73 
possible for breeding to be more nearly continuous. The instinct of 
this and other insects of recent southern origin is still to remain late 
feeding in the open, provided appropriate plants are available for their 
subsistence, or, to put it otherwise, they have not learned to seek 
shelter at the same time as native or acclimated forms do. 
Recent observations on this and other species of similar habits and 
origin suggest that the ancestors of those individuals which produce 
only a single generation were introduced in early times and are thor- 
oughly established and acclimated, while those which produce a second 
generation are the offspring of ancestors which have spread from the 
south more recently and have not yet become accustomed to the differ- 
ences in the weather in the North and in the South. 
The development of two generations by Melittia and other southern 
introductions in the District of Columbia and places having a similar 
climate is a matter apparently not so much dependent on the weather 
as upon the inability of the insects to find the appropriate food for 
their larve; for example, were cucurbits to be planted earlier and 
later, there would be no trouble in the vine borer producing two well- 
marked generations in spite of the fact that the vines of cucurbits are 
readily killed by frosts, the insect being able to survive upon stems 
which are not of the freshest. 
Certain species recently observed, e. ¢., Plutella cruciferarum, the 
diamond-back cabbage moth, there are the best of reasons for believing 
are able to produce an additional generation during the latter days of 
November and the first week of December, as many larve captured at 
this time were full grown and accompanied by numerous pupze, most 
of the individuals captured changing to pup before the end of the 
first week of December, in which condition they would naturally be 
less exposed to frost and better able to survive the rigors of winter. 
Still another generation, however, was attempted, as one moth cap- 
tured deposited its eggs at this time. This generation was, of course, 
doomed to failure. 
The effort on the part of so many introduced Old World species of 
producing extra generations would naturally lead to the belief that 
these insects came originally and in comparatively recent times from 
southern Europe or southern Asia, became acclimated farther north in 
Europe in the same manner that native Southern forms become estab- 
lished by migration to our Northern States, whence they were intro- 
duced in the Upper Austral portions of the United States, for the most 
part about our principal seaports, Boston, New York, and in some cases 
Baltimore, and in other large cities, such as Philadelphia and perhaps 
Washington, and after becoming adapted, more or less imperfectly 
perhaps, to the environment of those cities, have made their way still 
farther south, where they have again resumed what was probably their 
original habit of producing two, three, or more annual generations. 
