Experimental studies in insect parasitism—XII. The reactions of six exopterygote insects to an alien parasite

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When mature eggs of the ichneumon-wasp Nemeritis canescens were injected into crickets ( Acheta domesticus), cockroaches ( Blattella germanica), stick insects ( Carausius morosus), and earwigs ( Forficula auricularia), the parasite larvae were quickly encapsulated by blood cells and partly encased in melanin; and usually died within 48 hr. The hosts appeared to suffer no lasting disability. Eggs and larvae of Nemeritis were not encapsulated or melanized in larvae of Rhodnius prolixus, and only occasionally elicited any visible defence reaction; but the parasites did not feed or grow in this host, and died after 3 days. When the same parasite was injected into locust hoppers ( Schistocerca gregaria) no visible reaction took place until about the fifth day, after which the parasite was incompletely encapsulated but never melanized. Parasite larvae lived in these hosts as long as observations were continued (25 days). They fed and grew, but at a subnormal rate; and they remained in the first instar. In adult locusts no reaction was observed within 5 days, but the parasite larvae did not grow. Locust hoppers quickly encapsulated other foreign bodies. First-instar Nemeritis that had lived up to 25 days in Schistocerca caused no reaction when transferred to their normal host, caterpillars of Ephestia; but ganglia of Schistocerca transferred to Ephestia were at once encapsulated. The time-lag in the reaction of Schistocerca to Nemeritis is due to the parasite, which at first elicits no reaction but becomes evocative at a particular state of development within the first instar. First-instar larvae of Nemeritis did not ecdyse to the second instar in Schistocerca. Their failure to moult might be due to hormones in the blood of the host; but when Nemeritis was injected into decapitated hoppers they did not ecdyse and failed to feed or grow, acting as they did in adult locusts. Moreover, larvae retrieved from decapitated hoppers had lost their acceptability to caterpillars of Ephestia, in which they excited a haemocytic reaction. This propensity persisted through transfer at 48 hr intervals to three successive caterpillars. It appears that both the feeding and growth of first-instar Nemeritis and the haemocytic reaction of their normal host are affected by substances released from the head of locust hoppers.