Based on cicadas, Semis are small, yellow bug Badniks. They have six thin insect legs, a gray head with large black eyes, and yellow and orange-striped elytra.
In Sonic the Hedgehog CD, Semis appear in Wacky Workbench. In gameplay, they are found in small swarms on walls, usually in large hall sections with bouncing, flashing floors. When the player approaches, the Semis start flying away to the left or right (depending on which direction the player comes from) while dropping flashing spiked bombs in their wake which deal damage upon impact. The player can use the flashing floor to reach and defeat these Badniks with the Spin Jump.
In the Bad Future or sometimes the present timeframe of the Round, Semis show decay, their armor having partially fallen off, and cannot drop bombs. Like every Badnik in the game, they are powered by Little Planet flower seeds which will pop out of them when they are defeated.
"Semi" (セミ,"Semi"?) is the Japanese word for "cicada".