Experimental studies in insect parasitism. IX. The reactions of a stick insect to an alien parasite

Abstract

Eggs and larvae of an ichneumon fly, Nemeritis canescens, injected into adults of a stick insect, Carausius morosus, elicited two defence reactions. Dark material, probably melanin, was deposited on their surface; and they were invested with haemocytes which, within 24 h, formed unorganized clumps of irregular shape about them. The haemocytic reaction was less vigorous in confined spaces than in the main blood stream, and several observations support the view that the haemocytes did not flock to the parasite from a distance but accumulated by chance encounter of, at most, a small chemotactic field. The size of the clumps reached a maximum about 24 h after injection of the parasite. The melanin reaction, on the other hand, was as strong in confined spaces as in the main blood stream, and was progressive, eventually encasing the parasite in a brittle black sheath. In nymphs of $\frac{1}{3}$, and in young nymphs of $\frac{1}{20}$, adult weight, the two reactions were essentially the same as in adults; the melanin reaction was indistinguishable from that in the adult; the haemocytic reaction in the large nymphs was a little, and in the small nymphs was much, less massive. The deposition of melanin about the parasite was inhibited by simultaneous injection of phenylthiourea. The haemocytic reaction could not be inhibited by injections of chinese ink, because blood cells blackened by the carbon particles they had engulfed nevertheless invested the parasite. The two reactions were elicited both by living and by dead parasites; the haemocytic reaction appeared to be elicited by clean glass rods, but the melanin reaction was not. Most Nemeritis eggs failed to hatch in Carausius although the fully developed larva could be seen moving within them, and no Nemeritis larva was observed to live longer than 48 h in this host. But larvae that had hatched in Carausius and after 24 h were transferred to the normal host, Ephestia, lived and developed. Moreover, eggs hatched normally in the blood of Carausius in hanging-drop cultures, and the larvae lived as long as 75 h. The death of this parasite in Carausius does not appear to be due either to toxic properties of the blood as a fluid, or to its unsuitability as food, but to active processes, the haemocytic and melanin reactions, that take place in the living host.