Will my screens keep out every insect?

Standard 18 x16 grade mesh is designed to keep out most insects including mosquitoes, wasps, flies, bees and moths. In fact the majority of flying insects won't try and squeeze themselves through small gaps. However if you do have an issue with tiny insects such as midges, fruit flies, or thrifts then you might want to opt for a smaller 20 x 20 or 20 x 30 mesh.