Brachinus sp. (Bombardier Beetle).
Size: approx. 12 mm. Photo: 04/21/15, 10 pm.

Bombardier beetles have developed a chemical defense mechanism against predators. They can eject a foul-smelling, toxic gas from their abdomen. The gas is composed of hydrogen peroxide, hydroquinone, and catalytic enzymes. The ejection creates a loud popping sound. The larvae of the bombardier beetle are parasitoids of aquatic beetle pupae and scavenge body of host after its death  
bug guide (this photo):