Microfungi or micromycetes are fungi—eukaryotic organisms such as molds, mildews and rusts—which have microscopic spore-producing structures. They exhibit tube tip-growth and have cell walls composed of chitin, a polymer of N-acetylglucosamine. Microfungi are a paraphyletic group, distinguished from macrofungi only by the absence of a large, multicellular fruiting body. They are ubiquitous in all terrestrial and freshwater and marine environments, and grow in plants, soil, water, insects, cattle rumens, hair, and skin. Most of the fungal body consists of microscopic threads, called hyphae, extending through the substrate in which it grows. The mycelia of microfungi produce spores that are carried by the air, spreading the fungus.