20 NORTH AMERICAN ACRIDIID^. 



MACROPTEROUS AND BRACHYPTEROUS SPECIES OF LOCUSTS. 



The presence of apterous or brachypterous, that is to say, flight- 

 less, species of insects in a relatively large proportion is said to be 

 characteristic of islands and alpine mountain summits; and the 

 current explanation, that it is due to natural selection through the 

 agency of the wind, is widely known. While not questioning the 

 efficiency of this agent in a considerable degree with reference to 

 islands of small area, the evidence from North American locusts 

 requires a different explanation. According to this theory species 

 would be likely to show a tendency toward brachypterism along the 

 shores of continents. It would be interesting to make a quantitative 

 study of variation with reference to this point, but there is no evi- 

 dence at hand to indicate that such is the case. Furthermore, flying 

 species appear to be equally as common on alpine mountain summits 

 as flightless species, other conditions being the same. 



In North America the predominant group of Acridians is the 

 Melanopli. Of these many are brachypterous or apterous, in short, 

 flightless. The genera Bradynotes, Asemoplus, Podisma, and Para- 

 dichroplus are typically alpine or sub-alpine in distribution and 

 are flightless. The genera Gymnoscirtetes , Eotettix, Aptenopedes, 

 Phoetaliotes , Paratylotropidia, and others are of lowland distribution 

 and are likewise flightless. There are many flightless species of 

 Melanoplus, the dominant genus of the group, which are likewise of 

 lowland distribution, and others which are found at high elevations 

 in mountainous regions, some above and some below timber line. 

 In other groups there are numerous lowland genera containing 

 only brachypterous species (Rhadinotatum, Macneillia, Mesochloa > 

 Dichromorpha, Clinocephalus , Boopedon, Brachystola, Dictyophorus, 

 Pseudopomala , etc.). Flightless genera and species are distributed 

 impartially as to numbers over lowland and highland areas, in 

 Austral and Boreal life zones. Some factor other than the wind 

 must consequently be involved. 



The solution of this problem lies in the arrangement of the 

 Acridiidse in the two groups, campestral and sylvan, and the factor 

 involved is, adaptation of structure to habits brought about by a 

 sylvan environment. This has caused a change in structure through 

 disuse of the organs of flight. In short, Brachypterism in locusts is a 

 more complete adaptation to a leaping mode of progression brought about 

 by life in situations where flight is difficult or impracticable, and 

 consequently disadvantageous. That this is the true explanation is 

 indicated by the habits and haunts of the majority of the flightless 

 species (sylvan surroundings or tangled undergrowth wherever 



