362 W. HOVANITZ 



sissippi Valley in summer and southward in winter. The species 

 Danais plexippus does so also throughout the whole of its temperate 

 zone range. 



Nearly all plant-eating insects develop a high degree of specificity 

 in their food habits. One species or race of insect is commonly re- 

 stricted to one species or group of plants. Such restrictions naturally 

 limit the geographical ranges of the insects to the range of the nec- 

 essary plants. These restrictions of temperature or habitat further 

 narrow insects to but a portion of that range. The limited ranges of 

 many tropical genera, indicated before, are undoubtedly related to 

 the distribution of food plants. 



With few exceptions, all butterflies depend on direct solar radia- 

 tion for activity. The few exceptions are certain genera of the Brasso- 

 lidae Danainae, and Ithomiinae, which fly in the partial shade of the 

 tropical forest. A close relationship exists between temperature and 

 solar radiation, with regard to the tolerances of certain butterflies. 

 These tolerances are controlled even to groups as small as the color 

 phases of a single species. For example, the white and orange color 

 phases of Colias eury theme have been shown to respond differentially 

 to these two environmental factors, which operate in combination 

 (higher temperatures partly compensate for lower solar radiation 

 and vice versa). 



Areas with little or no direct solar radiation, especially where the 

 temperatures are low, do not support butterfly populations. This is 

 true in the Aleutian Islands, Iceland, and southwest Chile. 



CHANGING DISTRIBUTIONS 



Distributional maps, or the histograms here shown, give an undue 

 impression of permanence. The distributions change in time: over 

 many years (as with climatic changes), over one year (annually), 

 during part of one year (seasonally), or even daily. 



There is much circumstantial evidence for distributional changes 

 in past time. Restriction of colonies of northern species in southern 

 mountains with no recent contact suggests a once wider range with 

 continuity during a colder period. For example, the population of 

 Colias philodice in Guatemala is separated now from its nearest 

 relatives by the whole of Mexico; the population of Colias interior 

 in the southern Appalachians is separated now from the nearest 

 northern stock in the White Mountains of New Hampshire; Colias 



