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 pupse in the summer of 1909, but these 
pup aria, 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 larvse came in contact with these 
and died before entering. A few larvse 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. 
Tn 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. 
Tt 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- 
