THE DISTRIBUTION OF INSECTS IN NEW HAMPSHIRE. 333 



that they are chiefly influenced, so far as latitude is concerned, by the 

 temperature of the breeding season . . . ; whether a similar law con- 

 trols the distribution of mammalia, reptiles, insects, etc., can only be 

 determined by further investigation." 



Since insects are not regularly migratory animals ; as several genera- 

 tions frequently succeed each other during a single season ; and, as the 

 winter is passed in very various conditions, we can hardly expect their 

 distribution to follow exactly that of birds. Various causes may modify 

 unequally the distribution of insects belonging to a certain group: too 

 intense cold in our arctic winters ; the lack of snow during a less severe 

 season ; too excessive heat or too long a drouth in midsummer ; or, too 

 sudden changes of temperature at critical periods. Taking our butter- 

 flies only, they may be found at every season of the year, even in mid- 

 winter, of one species or another, in every stage of existence, from the 

 egg, through all the larval periods and the chrysalis, to the imago. The 

 distribution of butterflies is therefore much more complicated than that 

 of birds, whose early stages are always passed in comparatively warm 

 weather, under the guardianship of the mother; and, if more than one 

 brood appears during a season, the second is only the produce of the 

 same pair that raised the first. 



It is nevertheless true that the distribution of insects over continental 

 areas coincides in a remarkable way with that of birds. The northern 

 limits of the Alleghanian fauna, as laid down by Verrill, agree very 

 fairly with the northern boundary of the belt colored blue on Plate B ; 

 and this probably indicates pretty accurately the southern limit of Cyclo- 

 pides Mandan and the northern limit of Megisto Eiirytus, Grapta comma, 

 Argynnis Cybele, A. Aphrodite, and Euphyes Metacomet. It may be ques- 

 tioned, however, whether, as far as butterflies are concerned, this can really 

 be considered the northern limit of the Alleghanian fauna. If we trace 

 upon a map of the state the northern limits of the several Alleghanian 

 butterflies and the southern limits of the Canadian, they will be found to 

 mingle in a broad belt of country, which includes all the colored portions 

 of Plate B^ The northernmost Alleghanian and southernmost Canadian 

 species gradually decrease in numbers away from their metropolis, and 

 become confined to increasingly lower or higher altitudes in this belt, 

 according as they are Alleghanian or Canadian forms. 



