362 NEOTROPICAL PSELAPHIDAE 



Thus Euplectus, Reichenbachia, and Pselaphus would appear to have 

 entered North America from the Palaearctic long ago. Reichenbachia, 

 especially, seems to have great vagility. On the other hand Hamotus appears 

 certainly to have evolved within the neotropics, and to have spread in all di- 

 rections, reaching into the northern limit of the neotropics in Mexico and into 

 peninsular Florida via the Antilles. In this connection a general hypothesis 

 may be proposed regarding dispersal between the two American faunal areas. 

 The change in climatic and associated biotic conditions is quite abrupt in the 

 Mexican arm of the continent, between the arid southwestern United States 

 and Mexico on the one hand, and the northern extension of the rain forest in 

 Mexico. This faunal break appears to develop along the Tropic of Cancer. 

 The eastern or Antillean arm, from Venezuela to Florida, offers much less 

 change in climate and biotic conditions. 



In other words, on the west the change is relatively abrupt from neotropical 

 rain forest to nearctic semidesert; on the east the change is from neotropical 

 rain forest to nearctic subtropical forest. The western highway offers continuous 

 dispersal surface, with a good north-south highway in the Rocky Mountains- 

 Andes chain which allows vertical movements to compensate for changes in 

 latitude; but the climatic-biotic change is marked. The eastern highway is 

 subjected to frequent insular discontinuity, plus the vicissitudes of insular 

 climate; but the climatic-biotic overall change is much more gradual. It would 

 seem that these two dispersal paths have had to bear the north-south faunal 

 interchange over a long period of time, and a careful analysis should demon- 

 strate which path has been most successfully used. This requires a more 

 complete knowledge of pselaphid distribution than we now possess. 



The actual facts are meager. Separate cases can be made out for each 

 highway, largely depending upon the genus involved, which must mean that, 

 granting a sound taxonomy, both routes have been used by Pselaphidae. At 

 present we have not enough data to espouse either the Mexican or the Antillean 

 bridge as a major influence. 



Thus the wholly American genus Melba has about twenty-nine species. 

 Eleven species are nearctic, distributed north of the Rio Grande, from Arizona 

 to the Atlantic coast, and from Florida to New York. The preponderant num- 

 ber of these, however, is east of the grassland biome. The neotropics have 

 eighteen species, seven of which extend from central Mexico, through Guate- 

 mala, French Guiana, Brazil, and into northern Argentina; eleven Melba are 

 endemic to the Antilles. The genus Melba then, appears to have used both routes, 

 with a slight preponderance for the Antillean route. 



On the other hand, the important genus Hamotus, with eighty-nine species, 

 is distributed over the entire Neotropical Region and extends northwards to 

 the limit of the rain forest in Vera Cruz. Here the genus breaks off short, with 

 at least three species in Vera Cruz. The only non-neotropical species is 

 Hamotus opimus of southern Florida, which suggests colonization from the 

 Antilles. The difiiculty here is lack of facts because the Antilles have but one 

 species, Hamotus hirtus, of the Windward and Leeward Islands. This is a 



