PARASITES OF LARGER BROWN-TAIL CATERPILLARS. 299 



ever was felt concerning the recovery of the parasite, which was 

 considered to be as firmly established as the Calosoma or Compsilura 

 concinnata. The results afforded another example of the obtrusive- 

 ness of the unexpected. Not a single Parexorista puparium was 

 secured from any of the material included in this series of collections. 



Tins was, all things considered, the most serious setback of any 

 which the parasite work has experienced since its inception. It was 

 never doubted from the first that some among the parasites would 

 be unable to exist in America, and no species was really credited with 

 having demonstrated its ability to do so until it had lived over at 

 least one complete year out of doors. Parexorista had done this 

 and more, having gone through two complete generations, unless, 

 what was not at all likely, its puparia.had all been killed some time 

 during the fall or winter. 



Without indulging in unnecessary speculation as to the reason for 

 its disappearance, the following facts are presented for consideration: 



There is in America a tachinid known as Parexorista cJielonix, 

 wliich is morphologically identical with the European race so far as 

 may be determined through a painstaking comparison of the two. It 

 is a common parasite of the tent caterpillars Malacosoma americana 

 Fab. and M. disstria Htibn. The adult flies issue at the'same time 

 in the spring as do those of the European parasite of the brown-tail 

 moth. The same type of egg is deposited; the larvae are indistin- 

 guishable in any of their stages or habits during their several stages ; 

 the third-stage larvae issue at the same time and form puparia wliich 

 are apparently the exact copies of the European, and the hibernating 

 habits are the same. The one and only difference is that the Ameri- 

 can Parexorista chelonix does not attack the caterpillars of the brown- 

 tail moth, while the European Parexorista chelonix is perhaps the 

 most important of the tachinid parasites of this host. 



Mr. W. P. Thompson, whose excellent and painstaking work 

 makes possible the above comparison between the two races, went 

 a step further in his investigations. He found by actual experiment 

 that in confinement, at least, the European males would unite with 

 the American females with as much freedom as with those of their 

 own species. Granted that similar intermingling of the races takes 

 place in the open, and the reason for the nonrecovery of Parexorista 

 chelonise as a parasite of the brown-tail moth in the summer of 1910 

 is no longer a mystery. 



It was stated a few paragraphs back that the flies which were colo- 

 nized in the spring of 1908 were largely mated at the time of liberation. 

 Their progeny, which issued in the spring of 1909, would therefore be 

 of the pure-blooded European stock. Issuing at the same time were 

 a vastly larger number of the American race, because as it happened 

 there was an incipient outbreak of Malacosoma disstria in that very 



