80 
Psyche 
[March 
between faunas as distant as the south Texan and the northwest 
Argentine. This correspondence between Texan, Peruvian, and 
Argentine arid-adapted mesostenine communities also shows that 
the same versatile genera tend to survive anyplace in Middle and 
South America where local conditions become dry enough to elim- 
inate the wet forests which are the optimum habitat for most 
Neotropic mesostenines. Almost equally, it suggests a correlation 
between dry and cold tolerance, since nine of the ten Neotropic 
genera that reach well north in the eastern U.S. also occur in one 
or more of the dry areas studied (Crypt anura, Polycyrtus, Diapeti- 
morpha, Lymeon, Mai lochia, Acerastes, Pachysomoides, Messa- 
toporus, and Agonocryptus). 
Sonoran mesostenines also show much affinity among the three 
study areas. These genera are centered in Mexico and the south- 
western U.S. and evolved there in response to increased aridity and 
orogeny which affected that region in the latter half of the Tertiary. 
The Valley has four Sonoran taxa ( Joppidium , Lanugo, Compso- 
cryptus, and Mesostenus of the Longicaudis group) and two of 
these, Compsocryptus and the Longicaudis group of Mesostenus, 
are the only Sonoran mesostenines known from the Peruvian Des- 
ert and the Subandino. However, Lanugo reaches the Peruvian 
Andes and so might be found on the coast and Joppidium possibly 
enters both deserts, since it is known from Ecuador and turns up 
again in the Argentine Chaco. Most of these genera favor semiarid 
habitats, from thorn scrub to subtropical deciduous forests, and 
doubtless extended their distributions in the driest parts of the 
Tertiary and during interglacial xerothermic episodes. The present 
moderately wet interglacial has produced some notable Sonoran 
disjunctions, such as the above-mentioned case of Joppidium and 
that of Compsocryptus, which has many species in the western 
U.S. and Mexico, a single representative in Florida and Cuba, an 
isolated species in the Peruvian Coastal Desert, and another dis- 
junct species in northern Argentina. 
The Holarctic element, consisting mainly of genera adapted to 
temperate forests, shows more discontinuities among Middle and 
South American arid zones than do the Sonoran and Neotropic 
faunas. Gambrus, Trychosis, and Listrognathus reach only as far 
south as the Valley or northern Mexico. On the other hand, 
Trachysphyrus and the Transfuga group of Mesostenus occur in 
all three study areas. The Transfuga group is mainly Holarctic 
