about, run into each other and get into it! Almost like a hockey game.

When it was quiet I could actually
hear the fights and would carefully tiptoe over to shoot them.
My son spotted them first on the way back from the park, and was rather afraid of them (he's 3). I've always been fascinated by scarab beetles, so I hoped to get him intrgued in bugs bigger than ants as well. After 15 minutes he wanted to go inside for juice, so I went and grabbed my Pentax.
Originally Posted by 9thumbs
cool series, how long did you hve to watch these bugs to get those shots?
These are the "keepers" from around 100 shots, and I think I was shooting for maybe 30 minutes or so, going from fight to fight. Due to the AF confusion of the grass blades, I switched to MF and just shot a lot. They are all ISO 800 because it was a shady spot, and I still didn't get the shutter speed high enough. If they are still around this coming weekend, I'll break out the flash and radio triggers and
really freak the neighbors out!
Originally Posted by foxglove
"The Sigma 17-70 isn't stellar here"
I suppose these shots are not quite stellar, but they look pretty fine to me! I'd be delighted to have got them.
If you owned a macro, what are the chances you would have had it with you when you saw this?
Well, I actually had the 50-135 with me at first, and
went back home and got the Sigma when I couldn't even see the bugs at the minimum focus distance! So if I had a 100mm macro at home...
I do see your point, and actually this "close focusing" is one of the things I really like about the Sigma 17-70. I've actually considered a "macro" attachment lens in the past, but since I generally shoot landscapes this hasn't been a priority.