402 MK. J. B. GATENBY ON THE BIONOMICS, EMBEYOLOGY, 



detected, as already explained, by the jagged hole through which the insect 

 has escaped (compare PI. 24. fig. 1 and PI. 25. fig. 9). Mesocliorus is not a 

 Braconid or a Chalcid ; it belongs to another group o£ Hymenoptera, the 

 Ichneumonidge. Superficially it is (when the specimens themselves are 

 cursorily observed) very like the parasite Microgaster, but further examination 

 shows differences in the venation of the wings, etc. 



In PI. 24. fig. 4 the host caterpillar {Porthesia similis) is drawn, and near at 

 HP is the hyperparasite to the same scale. The parasitic larva, Microgaster, 

 lies inside the hsemocoel of the caterpillar : how does the hyperparasite 

 contrive to lay its eggs inside the body of the parasitic larva Microgaster ? 

 A certain percentage (about 10 per cent.) of the latter were hyperparasitized. 

 The only solution of the problem that I can suggest is that the Ichneumon 

 bores into the host caterpillar (Porthesia) till it feels a parasite (Microgaster) 

 with its ovipositor, and that it thereupon lays an egg inside the parasite. 



It is to be noticed that the hyperparasitic fly must be very skilful in 

 ovipositing eggs inside the parasites, for I judge that the host caterpillar 

 must be nearly full-grown when the parasites (^Microgaster) are hyper- 

 parasitized, and there is no doubt that unless the hyperparasite was most 

 circumspect the host caterpillar would struggle, and the movements of its 

 body-wall muscles would possibly prevent the Ichneumon from successfully 

 ovipositing in the body of the parasitic Braconid. If about 10 per cent, of 

 the Microgaster larvse are hyperparasitized, it must be noticed that this would 

 mean that the Ichneumon would need to pierce at least five holes in the 

 Porthesia caterpillar in order to lay its eggs in the parasites, and it "is 

 doubtful if at every piercing of the epidermis of the caterpillar, it would 

 locate a Microgaster parasite. The latter parasites lie here and there in the 

 fat-body of the host; the Porthesia sf??ii7e5 caterpillar is a stout, bristly 

 creature, and though the parasitic Microgaster was faced with a difficult 

 task in ovipositing inside the caterpillar, it only had to make one hole : how 

 much more difficult is the task of the Ichneumonid Mesocliorus ? It must 

 needs seemingly pierce a number of holes, and attack a much larger strong 

 caterpillar covered with bristly warts (PI. 24. fig. 4). 



All attempts to get either parasite or hyperparasite to oviposit while in 

 captivity completely failed, the insects were so timid. 



The Position of the Hyperparasitic Larva in the Parasitic Larva was never 

 constant, in some cases it lay in the thoracic or abdominal regions, in others 

 in the terminal vesicle (PI. 25. fig. 10). The youngest parasitic larva hyper- 

 parasitized was about the size of that in PI. 24. fig. 3 d. The hyperparasite 

 never killed the parasite till the latter had spun its cocoon ; this is why only 

 fairly old parasites become hyperparasitized. The parasite is used by the 

 hyperparasite till the former has spun a protective cocoon for the latter — not 

 only does the hyperparasite use the body of the parasite, but it makes use of 

 the latter's skill in spinning a beautiful strong silk covering. 



