5 
at different points, distant from each other less, perhaps, than a mile, 
there was by no means the same relative abundance of species. It can 
be seen at the time of this writing (January, 1897) that hibernating 
egg masses of the Orgyia are certainly four times as abundant at some 
places in the city as they are at others, a facet which points plainly toa 
greater scarcity of primary parasites at the first-named points. The 
tussock-moth caterpillar is itself a slow traveler. Its primary parasites 
naturally congregate at the points of greatest caterpillar abundance. 
At points where the caterpillars are scarcer they are thus less exposed 
to the attacks of their parasitic enemies, and it results that there may 
actually be an increase of the species at one point simultaneously with 
a decrease at another. This, then, at once suggests that ina small way 
artificial transportation of the Pimpla in particular may often be of 
some practical benefit. | 
The part played by the dipterous parasites in this instance was not 
great. Only 187 specimens in all were reared. All were of rather 
well-known Tachina flies, which are general and widespread parasites. 
Even though the work of these species was not important in this 
instance, as is well known, they frequently play a most important part 
in the reduction of the numbers of injurious larve.’ 
It is an extraordinary thing that these flies are by no means so 
restricted in their host relations as are the parasites which belong to 
the order Hymenoptera. The parallelism between structure and host 
relation, which is so striking among the Hymenoptera, seems to be prac- 
tically absent with these parasitic Diptera, as Brauer’s tables plainly 
indicate. 
‘It has been shown, for example, that Pimpla inquisitor, the most numerous of the 
parasites mentioned, is a very general feeder on lepidopterous larvie, and in such 
cases as this extensive parasitism of Orgyia in Washington, if the right moment 
were seized, a surplus of the parasites could readily be sent to such points as Boston, 
for example, where the tussock moth appears to be abundant nearly every year, and 
where, perhaps, the species would be found to attack even the gypsy moth, although, 
according to the reports of the gypsy moth committee, this species has not as yet 
been reared from this host. 
“The writer has searched for hours in grass fields overrun with army worms with- 
out finding a single specimen of the worm which did not bear upon its back the eggs 
of Winthemia 4-pustulata. It has long been known that many of these eggs fail to pro- 
duce any result, through the molting of the caterpillar before the hatching maggots 
have an opportunity to work their way into its body, but the observations made by 
Professor Fernald and his assistants in their work upon the gypsy moth in Massa- 
chusetts have thrown a new light on the number of failures in Tachina parasitism. 
On page 385 of the 1896 report upon the gypsy moth it is stated that during the sum- 
mer of 1893 Mr. Reid collected a number of caterpillars on which the eggs of the 
parasites had been laid. Two hundred and thirty-five of these caterpillars, having 
from 1 to 33 eggs on each, were fed in cages until they changed to pup, and from 
these 226 moths emerged, but only 4 dipterous parasites were secured from the entire 
number. The caterpillar which had 33 Tachina eggs on it molted before the eggs 
hatched, passed through all its transformations, and the moth emerged in good con- 
dition. In 1895, 50 larvie bearing eggs were isolated, and 43 moths resulted and no 
parasites. Later 252 caterpillars were taken in the field, each bearing dipterous eggs, 
and were fed and carried through their transformations without the appearance of 
a single adult Tachina fly. 
