biology zoology blog benno meyer rochow cave troglobites

Troglobites with Eyes?

Mr. Liddle solved that riddle

Our ancestors were troglodytes. They wore clothes of fur and lived in caves. To most people, however, the idea of living in a cave does not really strike them as terribly enticing: no sun, no flowers, no green plants, no breeze, no wide open spaces and always damp and cool. However, it is precisely the predictability of these conditions, their constancy, which are the characteristics of the cave habitat that work to the advantage of certain animals and make them actively search for caves to live in or even to embrace subterranean life on a permanent basis.

Troglobites are not troglodytes (the latter were human beings). It is a term used to refer to organisms that inhabit caves and that would not be able to survive outside a cave. Representatives belong to many very different groups of animals, e.g., terrestrial and aquatic flatworms, segmented worms, roundworms, snails, spiders and crustaceans; moreover insects, centipedes and millipedes, fishes as well as salamanders. Since plants other than fungi (which are not real plants) cannot exist in caves due to the lack of sunlight, troglobites are characteristically carnivorous or opportunistic feeders. Most can survive without food for months. Species that prefer to live in a cave, but in contrast to the troglobites can also survive outside the cave environment, are referred to a troglophiles.

In order to locate a food source in the dark, troglobites are often equipped with a very acute sense of smell. French scientists of the “Laboratoire souterrain” in Moulis, operating with a multi-chambered maze-like trap, in which one out of 64 chambers was baited with a bit of French cheese (what else?), found that the sparse population of tiny cave beetles had virtually in its entirety wandered unerringly into the correct food-containing, i.e., French cheese-containing chamber. Had they used English cheese, I am sure the result would not have been so convincing, for English cheese is particularly good for mouse traps.

Anyway, until recently it was assumed that loss of eyesight as well as body coloration and a lack of the daily activity rhythm so prominent in epigeic organisms living outside caves, were inevitable signs of an adaptation to the cave environment. However, in my old laboratory at the University of the Waikato in New Zealand, we could show that two species of spiderlike harvestmen, at least in those caves inhabited by luminescent glowworms (Arachnocampa lumiosa), were not blind but had eyes that were fully functional, larger and apparently more sensitive than the eyes of species known from caves that did not contain populations of light producing insects. Why?

The riddle was solved by Mr. Liddle, a student of mine. The two species of harvestman, restricted to caves and lacking body coloration as was expected from a true troglobite, were hunters with a taste (and an eye) for glowworms. And what in the dark would have been an easier method to locate a glowworm than to spot one visually? None. And so it came to pass that, henceforth, it is no longer correct to categorically include “eyelessness” and “being blind” in the definition of a troglobite. However, as important as this finding was, we unfortunately had to find out that it does take quite a while, before such information reaches the writers of speleology text books.

© Dr V.B. Meyer-Rochow and, 2020.
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