This may be more information than you want to know, but fly specks are a combination of excrement and undigested food. One thing you probably do want to know, however, is how to get the specks off your patio furniture, and while the same method won't work for all furniture types, one way works for most. You can use a combination of abrasive action and disinfecting power provided by two common household cleaners.
Saliva and Excrement
Flies don't have mouths, so they must liquify their food so they can lap it up with their spongy tongues. The chemical that does this is similar to the one that's in human saliva -- a starch-busting enzyme called amylase. After the fly spits this enzyme onto its food, the food turns mushy, and whatever the fly doesn't manage to consume remains behind as one source of fly specks. Of course, flies also leave evidence of the end results of their digestive processes, which is the other source. Both saliva and excrement contain harmful microorganisms; for example, the parasite that causes river blindness lives in the saliva of the black fly.
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A General Cleaning Regimen
Fly specks may be tenacious, but they aren't impossible to remove. You may be able to get most of them off of metal patio furniture with a solution consisting of an ounce of dishwashing detergent with a gallon of water. The reason the specks stick more persistently to wood finishes and plastic is that the amylase partially dissolves the substrate on which it lands, so you may need more abrasive action. Use baking powder to avoid damaging the finish on wood furniture or etching plastic furniture. Sprinkle the powder liberally on a sponge and rub the specks vigorously. You'll have more success if you've already washed with the detergent solution.
Disinfect and Polish
After you've managed to remove the fly specks, you should disinfect the piece with white vinegar to kill any lingering parasites that may have been left behind. Vinegar is acidic, and using it full strength on a wood finish or on plastic isn't recommended. Instead, mix a one-to-one solution of vinegar and water, soak the affected areas liberally, using a sponge or a cloth; then rinse with clear water and dry the area with a separate rag. If you notice some dulling of your plastic furniture after this treatment, spray some lubricating spray on the affected areas and wipe it off to restore the luster. Spray lubricant will also remove stubborn specks that won't come off any other way.
Not All Specks Are From Flies
A certain type of fungus -- Sphaerobolus stellatus, or the cannon fungus -- reproduces by ejecting its spores forcibly from the fruiting body, leaving 1/10-inch specks on windows, siding and outside furniture. These specks are as tenacious as fly specks, and you remove them in the same manner, although removal may require a significant amount of scrubbing on your part. If they are recurring frequently, you can take steps control them. Because the fungus lives on rotting wood, twigs, wood chips and dung, the main control strategy is to keep the patio clear of these growing substrates.