300 PARASITES OF GIPSY AND BROWN -TAIL MOTHS. 



locality, which was quite heavily parasitized by Parexorista. There 

 were easily 50 or 100 of the American flies to one of the European 

 race present in that general vicinity in the spring of 1909. The 

 chances that the pure-blooded European females were fertilized by 

 American males were therefore a good 50 or 100 to 1 at the most 

 conservative estimate. 



Being of the European race, their instincts led them to attack the 

 caterpillars of the brown-tail moth, and the attack was successful, 

 as witnessed by the number of pup aria which were secured from the 

 collected caterpillars and pupae in the summer of 1909, but these 

 puparia, instead of representing the pure-blooded European stock, as 

 was then supposed, represented the half-breed stock resulting from 

 the promiscuous mating of their mothers. Evidently the females 

 issuing from them in the spring of 1910 lost the cunning which is 

 characteristic of the European race, which makes possible the deposi- 

 tion of the soft-shelled eggs amongst the bristling poisonous spines 

 of the host without injury. 



Mr. Thompson, in his experiments with the American female 

 which had been fertilized by an European male, found that she was 

 neither anxious to oviposit upon the caterpillars of the brown-tail 

 moth nor able to do this successfully. A proportionately large 

 number of the eggs deposited upon this host were either pierced by 

 the poison spines or else the young larvae came in contact with these 

 and died before entering. A few larvae did succeed in gaining en- 

 trance, and one or two passed through their transformations, but 

 when the natural disinclination to attack the caterpillars of the 

 brown-tail moth was associated with a heavy mortality following 

 occasional attack the percentage of parasitism is reduced to the 

 minimum. 



In consequence of these observations in field and laboratory, the 

 name of Parexorista chelonise has been erased from the list of promis- 

 ing European parasites of the brown-tail moth and placed at the head 

 of the list of the imported parasites which are proved unfit. 



It is a pity, too, as has incidentally been stated, because it is about 

 the most common of any of the tachinid parasites in Europe, and, 

 moreover, is one which is entirely independent of any alternate host. 



Pales pavida Meig. 



There is a very considerable group of tachinid parasites of the 

 brown-tail moth which appears to be more commonly encountered in 

 material from southern European localities than from those in the 

 north. One of these, Zygobothria nidicola, has already been the 

 subject of lengthy discussion. The fact that though apparently 

 southern in its distribution in Europe, it has manifested a strong 

 tendency to become thoroughly acclimatized here, has lent encour- 



