A small insect or bug, or a small version of something resembling a bug.
From 'bug' plus the diminutive suffix '-let' (meaning small), following the pattern of words like 'booklet' and 'droplet' that indicate smaller versions of things.
The '-let' suffix is wonderfully productive in English—you can add it to almost any noun to make it cuter and smaller, which is why we have starlets, leaflets, and tiny buglets.
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