...the VICTORY BUG!
Cosmo appeared in The Secret Voice #1, written and drawn by Ronald Marcus in a story whose synopsis from the Grand Comic Database is worth quoting entirely:
An invisible super-intelligent insect uses his brain radio to direct Allied war strategy, send morale to Allies, drives Hitler to Hell with his voice, and helps build the atomic bomb.Comics are awesome.
You can find more about Cosmo's only appearance here.
Thanks Olaf, you've blown my mind.