i 4 o INSECTS 



insects may begin a migration beyond tlieir original 

 limits and may extend a long distance into adjacent 

 territory, only to be destroyed when a recurrence of 

 normal weather conditions renders the invaded area 

 unfit as a place of habitation. An example of that 

 character occurred in 1896 when the Harlequin cabbage 

 bug invaded New Jersey and Pennsylvania, its normal 

 range not extending north of Maryland along the At- 

 lantic coast. But although it was present in that year 

 in destructive numbers, it was completely killed off 

 during the winter following and has not been found 

 in New Jersey since. It is an example of an insect 

 rigorously restricted in distribution by climatic con- 

 ditions, although its food plants are widely distributed 

 outside of its own faunal limitations. 



But occasionally matters do not terminate as sim- 

 ply. It may happen that an insect long confined to 

 a definite faunal area may be started on a migration 

 along the line of its food plant, and may be found to 

 possess sufficient powers of adaptation to continue its 

 life under conditions varying materially from those in 

 which it started. A striking example of this is found 

 in the case of the Sphinx catalpa which in its caterpillar 

 stage feeds only on Catalpa and which, up to a few 

 years ago, did not range north of Virginia and Ken- 

 tucky, although its food plant extends into New York 

 and Pennsylvania. Somewhere about 1897 it began 

 to extend northward through Maryland and Delaware 

 into Pennsylvania, and year by year it has extended 

 that range until it has reached the headwaters of the 

 Delaware River, and has extended throughout New 

 Jersey into New York State. And this extension is 

 not of a few individuals only; but of a horde, capable 

 of causing defoliation and serious injury to the trees 

 attacked. At present it seems as if the species had 



