204 PARASITES OF GIPSY AND BROWN-TAIL MOTHS. 



necessary that it possess certain physiological characteristics which 

 force it to react in certain ways and no others to the stimulus of the 

 parasite's presence. Unless the host does react in the manner to 

 which the parasite is accustomed, the parasite which is unable to 

 accommodate itself to circumstances beyond a certain extent will 

 find itself in a position which would be comparable to that of a man 

 suddenly thrust into a world where all the commonest laws of nature 

 worked in an unfamiliar manner. 



To say that many of the tachinids are physiologically restricted in 

 their host relationships is equivalent to saying that they are restricted 

 to a limited number of hosts, and this is true; probably more true 

 than of the hymenopterous parasites taken as a whole, or of any large 

 group of the hymenopterous parasites if the Microgasterinse and a few 

 similar groups of genera are excepted. It is probably true also that 

 among those parasites which are the most closely restricted in their 

 host relationships are to be found those which are the most effective in 

 bringing about the control of their respective hosts. This is primarily 

 due to the fact that a correlation usually exists between the life and 

 seasonal history of such a parasite and some one or more hosts which 

 it is particularly fitted to attack. The existence of a correlation 

 between parasite and host of such intimate character makes possible 

 the continued existence of the parasite independently of alternate 

 hosts, and it is thus enabled to keep pace with the one species upon' 

 which it is peculiarly fitted to prey when other circumstances are 

 favorable to its increase. 



Some of the most interesting examples of correlation of this sort 

 which have yet come to attention are to be found among the tachinid 

 parasites of the gipsy moth or the brown-tail moth, and on this account 

 as well as on a purely empirical basis they are now considered much 

 more likely to become important enemies of these hosts than before 

 their characteristics were so well understood. 



THE REARING AND COLONIZATION OF TACHINID FLIES; LARGE CAGES 



VERSUS SMALL CAGES. 



In more ways than can be recalled without taking up and discussing 

 each species in turn has the necessity for a more complete knowledge 

 of the tachinid parasites impressed itself upon those most concerned 

 with their economical handling. The difficulties attending the 

 successful hibernating of the puparia of Blepharipa and the myste- 

 rious disappearance of Parexorista chelonise, after it was considered 

 to be thoroughly established in America, may be mentioned as con- 

 spicuous examples among the many oftentimes curious and sometimes 

 apparently inexplicable problems which have come up for solution. 

 Just at the present time there is pressing need of more and accurate 



