ICHNEUMONS AND THEIR ALLIES 107 



peering that an egg had been introduced at every prick, 

 Reaumur fed the caterpillar, and waited to see the issue. 

 After ten or twelve days the caterpillar pupated, but imme- 

 diately afterwards began to waste visibly. In four days it was 

 evident that all its internal organs had been devoured by the 

 parasites, which did not, it would seem, find enough food to 

 support themselves, for none of them were able to complete 

 their transformation. He subsequently learned that in the 

 ordinary course all the parasites quit their host after a certain 

 length of residence, and that they emerge before or after it 

 pupates, according as they were introduced early or late. In 

 December Reaumur found a number of cabbage-white cater- 

 pillars, and put thirty of them into a glass dish. More than 

 five-sixths proved to harbour parasites. When they were 

 ready to emerge, he observed a small, whitish prominence 

 on the flank of a caterpillar, which projected at right angles-. 

 Now and then the prominence was withdrawn a little, and 

 then pushed out farther than ever. Soon a second pro- 

 minence began to form, and this went on until, in less 

 than half-an-hour, fourteen or fifteen larvae had protruded 

 themselves from one side of the caterpillar, and about as 

 many from the other. When they had set themselves free, 

 they did not travel far, but remained close to the caterpillar 

 from which they had issued. During this painful operation, 

 the caterpillar remained motionless, as if dead. When it was 

 finally rid of its cruel enemies, it moved about a little, but died 

 within four days. Sometimes the parasites did not emerge 

 until the caterpillar had either suspended itself for pupation, 

 or actually pupated. No caterpillar which had contained para- 

 sites produced a butterfly. Reaumur estimated that not more 

 than a tenth of the various caterpillars which he kept under 

 observation completed their life-history. It is, however, quite 

 possible that in another year the proportion might have been 

 much more favourable to the butterfly. (See saw-fly, p. 98.) 

 These parasitic larvae are white, with smooth skins and no 

 legs ; the head is retractile. The digestive tube has no outlet, 

 for the food consists only of the blood and tissues of the catei'- 

 pillar, and contains no indigestible residue. Were any refuse 

 to be discharged into the body-cavity of the caterpillar, its 

 torments and the lives of the parasites would be speedily cut 

 short. All ichneumon-larvse are alike in this point of structure. 



