18 WILLIAM T. M. FORBES 



There is a marked difference between the Mississippi Valley forms 

 and those of the Atlantic Slope, but this may be due as much to the 

 barrier of the Appalachians as to any difference in climate. This 

 subdivision shows clearly only in the Transition and Upper Austral 

 xones. West of our territory there is an arid subdivision, which 

 invades Illinois in a few isolated stations. The sand region of the 

 Coast and in the vicinity of Albany and Peru, New York, forms a 

 sort of artificial arid subdivision, where live a few characteristic 

 western species. The Synedas may be examples; also Plagiomimicus 

 and its relatives. 



The control of physical barriers is best marked by the confining of 

 the colder-zone insects in New York to certain of the higher peaks, and 

 by the wider barriers of the St. Lawrence Plain (which stops Ccen- 

 onynpha inornata, for instance) and the Mohawk Valley, which is per- 

 haps the barrier for typical Pieris oleraca, and marks a varietal differ- 

 ence in certain geometers. These barriers, like that of humidity, are 

 much less effective with us than in the Western States. 



Distribution may also be viewed on a much smaller scale. Each 

 spot, or station, where a colony of a species occurs, has its own 

 peculiar characteristics of moisture and average temperature, and 

 its own surrounding barriers, more or less effective: of these factors 

 moisture is the most obvious variable, but fog or sunlight, close or 

 easily drained soil, high or low water-table, each has an effect at least 

 on the available food-plants and on the possibilities of pupation of 

 the insect. Standing water has its own characteristic types (largely 

 Xymphulinae), and even running water has its peculiar species 

 (Elophila fulicalis). Characteristic of the swamps are many Noctuids, 

 a large part of them recognizable by their striated wings, simulating 

 dead grass or reeds; as, for example, Leucania pallens and the Borolia 

 group, the Nonagrias and their kin, Senta, Ommatostola, and Euchalcia 

 venusta, as well as the species of Prenes, Ghrysophanus epixanthe, 

 Darapsa versicolor (which can only transform in wet moss), the 

 Raphipteras, Epimartyria, and many others. Dry, open fields are 

 relatively barren of Lepidoptera, but even they are the principal home 

 of the CrambinaB. 



It is a general rule that species in their most favorable local condi- 

 tions will range far beyond their principal zone, and at optimum 

 conditions of temperature will invade abnormal types of environment. 

 This doubtless explains the curious mixture of northern and southern 

 types, for instance, in the peat-bogs, where such arctic genera as CEneis 

 find their only chance in the Canadian zone, and where the typically 

 southern Exyras invade New England. So also the Nymphulinae, 

 which are richly developed in the tropics, find protection from the 

 frost only in our ponds and streams, where they winter below the ice. 



