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Caves are perhaps the most distinct and well-defined of insect habitats. A number of insects are permanent habitual inhabitants of caves, characterized by marked specializations for the extreme conditions. These are the true cavernicole species. Most caverniculous insect species are severely restricted wholly to certain caves or occur in generally similar caves of the same region. Some cave insects such as grasshoppers and Collembola are, however, rather widely distributed and may be found in caves in different areas. Most of these true cave dwellers have no closely related groups on the open ground. The caves appear to have become the last refugium for many ancient types of insects, which are not found any more free in the open above ground in the region. The cave fauna thus represent, at least in part, relicts. Cave insects, when suddenly exposed to the outside world, often succumb very rapidly.
True caverniculous species are found not only among insects but also in diverse other groups like planarians, Oligochaeta, Polychaeta, leeches, Mollusca, fish, many Crustacea (such as Isopoda, Amphipoda, Syncardida, Decapoda, and Copepoda), predatory Chilopoda, mites, opilionids, chermetids, spiders, etc. Insects are of course very abundant and range from Campodea and numerous Collembola to Carabidae, Silphidae, Curculionidae, some Orthoptera, blattids, Trichoptera and Diptera.
The cave environment
The general climate in a cave is remarkably unvaried and without distinction of day and night. The differences between summer and winter conditions are also greatly moderated. Violent winds and storms are unknown, though there are steady air currents. The humidity condition changes but very little. Communications with the outside world are on the whole minimum. The river flowing or water seeping in from the outside insures regular quantities of nutritive material. The bats that feed outside but sleep in the cave likewise provide massive deposits of organic matter by way of fecal droppings and dead bodies. In addition to bats, a variety of other animals penetrate caves for sleeping or for over-wintering: butterflies, flies and other insects, bears, hyenas, and even humans. The cave environment is thus recognized largely by negative characteristics, viz. absence of light, lack of rhythm of day and night and of seasons, scarcity of available food, limitation of living space, restricted freedom of movements, absence of contrast, absence of marked changes in temperature and humidity.
The ultimate source of food for all cave animals lies naturally outside the cave. Rivers carry in cadavers and other organic debris. Fungi which develop on this detritus provide food for many cave dwellers. Bat guano represents another source. Lepidoptera that enter caves for sleeping are preyed upon by cave grasshoppers. The caverniculous Collembola feed on colloidal matter on the water. The Collembola and cave beetles are devoured by spiders and myriapods. All these activities go on in total darkness.
In individual caves not all the numerous cavernicol insects, the most conspicuous and perhaps also universally met with peculiarity is the reduction in body pigmentation. This is particularly marked in Coleoptera. The reduction or total loss body pigmentation are without doubt correlated with the absence of sunlight. This is demonstrated by the fact that all these unpigmented cave beetles readily develop the characteristic pigment when they are exposed to sunlight. A second peculiarity is the more or less pronounced reduction of eyes in all caverniculous species. Nearly all cave insects are characterized by an abnormal elongation of appendages, especially the antennae, as compensation for loss of eyes. There is also increase and elongation of sensory setae, as for example, in the beetle Scotoplanetes arenstorffianus from Herzegovina. In contrast, none of the free-living related carabids have such sensory setae on the elytra. True cave insects are generally all characterized by wing reduction. Among the cave beetles the hind wings are reduced or even lost.
The general appearance and attitude of body of cave insects often differ conspicuously from those of free-living relatives. This is particularly observed in Silphidae, through nearly every other cave insect also exhibits this peculiarity. All these are evidence not of selection, as commonly assumed, but of direct functional adaptation and correlation to the immediate environment.
Categorization of cave dwellers
The cave dwellers fall under one of the following categories:
- Troglobiont species are true cave dwellers, occurring exclusively in caves and never in the open.
- Troglophile species are insects which can and sometimes occur outside the cave, but prefer the cave habitat.
- Trogloxene insects are incapable of living long or permanently in caves, but do occasionally penetrate the caves and manage to survive the extreme environment.
Among the troglobionts insects the most important include Coleoptera, Stenopelmatidae and some Diptera (many of which are perhaps only troglophiles). Nearly all other insects found in caves are perhaps only trogloxene. The troglobiont grasshoppers of Europe belong to Dolichopoda and Troglophilus. These species are completely apterous, but are provided with well-developed and pigmented compound eyes. The legs and antennae are markedly elongate as in Dolichopoda. Numerous Carabidae are true caverniculous forms.
On the other hand the conspicuously microphthalmous staphylinids Glyptomerus cavicola, Atheta absoloni, Colydiidae Anommatus titanus, Curculionidae Troglorrhynchus and Absoloniella, some blind carabids, staphylinids and Bathysciinae found commonly in caves in Europe, are really only troglophiles. All true troglobionts are apterous, yellowish-brown or pale reddish-brown, with eyes mostly atrophied or absent. Two European aquatic beetles, Dytiscus balsetensis and Hydroporus aveniovensis, are pale yellowish-brown and with eyes greatly atrophied and unpigmented.
Some important cave insects from Europe include the following: Paraoalyscia wollastoni, Bathysciola fauveli, Trechus (Trichaphaenops) sollandi, Royerella villaridi, Trechus (Trichaphaenops) angulipennis, Trechus (Duvalius) pilosellus stobieckii, etc. The beetle Leptodirus hochenwartii, found in the Postojna cave system in Slovenia, was the first animal to be recognized as a true cave dweller.
The cave insects found in the Atlas Mountains include blind Trechus jurijurae, Aphaenops iblis, Nebria nudicollis with very long antennae and legs, the staphylinids Paraleptusa cavatica and Apterophaenops longiceps, and the curculionid Troglorrhynchus mairei. The carabid Laemostenus fezzensis is a troglophile. Neaphaenops tellkampfi occurs in caves in Kentucky. The American stenopelmatid Hadenoecus subterraneus is recorded from Kentucky caves. The remarkable carabid Comstockia subterranea is a true cave species found in Texas. The exclusively cave-dwelling silphid Adelops hirtus occurs in Kentucky caves and has very minute, unpigmented, atrophied eyes.
- Wilson, J.M. (1982). "A review of world Troglopedetini (Collembola) including an identification table and descriptions of new species". Cave Science: Transactions of the British Cave Research Association. 9 (3): 210–226.
- Polak S. (2005). "Importance of discovery of the first cave beetle Leptodirus hochenwartii Schmidt, 1832." . Endins 28 (2005).
- Chapman, R.N. 1931. Animal Ecology. London and New York
- Graham, S.A. 1933. The influence of civilization on insect fauna of forests. Ann. Ent. Soc. Amer., 26: 497–503
- Hubbard, H.G. 1898. Insect life in Florida caves. Proc. Ent. Soc. Washington, 4: 394
- Mani, M.S., 1968. General Entomology, Oxford & IBH Publishing Co. New Delhi, chp. XIII, pp 308–312