236 HYMENOPTERA. 



consequence, by this part of the body to the extremely 

 long ovipositor (^^), is well shown in the drawing. 

 It was generally supposed until two years ago that 

 this insect bored into the trunks of trees and pierced 

 the bodies of grubs, especially those of Tremex, for 

 the purpose of laying its eggs. Definite knowledge 

 on the subject was wanting until Dr. C. V. Riley ^ pub- 

 lished his observations. His detailed description, with 

 figures illustrating the method of oviposition in Thal- 

 essa, the structure of the ovipositor, etc., are of great 

 value. The trees in which the eggs are laid are in 

 most cases somewhat affected, so that the wood is not 

 firm and healthy. The larvce of Thalessa are found in- 

 variably external to the Tremex grub ; i.e. " not within, 

 but holding on to its victim and sucking the latter's 

 life away, without in any case entering its body." After 

 careful observations of the female while ovipositing, 

 Riley came to the conclusion that she did not attempt 

 to reach the Tremex larva, but only its burrow, and 

 that the young parasitic larva after hatching must in- 

 stinctively seek its victim. He continues, " The truth 

 of the whole matter is, that Thalessa, like all other 

 insects, is liable to suffer from fallible instinct, and 

 that while she doubtless has better means of distin- 

 guishing a tree infested by Tremex than we have, she 

 nevertheless often makes mistakes, and the unerring 

 instinct, which book entomologists are so fond of 

 dwelling upon, is often at fault." 



The ovipositor is extremely long, measuring not less. 

 than four and a half inches ; it is protected by two 



1 See Tnsect Life, Dept. Ag., Vol. L, No. 6, December, i888. 



