1976] 
Peck — Beetle Genus Echinocoleus 
59 
generic characters as apotypic; the limuloid shape, the lack of a 
mesosternal carina, and the long, semi-erect hairs. These are ob- 
vious adaptations to myrmecophily. Other characters vary between 
the species and are judged to proceed from a plesiotypic to an apo- 
typic state as follows: long dorsal hairs on all dorsal surfaces to on 
elytra only; pronotal striae present to absent; elytral striae from 
closely spaced to widely spaced; aedeagus tip from narrow to broad 
in dorsal view; color dark brown to light brown; and spermatheca 
crest from narrow to elongate. Comparison of these states with 
data given in the species descriptions leads to the conclusion that 
£. sonorensis possesses the greatest number of apotypic characters, 
E. chihuahensis possesses a mixture of pleisotypic and two inter- 
mediately apotypic characters, and E. setiger possesses all but one 
character in a pleisotypic condition. A parsimonious splitting se- 
quence is determined by that involving the fewest convergencies 
(Ball, 1976). Thus, sonorensis is the sister group of the stock that 
gave rise to the other two species. In this scheme the reduction of 
pronotal striae, coarsening of elytra striae, and pronotal broaden- 
ing have each occurred twice. 
Using this phylogenetic interpretation, applying the zoogeo- 
graphic and evolutionary clues of Darlington (summarized by 
Erwin, 1970: 184), and considering known and surmised paleo- 
botanic and paleoclimatic conditions, I propose the following 
scenario of sequential events. 
1. The aridity of the southwest increased in the second half of the 
Tertiary. Grasslands and thorn scrub desert areas developed in the 
Miocene, especially in the rain shadows of the developing mountain 
ranges (Axelrod, 1958; Cohn, 1965). Since I know of no specula- 
tion of the subject, I suggest that Pogonomyrmex and Novomessor, 
both comparatively primitive myrmecine ant genera came to occupy 
these developing arid habitats at this time. The ants were already 
present because Pogonomyrmex, Aphaenogaster, and Messor (in- 
terpreted by Creighton (Wheeler and Creighton, 1934: 360) (as 
Pogonomyrmex) occur in the Upper Oligocene Florissant shale 
of Colorado (Carpenter, 1930). 
2. The increasing aridity of the late Pliocene and especially of 
the Pleistocene led to development of part of the already existing 
southwestern Madro-Tertiary Geoflora into the diverse and distinct 
Sonoran desert vegetation (Axelrod, 1958). 
3. A species of Ptomaphagus in the consobrinus group invaded 
