62 



exhibiting both these traits were from the same moth ; the larger size 

 and paler color of those acting as Cut-worms shows distinctly that 

 they belong to a different race than the traveling worms, although 

 belonging to the same species. It is very probable that the travel- 

 ing worms were the progeny of moths which formerly inhabited 

 some of the low marshes along the banks of a stream which has 

 its source in the mountains. When the weather becomes warm in 

 the early summer the snow on the mountains melts, and when this 

 takes place at all rapidly the streams which flow from them become 

 greatly swollen, and overflow the low, marshy meadows along their 

 banks. But before this occurs the Army- worms which inhabit these 

 low meadows — prompted by an inherent instinct — migrate to the 

 higher land. That insects of this kind do perform such migrations 

 is confirmed by the observations of Dr. Harris, who, in giving an 

 account of the Salt Marsh Caterpillar {Arctia acraca, Drury), which 

 inhabits the marshes around Boston, says :* 



"During this month (August) they come to their full size, and 

 begin to run, as the phrase is, or retreat from the marshes and 

 disperse through the tuljacent uplands, often committing extensive 

 ravages in their progress." 



A brother of the writer, who has spent about one and a-half 

 years in Merrick Co., Nebraska, says that the Platte River — which 

 has its source in the Eocky Mountains — reaches its highest point 

 during the month of June. According to the Isothermal Map fur- 

 nished by the Smithsonian Institution, and published on plate IV 

 of the Patent Office report for the year 1856, the climate of Ne- 

 braska in the summer is about that of Missouri and Tennessee dur- 

 ing the same season ; and as the Army-worms in the latitude of 

 these States usually make their appearance in the month of June, 

 their coming would be at about the same date that the mountain 

 streams in the same latitude attain their greatest height. Now it 

 naturally follows that insects which for many generations past had 

 been in the habit of migrating at a certain season of the year, 

 would be very likely to manifest this habit, even when reared in 

 localities where it would be unnecessary to perform these migra- 

 tions ; and as moths are known to travel long distances in their 

 migrations it is only reasonable to suppose that the Army-worms 

 which devastate our fields are the progeny of moths that were bred 

 in these marshes and have migrated to the infested locality ; or at 

 least are the progeny of moths whose ancestors were reared in these 

 marshes and which had not yet lost the migrating habit. That an 

 insect which, at one time of its life, possesses the sedentary habit 

 and pale color of the Cut-worm, should suddenly acquire the mi- 

 grating habit and dark color of the Army-worm, is not at all pro- 

 bable; and we can account for this difference in habit and color of 

 the same species of insect only by supposing that they belong to 

 two distinct races, which originally inhabited two different portions 

 of our country, and whose habits were largely influenced by their 

 environments. 



♦Insects Injurious to Vegetation, page 351. 



