14. Visible to the naked eye. It is assur d’oraisa to eat an insect even if it is as small as a hair (רמב''ם פ''ב מאכ''א הכ''א ), e.g., “milbin” [mites] or the like, provided it is visible with one’s natural vision. If it is invisible to the unaided human eye, it is not assur. The Torah does not forbid microscopic things, as the Torah was not given to malachim (ערוה''ש סי' פ''ד סל''ו, שו''ת שבט הלוי ח''ז סי' קכ''ב, שו''ת מחזה אליהו סי' צ''א ). Thus, microscopic bacteria are obviously not assur (שו''ת אג''מ יו''ד ח''ב סי' קמ''ו ).
15. An indistinct dot. If one can see a dot but cannot tell if it is a bug or something else, and a magnifying glass can help identify it as a bug, it is assur; the magnifying glass is merely clarifying what the eye is seeing. However, there is no chiyuv to inspect every dot with a magnifying glass, as we do not assume the presence of issur, unless there is a reasonable concern or something has a suspicious appearance suggesting it might be a bug.
16. Visible in sunlight. Some bugs are extremely small or the same color as the food they are on. It is very difficult to spot them unless they are held up to the sun or another light source and inspected. If it is the type of thing that must be checked for, one must check by sunlight or with a light box designed for checking for bugs, as that is the way to spot them (פר''ח סי' פ''ד סקל''ד ).
