Observations on cellular immunity and parasitism in the tussock moth

Cellular responses to the introduction of foreign objects into the haemocoele of both control and parasitized tussock moth larvae were examined. In normal larvae, the response to large foreign objects such as Hyposoter fugitivus eggs and Sephadex beads was encapsulation, accompanied by a rapid and sustained increase in the total haemocyte count. Smaller objects such as yeast cells were cleared into nodules within a matter of minutes; nodulation too was accompanied by an increased total haemocyte count. In larvae parasitized by the braconid wasp Cotesia melanoscela, both encapsulation and nodulation were permanently suppressed. Inhibition of these normal cellular defence reactions was accompanied by a reduction in the total haemocyte count, the appearance of debris in the haemocoele, and by nuclear pycnosis in an unidentified population of cells; however, since extensive nuclear pycnosis also occurred in haemopoietic tissue, it is assumed that prohaemocytes may represent a target cell population. All of the observed effects required the presence of both C. melanoscela calyx fluid and venom in the host animal. Phagocytosis as an immune response remained essentially intact, and was capable of completely clearing both yeast and Escherichia coli cells injected into the haemocoele.