70 
Psyche 
[June 
adapted for cave life. Ants are not exceptional in this regard. Other 
adventitiously cavernicolous animal taxa are often very varied in their 
ecology; some other arthropod groups, such as the European copepods, 
resemble the ants in that the cave species are typically widely distrib- 
uted on the outside (Hesse, Allee, and Schmidt, 1951 ). 
The question can now be raised, whether there are any ant species 
that are troglobitic, i.e. limited to caves. Erebomyrina urichi is clearly 
excluded on the basis of information just given, and the accumulated 
evidence makes it improbable that any other member of the genus is 
troglobitic. Paratrechma (Nylanderia) troglodytes Weber of Cuba, 
originally recorded from a shallow cave, has been shown to be a 
junior synonym of P. (N.) myops (Mann), which has been found in 
soil away from caves in two montane localities in Cuba (Brown, 
1955). A related species, microps M. R. Smith, occurs in places 
remote from caves on Puerto Rico. Proceratium cavernicola (Borg- 
meier), described from a queen found in the Chilibrillo Caves of 
Panama, has been recognized as a synonym of P. micrommatum 
(Roger), known from several non-cavernicolous series collected in 
Central America and Cuba (Borgmeier, 1957; Brown, 1958). 
There remain at least four ant species known only from single 
collections made in caves. Brachymyrmex cavernicolus Wheeler 
( 1938) was found beneath a stone near the mouth of Balaam Canche 
Cave, Chichen Itza, Yucatan. It is pale yellow in color and has small 
eyes, which perhaps suggest a normally cavernicolous habit. But it 
belongs to a genus the taxonomy of which is in outstandingly poor 
condition, so that the possibility exists that non-cavernicolous series 
have been recorded in the literature under other names, as in the cases 
of the Paratrechma and Proceratium species just mentioned. The 
African cavernicoles Leptogenys jeanneli , Stru?nige\nys stygia, and 3 
Smithistruma marginata have already been discussed. They belong j 
to otherwise non-cavernicolous groups and show no apparent morpho- 
logical adaptation to cave life themselves. Clearly, until stronger 
evidence is produced, the existence of troglobitic cave ant species must 
be considered in great doubt. 
Enlarging on this point, let us ask why there are so few (if any) 
troglobitic ants or, for that matter, troglobitic social insects of any 
sort. The following hypothesis is proposed. The reproductive^unit 
in social insects is the colony, and it follows that natural selection 
operates on the colony as a unit. Further, the singly prefertilized 
mother queen of a monogynous colony ordinarily contains thef whole 
of the transferable genetic material, and, since she stores the spferm of 
