I 



171 



In the discussion which followed the reading of our letter at the No- 

 vember (1884) meeting of the Brooklj^n society, as reported by Mr. John 

 B. Smith, Mr. Gade announced himself as "positive that many of the 

 logs frequented by the RJiyssa are not infested by Tremex or other 

 wood-boring larva." 



It follows from the accurate observations here brought together, and 

 which do not depend upon inference, that Mr. Gade (as all those who 

 support him) was entirely wrong in his conclusion that Thalessa is 

 lignivorous; and though further observations were j^romised the en- 

 suing year we have looked in vain in the reports of the meetings of 

 the Brooklyn society for any subsequent statement or admission of 

 error. 



We have had in our collection since 1872 alcoholic specimens of T. 

 Innator, as well as Tremex columba in all stages, taken from the trunk of 

 a Box Elder {Negiindo aceroides) on Mr. William Coleman's farm, near 

 Merrimac, Mo. We took these on the 4th of July, 1872, and made notes 

 as to the habits of the larva and pupa of both species. The tree was al- 

 ready partly dead, and, in fact, our experience in this as in subsequent 

 observations, shows that in most cases the tree has been somewhat af- 

 fected, so that the wood was not firm and healthy. This stump fur- 

 nished an excellent opportunity for investigation, because it was so 

 easily split, and we examined the burrows very carefully and found 

 Thalessa in all stages at that time — larvae, pupte of both sexes, and 

 imagines of both sexes withjn the tree, the larvae being of various sizes 

 and invariably external to the Tremex, i. e., not within, ^wt holding on 

 to its victim and sucking the latter's life away, without in any case en- 

 tering the body. At this same time females were also actively engaged 

 in ovipositing, and by carefully tracing the ovipositor in several cases 

 we came to the conclusion that she did not attempt to reach the Tremex 

 larva but only to reach its burrow, and that the young parasitic larva 

 after hatching must instinctively seek its victim. Thalessa, therefore, 

 is not an internal parasite and in this it agrees with a great many 

 other parasites both Hymenopterous and Coleopterous, e. g., Ophion, 

 Typhia, Eupiectrus, Elachistus, Elasmus, Polyspbincta, Acrodactyla, 

 Khipiphorus, etc., which are all external, as we know from our own ex- 

 perience and Mr. Howard's; while Tryphon, Sphiuctus, and Paniscus 

 are mentioned by Westwood as having the same habit. In fact, external 

 parasitism is far more common among the larvre of the Ichneumonid^e 

 and the Chalcididji) than has hitherto been supposed, and may be said 

 almost to be the rule with all parasites upon trueEndophytes, and with 

 secondary parasites. The truth of the whole matter is, that Thalessa, 

 like all other insects, is liable to sutler from fallible instinct, and that 

 while she doubtless has better means of distinguishing 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 



