1971] Willey & Willey — Populations of Grasshoppers 
341 
of the mesa will be changed. It is well to have a baseline for evaluat- 
ing the changes which will occur in the zoogeography of this zone 
of hybridization. It should be instructive to examine this area soon 
for evidence of other species- and morph-pairs in the process of hy- 
bridization, since climatic conditions and geography at present seem 
ideal for the development of a common “suture-zone” between spe- 
cies-pairs of diverse organisms (Remington, 1968), especially between 
those which are spring-brooded, ground-inhabiting poikilotherms. 
Conclusions 
On Black Mesa, Colorado, the grasshopper /lrphia conspersa ex- 
hibits a phenotypic cline in wing-color variation which separates 100% 
orange-red denies from 100% yellow demes by two to five miles. The 
cline seems to be maintained by three factors at least : 1 ) limited 
vagility of the individuals in a deme, 2) unsuitable habitat between 
the geographically discontinuous populations, and 3) periodic, vir- 
tually complete, exterminations of the hybrid demes by climatological 
catastrophes related to the fact that the mesa-top is at the altitudinal 
limit for the species. 
Summary 
Arphia conspersa varies in wing color on Black Mesa in southwest- 
ern Colorado. The phenotypes exhibit an orderly but steep step-wise 
cline from 100% orange and red to 100% yellow over a distance of 
only two to five miles, ascertained by sight-census and mark-recapture 
methods over a period of eight years. Black Mesa slopes gradually 
from 9000 ft elevation in the south to 11,000 ft in the north. Since 
A. conspersa has successfully colonized only a few areas above 1 0,000 
ft, this also affords an opportunity to observe the ecology of a species 
at its altitudinal limit. 
In 1970, 50 demes which had been located on the mesa were vir- 
tually exterminated, probably by unseasonal weather conditions prior 
to and after a prolonged winter snow pack lasting more than 8 
months. However, these high-altitude populations have a two-year 
life cycle with both eggs and nymphs overwintering. The alternate- 
year brood, which had over-wintered eggs in 1969-70, was unaffected 
and matured normally in 1971. Census of nymphs in the fall of 1971 
indicated no recovery of the nearly exterminated brood. This catas- 
trophe strongly suggests that maintenance of this narrow hybrid-zone 
depends not only on the barriers produced by unsuitable habitat, but 
also on periodic extermination of the mixed populations. It also indi- 
cates the survival value of a two-year life cycle at the altitudinal 
limits of a species. 
