342 ANIMAL STUDIES 



for these insect pests. The adult ichneumon fly is four- 

 winged and lives an active, independent life. It lays its 

 eggs either in or on or near some caterpillar or beetle grub, 

 and the young ichneumon, when hatched, burrows about in 

 the body of its host, feeding on its tissues, but not attacking 

 such organs as the heart or nervous ganglia, whose injury 



FIG. 205. Parasitized caterpillar from which the ichneumon fly parasites have 

 issued, showing the circular holes of exit in the skin. 



would mean immediate death to the host. The caterpillar 

 lives with the ichneumon grub within it, usually until 

 nearly time for its pupation. In many instances, indeed, 

 it pupates, with the parasite still feeding within its body, 

 but it never comes to maturity. The larval ichneumon fly 

 pupates either within the body of its host (Fig. 205) or 

 in a tiny silken cocoon outside of its body. From the 

 cocoons the adult winged ichneumon flies emerge, and 

 after mating find another host on whose body to lay their 



One of the most interesting ichneumon flies is Thalessa 

 (Fig. 209), which has a remarkably long, slender, flexible 

 ovipositor, or egg-laying organ. An insect known as the 

 pigeon horn-tail (Tremex columba) (Fig. 207) deposits its 

 eggs, by means of a strong, piercing ovipositor, half an inch 

 deep in the trunk wood of growing trees. The young or 

 larval Tremex is a soft-bodied white grub, which bores 

 deeply into the trunk of the tree, filling up the burrow be- 

 hind it with small chips. The Thalessa is a parasite of the 

 Tremex, and when a female Thalessa finds a tree infested 

 by Tremex, she selects a place which she judges is opposite 



