How do mosquitoes bite through fabric?
A mosquito's mouthpart, called a proboscis, is a needle-like structure roughly 100 microns wide, about the same diameter as a human hair. It's designed to pierce skin and draw blood, and it can still do this through thin or loosely woven fabric when pressed close to the skin.
The key factors are fabric thickness and the tightness of the fiber weave. If there's enough space between the fibers, or if the fabric is thin enough, a mosquito's proboscis can reach through to the skin beneath.