I saw several of these beetles marching through cut grass in the park. They seem to be a regular sight in the fall.

An American oil beetle marching grimly along.

They are known as American oil beetles or blister beetles because of an oil secreted from their legs when disturbed. This oil is a skin irritant and soon teaches victims to leave the beetles alone. This isn’t the only example of an ability to use chemicals to their advantage. In fact the life cycle of blister beetles stretches all credulity in addition to illustrating an amazing balance between parasite and host.

This BBC short tells the story of these amazing creatures (in this case in the Mojave Desert) better than I can.