This past June, I was in the field with a coworker at a mitigation site in Lake County, Indiana, and we came across this odd larva moving across a gravel drive between weedy field and emergent marsh.
It may be a bit difficult to get an idea of scale from this photo, but this larva was approximatley 1.5 inches long. I searched online and through all of my references, but I couldn't figure it out, so I put the image aside, stored as an unknown beetle larva. Fast forward to yesterday, when I was going through photos in the project file while working on the report. I decided to try out http://www.bugguide.net/. I have visited the site on numerous occasions, but I had never sent them an unknown photo. After signing up for the webpage, I sent the photograph above, and within the next couple of hours the mystery was solved. This is the larva of a water scavenger beetle in the genus Hydrophilus. Adults are shiny black, shaped a bit like an elongated football, and are approximately 1.5 to 3 inches long. Being that large, they are pretty conspicuous, and can often be found at night near lights.
I was quite impressed with how quickly my unknown was identified. The folks at http://www.bugguide.net/ have a good thing going. Be sure to check out their site when you have a chance, and if you have any photos of unknown insects, submit them and there is a good chance that they won't be unknowns for very long.