Ant-Man fights a beetle made super-intelligent by radiation. It uses Ant-Man's growth gas to become giant, but he is able to defeat it.
Reprints
from Tales to Astonish (Marvel, 1959 series) #39 (January 1963) [Extensively edited to bring the story length down to 3.5 pages. Prints, in this sequence, detail from cover, used as story panel; pages 6-7; page 8, panels 1-5; page 9, panel 1; page 1; page 8, panel 6; page 9, panels 2-6; page 10. panels 1-5 & 7-8]
from The Eternals (Marvel, 1976 series) #3 (September 1976) [Extensively edited to bring the story length down to 4 pages. Prints page 11, panels 3-5; pages 12-14 & 16; page 17, panels 1-4 & 6]