Acacia collinsii trees contain a limited number of domatia that are inhabited by Pseudomyrmex spinicola ants. Pseudomyrmex spinicola creates entrances to the uncolonized domatia by chewing a hole into them. We tested the null hypothesis that entrance orientation would be non-random. However, as predicted, there were more domatia with downward facing entrances. Also, more ants were found inhabiting domatia with downward facing entrances than upward entrances and when experimentally manipulated, more ants colonized emptied domatia that were oriented downward. Simulated rain sealed upward entrances more often than downward entrances. Observations suggest that ants inside were trapped, and ants outside could not enter, implicating rainfall as the primary factor causing P. spinicola to create, inhabit and colonize domatia with downward facing entrances.