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A new species of hymenostome ciliate, Tetrahymena dimorpha sp.nov., is described. This ciliate occurs as a parasite in the haemocoel of larval, pupal and adult Simuliidae (Diptera). In larval hosts the total number of parasites never exceeds about 240 and the infection is benign. Within larval hosts the ciliates are large and broadly oval and possess an unusually wide range of somatic kineties, from 30 to 66; moreover a variable proportion of these kineties are characteristically disorganized, being incomplete, meandering or branched. Metamorphosis of the host to the adult fly is accompanied by a dramatic increase in the number of ciliates, which reach pathogenic intensity. Adult hosts may contain up to 19000 ciliates and the flies soon die from this heavy burden. Associated with ciliate population growth during host metamorphosis is a startling morphological transformation of the ciliates themselves. In adult hosts the ciliates are smaller and pyriform in shape and the cortex is greatly modified; the total number of somatic kineties is considerably reduced and has a limited range of 19—22. Most significantly, the kineties are ordered with typical tetrahymenine precision. By application of appropriate culture conditions to ciliates isolated from any host stage, either of the distinctive morphological forms of T. dimorpha may be selectively induced in vitro . In bacterized infusions, ciliates are produced that have the general form and cortical characteristics of those found naturally in adult hosts. Sterile culture in serum-supplemented Mitsuhashi and Maramorosch insect tissue culture medium produces a population showing features characteristic of ciliates from larval hosts. Sterile culture in proteose-peptone-yeast-extract medium results in populations exhibiting concurrent dimorphism, even after cloning. The extreme nature and multiple facets of the dimorphism together with the ease of its manipulation in vitro afford opportunities for the experimental investigation of many problems, particularly those related to cell surface patterning in ciliates, and these possibilities are discussed in relation to current concepts of ciliate morphogenesis.

A new species of hymenostome ciliate, Tetrahymena sialidos sp.nov., is described and proposed as a model for studies upon insect-ciliate interactions. This ciliate occurs as a parasite in the haemocoel of larval Sialis lutaria (Megaloptera) and exhibits field and laboratory characteristics favourable for two particular areas of study, the regulation of invertebrate populations by parasites and insect immunity to parasitic infections. The prevalence of T. sialidos infection in S. lutaria from a small lake in mid-Wales has been monitored during a four-year period. Each summer, early in the first of their two-year aquatic development, 40-70% of S. lutaria larvae become infected. Death of infected larvae due to parasite proliferation is inevitable but does not occur until 11-12 months after infection, when the mean intensity has reached 55000 ciliates per infected host. Ciliates escaping from moribund or dead host larvae undergo a distinct morphological transformation followed by a period of synchronous conjugation. Death of infected host larvae and release of ciliates coincides with the appearance of a new generation of host larvae in which the prevalence of infection rises rapidly. The possible significance of this substantial and persistent regulatory influence upon host population dynamics is discussed in relation to current ideas of parasite regulation of invertebrate populations. Sialis lutaria is not able normally to mount a successful host response against T. sialidos infections. A laboratory thermalshock technique is described by means of which parasitic ciliates may be killed in situ without incurring physical damage to the host. Infected larvae subjected to thermalshock at 39 °C for 1 h subsequently encapsulate and melanize the dead ciliates. The use of this technique is discussed in relation to recognition of non-self in insects and survival strategies of parasites infecting insects.

SUMMARY A method is described for the adaptation of a commercially‐available photomicroscope in order that the automatic facility of a microprocessor electronic flash may be used.