ABSTRACT
Morphological adaptation is used to mean evolutionary modifications of the external morphology of lineages of organisms, which are associated with their existence in caves. While some of these modifications may be non-adaptive, most putatively increase their chances of survival or competitiveness in caves. Selected works describing such adaptations in specific groups and reviews of general cave adaptation are listed in the references and further reading. A good example of morphological changes can be seen in cave Collembola of the family Entomobryidae where members of the genera Pseudosinella and Sinella go through very similar evolutionary changes in the caves of Japan, North America, Central America, and Europe (Christiansen, 1961; see Figure 1). These involve increase of adult size, expansion of mesothorax and elongation of furcula, and associated muscle changes producing an enlarged mesothorax, all associated with increased jumping ability. They also involve elongation of legs (also probably associated with increased predation escape), elongation of antennae, and enlargement of antennal sensory organs (both probably associated with increased olfactory and tactile sensitivity). The most studied changes are major changes in the foot complex. These have been shown to be associated first with adhesion on smooth wet rock surfaces and penetration into wet clay, and then with movement over water surfaces. In addition, there is the much-debated loss of eyes and pigment.
