All known hosts of trichomycetes belong to the Subphylum Mandibulata of the Arthropoda. To the extent that these fungi have been studied, no assimilative stage occurs while they are disassociated from their hosts, although maturation of some resistant spores and the development of cysts in the Amoebidiales may take place in the shed molt or on a nonhost substrate. Three classes of arthropods are involved: Crustacea, Diplopoda, and Insecta (Figs. 4.1 and 4.2). Asellaria scutigera (nom. nud.) was reported by Manier in 1954 from the rectal cuticle of a centipede (Chilopoda), but the description was not sufficient to determine if it was, in fact, a trichomycete. Lists of known arthropod hosts and their habitats are given in Appendix B. It must be stressed that these lists, as well as the discussion presented in this chapter, are based on current knowledge of the range of hosts, and will undoubtedly have to be modified as new discoveries are made. When one considers that the Arthropoda may represent more species than all other groups of living organisms combined, and that the trichomycete flora has been sought in relatively few parts of the world, it becomes evident that generalizations must be to some extent tentative.
The mandibulate hosts of trichomycetes feed primarily on decaying vegetation of various kinds or living algae, or they are omnivorous. Obligately parasitic, predaceous, or carnivorous arthropods, with rare exceptions (see Chapter 6), do not appear to be suitable hosts for the fungi, nor are those that feed only on the living tissues of higher plants. We are excluding from consideration the ectocommensal trichomycete species, Amoebidium parasiticum, whose wide range of hosts does include the predaceous nymphs of dragonflies (Odonata). Lichtenstein (1917a) described A. fasciculatum [which appears to be A. parasiticum (Tuzet and Manier, 1951b; Manier, 1969b)], growing in the rectum of Anax imperator (Odonata) nymphs, but stressed that the rectal environment of this aquatic dragonfly nymph is really comparable to the outside medium and contains a variety of free-living and attached bacteria, amoebae, flagellates, vorticellans, etc. It thus appears that the gut-inhabiting trichomycetes, at least, are restricted to arthropods that feed wholly or in part on decaying plant materials or living algae or that may be only incidentally predaceous, as are some stonefly nymphs containing Paramoebidium sp. A case in point is that among Chironomidae larvae one finds predaceous species that feed on herbivorous species that harbor trichomycetes, yet the predaceous species have not been found infested with trichomycetes. Restricted fungal host specificity alone cannot account for the lack of infestation in these predaceous forms, because the chironomid trichomycetes in question (several species of Smittium and Stachylina; Trichozygospora chironomidarum) are not species specific, for they are known to live in species of several different genera of herbivorous Chironomidae.
As a general rule, in crustaceans and millipedes in which immature forms resemble their adult stages, feed on the same kinds of substances, and live in combined populations, both immature and mature stages can be infested by the same species of trichomycete. Immature individuals, especially young ones, may be less infested, however. This probably is due primarily to the shorter intermolt period, which may not allow time either for the chance ingestion of spores and the establishment of new thalli or, when thalli do become established, for the production of spore types (Eccrinaceae and Parataeniellaceae) that function to increase infestation endogenously. Young immature specimens in some situations may ingest abundant spores, but the thalli that develop may not sporulate in time prior to being shed with the molt (Lichtwardt, 1961b).
Harpellales are found in immature forms of their aquatic insect hosts, whether metamorphosis is complete (Diptera) or incomplete (Plecoptera, Ephemeroptera). Adults of these insects are not aquatic, and they feed on blood, plant juices, etc., or do not feed at all. In the usual phase of growth and reproduction only larval guts contain fungi, but it is now known that several Harpellales are capable of invading the developing ovaries in larvae, resulting in adult females whose ovaries are packed with fungal cysts in lieu of eggs. This pathogenic stage serves as a means of dispersal of the fungi to larvae beyond the immediate population when the flying adult "oviposits" these propagules in other breeding sites. See section on Pathogenicity in Chapter 8.
Beetles offer examples of several patterns of infestation. Lajasiella aphodii, Enterobryus pentodoni, and several unnamed eccrinids (Thèodoridés, 1955) are known only from the larval stages of Scarabaeidae, whereas Eccrinopsis leidyi (nom. dub.) and several species of Enterobryus have been found only in adult Hydrophilidae. The Passalidae are widely scattered in the Americas, and most of the adults in the many populations that the authors have examined contain Leidyomyces attenuatus. Their larvae may also contain the same eccrinid, and this infestation of both developmental stages is undoubtedly related to the fact that larvae and adults live communally in tunnels excavated in logs by adults. The tunnels contain considerable adult fecal matter that one can presume includes sporangiospores, thus permitting ready transfer of inoculum from adult to larva.
One might expect the adaptability of the fungi to different external environments to be dependent primarily on the tolerances of the host, sheltered as the fungi are within the guts of their hosts where the gut ambient is to some extent under homeostatic control. At the same time the possibility cannot be ruled out that the absence of trichomycetes in some populations of the appropriate host species may be due to specific chemical substances such as pollutants or other factors in the environment that are deleterious to the fungi but not necessarily to the arthropods.
The degree of adaptability of arthropods and their fungi can be illustrated by several examples. Blackfly larvae of the Simulium vittatum complex are infested with a number of trichomycetes, among them Harpella melusinae, Genistellospora homothallica, and Paramoebidium curvum. This blackfly complex of species is widespread in North America. At one environmental extreme, larvae can be found in the clear headwaters of the northern Continental Divide in the U.S.A. that are constantly cold during the relatively brief season of larval development. At the other extreme, larvae can be found throughout the year in streams of the southern Great Plains, such as in southern Kansas, which tend to be more silted and have temperatures that fluctuate considerably from summer to winter. The authors have found all three of the above trichomycete species in S. vittatum populations from both environmental extremes. Furthermore, those fungi in southern Kansas may be found throughout the year. Since the hosts are poikilothermic, the fungi must also maintain their functions over a considerable temperature range. More important, perhaps, is that the algal flora on which the larvae predominantly feed appears to be different in the two types of habitats cited.