264 J. A. G. REHN 



tropical origin, and it has broadly established itself, with a related 

 genus of similar Neotropical relationship, in suitable environments, 

 across parts of the southern United States. However, Leptysma 

 occurs across the entire southern border of the western United 

 States, and into Mexico, while the other genus (Opshomala) is 

 found within North America only in the southeastern United States, 

 It would appear probable that the presence of Leptysma in localized 

 areas in the southwestern United States is a reminder of a broader 

 dispersal, probably in Pleistocene times of a greater degree of 

 precipitation. 



Of the Cyrtacanthacridini, to which tribe belongs the striking and 

 often exceedingly destructive "bird locusts," but a single genus, 

 Schistocerca, lives in the Western Hemisphere, and while it has 

 developed there a considerable number of endemic species, a single 

 member of the genus is limited to the Old World, and is there one 

 of their most serious plague forms (the desert locust, Schistocerca 

 gregaria). While this species is known to reach as far across the 

 Atlantic Ocean from the West African coast as the island of As- 

 cension, and has also been captured landing on a ship midway 

 between Africa and South America, there is no certainty that the 

 numerous New World species of Schistocerca, representing at least 

 ten diverse lines of the genus, have entered the New World by 

 flying the South Atlantic, as a number of our species are not addicted 

 to extensive flights, and also some are definitely localized in their 

 distribution and ecological preferences. The optimum differentia- 

 tion of the Cyrtacanthacridini clearly took place in the Old World, 

 and our stock was certainly derived from progenitors there. It 

 is possible that Schistocerca, from a basic ancestral stock of the 

 tribe, developed its various lines in the New World, and that the 

 one to which gregaria belongs, which is well represented in the 

 Americas, North, Central and South, later reentered the Old World, 

 giving it its troublesome S. gregaria. If the latter were a local or 

 restricted type we could conclude that the genus was Old World in 

 origin and is there dying out, but with gregaria in an entirely 

 different category, and the genus greatly diversified in the Americas, 

 it is plausible to conclude that its ramifications had their base in 

 the New World. 



Of Schistocerca five lines occur within western North America, 

 and they are restricted to the same general area except that certain 



