96 
Psyche 
[June 
However, the similarities suggest a comparable level of cave adapta- 
tion for the two species. 
The last column of the table presents the data on the lateral width 
of the eye compared to the lateral width of the head between the 
antenna and the vertical carina, forming the side of the head. The 
eye of troglomexicanus is significantly larger than of loedingi. 
Appendage elongation may be a positive adaptive response to com- 
pensate for the loss of visual environmental information. With 
longer appendages covered with more sensory hairs, a beetle could 
receive information from a greater volume of its environment. Van- 
del (1965) has reviewed this argument, citing organisms which both 
favor and refute it. He notes that since many groups of cave animals 
do not possess appreciably lengthened appendages, the argument is 
inadequately supported. However, we should not assume that all 
taxa, after adaptation, will exhibit the same degree of morphologic 
specialization. A situation may be adaptively solved by more than 
one answer. Various physiological, behavioral, or other character- 
istics may be of greater significance in some taxa not showing ap- 
pendage elongation. Laboratory experimentation on the beetles’ 
sensory physiology may possibly tdl us if the tendency towards 
appendage elongation is really the result of a positive selection for 
greater sensory capability. 
In a totally dark environment, eyes would seem to be of no selec- 
tive value. With relaxed selection, loss of the organ system will 
follow the accumulation of degenerative mutations. Sadoglu (1967) 
has investigated this phenomenon in the Mexican cave fish Ano- 
ptichthys. The eyes of P. troglomexicanus , when compared to P. 
loedingi , are not reduced in proportion to the degree of lengthening 
of the appendages. This undoubtedly reflects the expected discordant 
selective forces acting in the two sets of organ systems. The gradual 
loss of eyes may be considered as a selectively neutral byproduct of 
the genetic reorganization experienced during adaptation to a cave 
environment (Barr 1967). 
Compared to temperate areas, very few troglobitic beetles are 
known from tropical areas. The present view on the origin of 
troglobitic beetles is that they evolved from epigean ancestors living 
in the areas of the caves during the cold-wet glacial periods. The 
entry into caves was brought about by changing climatic conditions; 
the unsuitable warm-dry periods of the interglacials (Barr 1968, 
Jeannel 1949). The interglacial epigean environment became un- 
favorable for cold and moisture demanding species. They were 
