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milliped, laniatorid harvestmen, and collembolan.Two specific examples may be cited, both belongingto comparatively rare arachnid orders. The schizomidSchizomus mexicanus Rowland and the ricinuleidCryptocellus pelaezi Coronado are abundant, both incaves and on the surface in the caves of the Sierra deEl Abra. During the wet season both species may befrequently collected under rocks in this area, as wellas in caves; but with the onset of the dry season bothspecies retreat into the soil and may be found only incaves. Another interesting species with respect to itsdistribution is Schizomus portoricensis (Chamberlin).This species is known throughout many parts oftropical America. Sexual populations are knownfrom the surface in mainland Mexico and from cavesin Chiapas; peripheral populations are apparentlyparthenogenetic. An analysis of cavernicole populationsin Yucatan indicate that facultative parthenogenesishas been a significant factor in the abundantcolonization of the caves of that region (Rowlandand Reddell, 1977).In some arid regions of northern Mexico speciesoccur only in caves, while being found on the surfacein more mesic areas of their ranges. Examples of thistype of distribution are to be found in the pholcidspiders, and most notably Physocyclus enaulusCrosby and P. hoogstraali Gertsch and Davis. Althoughit would he premature to do more than speculate,it is possible that cave populations of these andother species may now be isolated and will eventuallybecome genetically distinct from the parental stock.Numerous species of troglophile in the caves ofthis region are known only from caves. This is particularlytrue with respect to many arachnids andmillipeds. There can be little doubt but that many ofthese will eventually be found in endogean situations;a bias both for cave collections and the study ofcavernicoles by taxonomists has emphasized thecavemicole fauna at the expense of the related endogeanforms. Among the groups in which this occurrenceis most notable are schizomids, pseudoscorpions,opilionids, the smaller spiders, and the moredelicate millipeds. These groups tend to seek a moist,dark habitat of a sort frequently overlooked by thecasual collector. Collecting during drier periods inareas with marked dry and wet seasons will also failto turn up the more mesic-adapted forms. Thecavernicole habitat, with its nearly constant con·ditions of temperature and humidity, provides astable environment at almost all times and thus thechances are greater for finding these forms. Also inmany caves food is scarce in comparison with that onthe surface and even the casual visitor to a cave canquickly locate troglophile and troglobite speciesbecause of their concentration in a few areas whereorganic matter has accumulated. In the case of thesmaller, more secretive forms, such as the pyrgodesmidmillipeds and most pseudoscorpions, only intensivesearch, including the use of special extractiontechniques, will result in their discovery on thesurface.68