78 
Psyche 
[March 
when temperatures there, if scarcely tropical, were milder than at 
present, most of the Middle American mesostenine genera prob- 
ably ranged far up the Gulf and Atlantic coasts. All of these 
moisture rather than temperature-controlled taxa tolerate winters 
with repeated frosts (as shown by their present-day altitudinal and 
latitudinal distribution in Mexico) and, in fact, there are few, if 
any. Neotropic mesostenines which are “tropical” in the sense of 
requiring frost-free winters. This is why a fair number of Neo- 
tropic mesostenines survived glaciation in the southeastern U.S. 
This is also why the Valley should be viewed not only as a present- 
day northern outpost of the main Neotropic fauna but also in 
historical perspective as an area once situated deep within the 
vaster Tertiary Neotropics. 
Comparison with other Neotropic Deserts 
Recent hand collecting surveys in the Peruvian Coastal Desert 
(Porter, 1975b) and net and Malaise sampling in the northeast 
Argentine Subandino (Porter, 1975a) allow comparison of Valley 
mesostenines with those of two ecologically somewhat similar 
areas in remote parts of the Neotropics. 
The Peruvian Desert reaches from south Ecuador to north Chile 
on the Pacific Coastal Plain and adjacent Andean foothills of west- 
ern South America. Its rainfall varies from 80 mm. per year in 
northern Peru to 0.5 mm. at Arica, Chile but, near the coast, 
massive fogs supply additional humidity. This desert is frost-free, 
at least on the Coastal Plain, but the cold Humboldt Current run- 
ning just off shore keeps temperatures moderate (at Arica, Chile 
the summer day-night range is about 20-30 degrees C. and the 
winter range approximately 10-22 degrees C.). Thus we confront 
the paradox of a humid but almost rainless, cool but frost-free 
tropical desert. 
Hand collecting during June-July of 1 974— ’76, mainly around 
Chiclayo, Trujillo, and Lima, Peru and Arica, Chile, has obtained 
from this desert in fertile valleys between sealevel and 2000 m. a 
mesostenine fauna of 12 genera and 31 species. There are nine 
Neotropic genera ( Biconus , Cyclaulus, Diapetimorpha, Basileucus, 
Lyme on, Acerastes, Polycyrtidea, Messatoporus and Agonocryp- 
tus ), one Pantropic genus ( Baltazaria ), two Sonoran genera ( Comp - 
socryptus, Mesostenus of the Longicaudis group), and two Holarc- 
tic genera ( Trachysphyrus , Mesostenus of the Transfuga group). 
