From Sonic Retro

Semi (セミ) is a badnik in Sonic the Hedgehog CD's Wacky Workbench. It isn't listed in any instruction manual, but is referred to as "semi" (the Japanese word for cicada) in the game's source code. It is a bug-like badnik found clinging to walls in small swarms near the bottom of the level. When Sonic approaches, they unfurl their rotating wings and drift slowly right-to-left, occasionally dropping spiked balls in the style of Sonic Triple Trouble's Bombaberry badniks.

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