SUBTERRANEAN ANIMAL LIFE 533 



tails and mites. Whether this is due to the chemical action of light or 

 in part also to the heat rays was not determined. 



Sense organs of cave animals. — Degeneration of eyes, optic 

 nerves, and optic ganglia is widespread among cave animals. Many 

 cave turbellarians have no external eyes. In Planaria montenegrina 

 from caves the eyes seem reduced in comparison with specimens from 

 without. The dark planarian, PI. vitta, still has eyes. The leech, Dina 

 absoloni, is without pigmented eyes. In may cave snails, especially in 

 Lartetia, the eyes are reduced. The cave crustaceans Niphargus, Asel- 

 lus, and Cambarus have reduced eyes. Numerous cave beetles* are eye- 

 less. The olm and the American cave salamanders Typhlotriton and 

 Typhlomolge have much-reduced eyes. Cave springtails may be eyed 

 or eyeless, like those of the surface. The same is true of mites, but the 

 gamasid mites of caves are uniformly without eyes. The beetle, Ma- 

 chaerites mariae, is found with or without eyes according to the distance 

 at which it lives from the cave entrance. 18 The species of Chologaster 

 have perfect eyes, but they become smaller with increased fixity of 

 cave life. In the fishes Amblyopsis, Typhlichthys, and Troglichthys, 

 the eyes are reduced in size, the lens is minute, and the retina more or 

 less vestigial, little pigmented or unpigmented (Fig. 134) , 19 



The question has been raised as to whether the eyes were not al- 

 ready on the road to reduction before these animals took to subter- 

 ranean life. This is pertinent among such forms as the springtails or 

 mites, but does not apply to such forms as the beetle Machaerites 

 mariae or to Planaria montenegrina. Kammerer's experiment 20 on the 

 olm, in which he found that larvae maintained in red light would 

 produce adults with well-developed eyes, also speaks against it. The 

 larvae of the cave salamander Typhlotriton spelaeus, which develop in 

 waters outside the caves, have functional eyes. These degenerate in the 

 course of normal metamorphosis, which takes place in the completely 

 dark caves; but larvae kept in the light will produce adults with func- 

 tional eyes. 21 Vire's experiment with Gammarus fluviatilis produced no 

 reduction of eyes after a year. Obviously, animals react differently to 

 darkness, both with respect to loss of pigment and reduction of eyes. 

 Any conclusion as to the relative length of cave life of a species on 

 this basis is consequently untrustworthy, except perhaps in respect to 

 series of closely related forms, like the blind fishes. 



The loss of eyes in cave animals, as in the deep sea, is accompanied 

 by compensating strengthening of other sense organs. Many insects, 

 myriapods, and arachnids of the cave world have legs and antennae of 



* Trechus (Anophthalmus), Caulomorphus. Bathyscia, and Orostygin. 



