Fungi belonging to the class Trichomycetes lie hidden within the digestive tract of several types of insects and other arthropods. Upon 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.
Regardless of where trichomycetes are attached to their respective hosts, they have an obligate association with them. That is, the fungi are not capable of growing, metabolizing, or reproducing in the natural environment other than in association with their living hosts, at least insofar as we currently understand these microorganisms. In a few instances released propagules functioning as dissemination agents may undergo some development while separated from their arthropods. These include the amoebae and cysts of the Amoebidiales and the resistant primary infestation spores of some Eccrinales.
The existence of these gut fungi in many groups of commonly encountered arthropods is not generally realized, nor is the fact that they have been found on all continents of the world (except Antarctica) as well as on many larger and smaller islands. As presently known, their distribution ranges from north of the Arctic Circle to south of the Tropic of Capricorn and from deep sea level to mountain altitudes as high as the larval stages of some insects are capable of development.
The majority of arthropods known to be associated with these gut fungi live in freshwater habitats. They include, among others, larvae of a number of families of flies (Diptera), mayflies (Ephemeroptera), and stoneflies (Plecoptera), and adult stages of some isopods, amphipods, cladocerans, and copepods (Crustacea), and even a few species of springtails (Collembola). Certain arthropods from terrestrial habitats, especially millipedes (Diplopoda), but including some beetles (Coleoptera), and isopods are frequently infested with trichomycetes. Trichomycetes in marine habitats are known to be associated with several kinds of true crabs (Brachyura) and shrimplike crabs (Anomura), as well as isopods and amphipods. Most of these marine animals live in the intertidal or subtidal zones; others live on higher reaches of marine shores. One marine fungal species, Arundinula abyssicola, has been found at depths of 2600 m.
Virtually all of the arthropod hosts of trichomycetes are detritivores, scavengers, or consumers of living aquatic algae, and they all have chewing mouthparts (are mandibulate). Most of the fungal species live in the hindgut region; some attach to the peritrophic membrane in the midgut of Diptera larvae; a few grow in the foregut of crustaceans. They exhibit a greater or lesser degree of host specificity, but all develop spore types to ensure successful host-to-host transmission. Further, they have adapted in special ways to integrate their development with that of their arthropod hosts, including dealing with the molting process that all arthropods periodically undergo during growth and which in turn causes the fungi to be expelled from the gut.
Although trichomycetes are obligately associated with their particular hosts, the reverse is not necessarily true. Natural populations of arthropods that might be infested can be devoid of trichomycetes, or they may have a low percentage of infested individuals. In some species of arthropods, such as certain millipedes and passalid beetles, virtually all adult individuals contain trichomycetes in their guts. Because these gut fungi do not usually affect their hosts adversely or beneficially in any obvious way, they are often called commensals. Nevertheless, there are several reports, one well substantiated, that at least one species of the fungal genus Smittium (Harpellales) can cause high mortality to mosquito larvae cultured in the laboratory. On the other hand, there is limited experimental evidence that mosquito larvae grown on sterol- and vitamin-deficient media develop faster and go through more instars when they are infested with another species of Smittium. Thus, there may be a range of symbiotic relationships between these fungi and their arthropod hosts. This important biological question has not been addressed to the extent it should be, in part because only about 6% of the recognized species of trichomycetes have been cultured axenically, and those that have been represent only two of the seven trichomycete families.
It has now become evident that some Harpellales affect the fitness of their hosts by growing at particular times from the gut into the developing ovaries of the larvae, with the result that infected adult ovaries contain masses of fungal cysts and no eggs (see Chapter 8). Though sterile, the female flies to new sites where she "oviposits" the cysts, thus providing a dispersal mechanism for the fungi. This is currently known to occur in some blackflies (Simuliidae) and midges (Chironomidae), but it is conceivable that all Harpellales, living as they do in the guts of aquatic, non-flying larval insects (with one exception), have evolved a similar means of dispersal.
The Class Trichomycetes is placed in the Zygomycota, among the lower fungi. There are three orders according to the current classification (see Table 1.1). The unrelated protistan Amoebidiales (with two genera) are treated here as "trichomycetes" (lower case t) because of certain parallelisms, tradition, and frequent association with Harpellales. Included in the true Trichomycetes at this time are 7 families and more than 55 genera and 181 species. They are morphologically and ecologically quite distinct from all other fungi. The known trichomycete mycota probably represents only a fraction of the actual extant species. Revisions of this monograph will undoubtedly include many new fungal and arthropod host taxa, as well as the provision of new taxonomic arrangements and new phylogenetic assessments.