40 
Psyche 
[March 
founding females. If her main concern at this time is to put herself 
into a safe position as rapidly as possible, it is logical to believe that 
the nest-founding female of texanus would accept any cavity in plant 
tissue whose opening she could occlude. In securing her own safety 
the female has also provided for the initial development of the colony, 
since the occlusion that protects her from predators gives equal pro- 
tection to her developing brood. But, thereafter, there should be a 
high mortality among these incipient colonies for, unless the female 
has occupied a cavity in a live-oak or in some other tree on which 
Tillandsias are growing, there is scant chance that the colony will 
reach maturity. If this view is correct the selection involved is not 
the choice by the female of a suitable host plant but the much more 
commonly encountered phenomenon which eliminates any organism 
that has placed itself in a position where survival is impossible. 
There are features in the distribution of texanus which accord well 
with the above view. In the past twenty years the number of records 
for texanus has more than doubled and its range has been consider- 
ably extended by the addition of records from Mexico. With this 
additional information it has become clear that in the lower Rio 
Grande Valley the incidence of texanus is far less than it is in other 
parts of the range. This region of low incidence extends both north 
and south of the Valley proper and forms a band, about a hundred 
miles wide, in which it is exceedingly difficult to find colonies of 
texanus. During six winters of collecting in this area the writer has 
failed to take a single colony. Indeed, che one record of texanus from 
the Rio Grande Valley appears to be the Brownsville record published 
by M. R. Smith in 1936 (5). The only live-oaks in the lower part 
of the Valley are a few trees which have been brought in and planted 
around houses as ornamentals. For this reason alone it might be 
expected that texanus would not find the lower Valley a particularly 
favorable area in which to nest. But there is no lack of mesquite, 
Texas ebony or hackberry trees in the lower Valley and it is hard 
to see why these trees are so seldom utilized if nothing more than 
their presence is required. But while all three of these trees are 
widely and uniformly distributed throughout the lower Valley, the 
great majority of them are free from Tillandsias. The latter are very 
sporadic in the Valley, most of them being confined to a few isolated 
pockets near the river. If it is true that this restriction makes most 
of the trees in the Valley unsuitable as nest sites for texanus there 
is no need to look further for an explanation of its low incidence 
there. 
