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 I 
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 Microgasterinae 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 chelonix, after it was considered 
to be thoroughly established in America, may be mentioned as con- 
spicuous examples among (he ninny oftentimes curious and sometimes 
apparently inexplicable problems which have come up for solution. 
Just at the present lime there is pressing need of more and accurate 
