Fungi belonging to the class Trichomycetes live hidden within the digestive tract of several types of insects and other arthropods. On dissection of the gut, they may be seen as small unbranched or branched fungal bodies (thalli) firmly attached to the gut lining and lying within the gut lumen from which they obtain their nutrients. Many are minute and require suitable preparation to be seen with the microscope. Some are larger, so that in guts that have been cut open they may be observed, with the unaided eye or with a low-power dissecting microscope, growing as individual thalli or in dense clusters. In the aggregate, longer thalli may make the inside of the gut look fuzzy or hairy, hence the name trichomycetes (“hair fungi”). In a very few species, part of the fungus may protrude from the anus in such a way that its presence can be detected without cutting open the gut. One trichomycete (Amoebidium parasiticum) grows externally on the exoskeleton of a variety of aquatic arthropods, usually daphnids and insect larvae, but the thalli are small; even when they are numerous, some magnification may be required to distinguish the trichomycete from bacteria and protozoans that sometimes occupy similar sites.
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