360 EVOLUTION AND ANIMAL LIFE 



of its body. From the cocoons the adult winged ichneumon 

 flies emerge, and after mating find another host on whose 

 body to lay their eggs. 



One of the most remarkable ichneumon flies is Thalessa 

 (Fig. 219), which has a very long, slender, flexible ovi- 

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

 horntail (Tremex columba) (Fig. 220) 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 behind 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 



FIG. 218. Parasitized caterpillar from which the ichneumon fly parasites have issued, 



showing circular holes of escape in skin. 



which she judges is opposite a Tremex burrow, and, elevating 

 her long ovipositor in a loop over her back, with its tip on the 

 bark of the tree (Fig. 221), she makes a derrick out of her body 

 and proceeds with great skill and precision to drill a hole into 

 the tree. When the Tremex burrow is reached she deposits 

 an egg in it. The larva that hatches from this egg creeps 

 along this burrow until it reaches its victim, and then fastens 

 itself to the horntail larva, which it destroys by sucking its 

 blood. The larva of Thalessa, when full grown, changes to 

 a pupa within the burrow of its host, and the adult gnaws 

 a hole out through the bark if it does not find the hole already 

 made by the Tremex." 



The beetles of the family StylopidaB present an interesting 

 case of parasitism. The adult males are winged, but the adult 

 females are wingless and grublike. The larval stylopid at- 

 taches itself to a wasp or a bee, and bores into its abdomen. 

 It pupates within the abdomen of the was}) or bee, and lies 

 there with its head projecting slightly from a suture between 

 two of the body rings of its host. 



