What physical structure might mites form on plants?

Mites are known for their ability to create webs, particularly on the undersides of leaves. This webbing can serve multiple purposes: it provides shelter from predators, creates a microenvironment for the mites, and can facilitate feeding by helping to secure a food source. The fine silk threads produced by spider mites, a common type of plant mite, can be particularly noticeable and are often a sign of their presence on plants. The webs can also indicate that the plant might be under stress, which may lead to decreased health and yield.

The other options refer to different phenomena or structures that are not typically associated with mite activity. Fungal growth is related to fungal infections, thorns are a structural defense mechanism of certain plants, and brittle exoskeletons pertain to the physical features of invertebrates rather than structures formed by mites on plants. Thus, understanding the behavior of mites and their interactions with plants highlights why the formation of webs on the lower leaf surface is the correct response to the question.