OF WASHINGTON, VOLUME XVIII, 1916 19 



The second is confined to Europe, and has apparently never 

 reached America. Such cases may be multiplied. 



Just the opposite of the preceding are the cases of the parasitic 

 muscoid species whose larvae live in caterpillars and other insect 

 hosts, but whose pupae may remain for a short or long time dor- 

 mant, according to temperature, in earth or other material com- 

 monly shipped. The first set of conditions is present in these 

 cases; but the second is lacking, since the natural hosts do not 

 exist in the country of introduction. Even when such hosts do 

 exist in number, establishment is no easy matter for the species to 

 effect, unaided. And still again, even when intentionally aided 

 by man, establishment is a very complicated and difficult matter, 

 not at all easily effected. This has been well illustrated by the 

 work of the U. S. Bureau of Entomology with muscoid parasites 

 at the Gipsy Moth Laboratory in Massachusetts. Although the 

 natural hosts, the gipsy and browntail caterpillars, existed in 

 great quantity in the country of introduction, yet colonization of 

 over 68,000 individuals of at least 16 foreign species of the para- 

 sites had resulted up to the end of 1910 in the complete estab- 

 lishment of only two species, Compsilura concinnata Meigen and 

 Zyciobothriti nidicola Townsend, and that after many years of per- 

 si-tent effort, carried out with the utmost regard for securing 

 the requisite conditions at the right season. It is significant 

 that the first species has a much wider host-range than the others , 

 except Exorista larvarum Linn, with whose establishment a closely 

 similar American species has interfered ; while the second species is 

 independent of alternate hosts, and finds practically no compe- 

 tition inasmuch as it is the only tachinid breeding in the hibernat- 

 ing browntail caterpillars. 



I have repeatedly insisted that very few if any of the tachinid 

 species hitherto announced as common to Europe and North 

 America are in reality so. This is demonstrable in main" cases 

 by a comparative study of specimens. The reason, as above 

 explained, is quite readily understood, once it is pointed out. 

 To the cases of certain species, which appear to differ only in 

 physiological characters, this reason lends emphasis. For ex- 

 ample there is a species of Paraexorista in New England which 

 Mr. W. R. Thompson was unable to separate on anatomic char- 

 acters from the European Paraexorista cheloniae, yet he proved it 

 a distinct species by the fact that it would not attack the browntail 

 caterpillars, of which the European species is one of the most 

 efficient parasites. In this connection, the fact nuisi be empha- 

 sized that the existence of a closely similar species in ihe country 

 of introduction constitutes a. bar to esta.bli.-hinenl . since the foreign 

 species is often practically swallowed up by the native spec'n - 

 through interbreeding, the issue apparently inheriting the physio- 

 logical characters of the hitter. 



