What is the secret to the evenly balanced exposure across both the foreground and background in this fisheye photograph?

It is not a clever processing job in photoshop. Rather, I used a handheld flash set to sufficient power to bring the foreground caterpillars up to the same light levels as the sky. Fill flash is an effective technique. In the absence of a flash I could either expose for the sky, leaving the caterpillars in the dark:

Or, I could expose for the caterpillars, leaving the sky blown out:

Careful use of flash allows the best of both worlds, with a minimum of digital correction.

photo details:

Sigma 10mm f/2.8 EX DC HSM Fisheye Lens on a Canon EOS 6D

Lighting: Canon 430 EXII Speedlite with a Lastolite Ezybox diffuser