Cave-dwelling insects are among the most widespread and prominent troglofauna (cave-dwelling animals), including troglobites, troglophiles, and trogloxenes. As a category of ecological adaptations, such insects are significant in many senses, ecological, evolutionary, and physiological.
Introduction
A cave is an unusually well-defined ecological habitat in terms of its nature, time, and place. Accordingly, it is not surprising that a number of insects permanently inhabit caves, especially at the deepest levels, and are markedly specialised for niches in some of the extreme conditions. These are the true cavernicole species; troglobites rather than troglophiles or trogloxenes. These include; spiders and insects.
Cavernicolous insect species rarely are adapted to move from cave to cave, so each species or community generally will be restricted wholly to certain caves or cave systems each, and commonly will have evolved in their respective home cave systems. Exceptions commonly are those that have been carried by mobile vertebrate trogloxenes or troglophiles, though in some cases a number of populations may have evolved from a single mobile troglophile population. Caves tend to be geologically short-lived, so most of the specialised adaptations are correspondingly young in evolutionary terms and to have arisen rapidly and in parallel from similar ancestors that began as similar troglophiles in separated caves. Many insect troglobites are Orthopteran, Collembolan, or Blattodean, for example, and given the nature of their open-air ancestral species, it would be in no way surprising that where a cave becomes available, it soon is invaded by opportunistic troglophiles that may be widely distributed and may evolve similarly in separate caves in different areas.
Caves also appear to have become the last refugium for many ancient types of insects, which are no longer found free in the open in surrounding regions. Such cave fauna thus represent, at least in part, relicts. It does not follow that they had been in those particular caves since ancient times though. For example, modern troglobitic Onychophora had not been occupying their current caves since the Carboniferous period but had entered new caves comparatively recently and flourished by exaptation.
Once adapted to troglobitic existence, cave insects become specialised and dependent on the cavernicolous conditions; when suddenly exposed to the outside world, they are likely to succumb rapidly.
True cavernicolous species, troglobites, include many animals apart from insects. There are various troglobites among the planarians, Oligochaeta, Polychaeta, leeches, Mollusca, and fishes. Troglobitic Crustaceans include species of Amphipoda, Cladocera, Copepoda, Decapoda, Isopoda, and Syncarida. Many troglobites are predatory, including Chilopoda, Acari, Opiliones, Chernetidae, and spiders.
Troglobitic insects include Apterygota such as Campodea and various Collembola. There are many species of beetles in families such as Carabidae, Curculionidae, Leiodidae, and Silphidae. Some Orthoptera are troglobitic and some are trogloxenic. The order Blattodea includes troglobites, and so do the Trichoptera and Diptera.
Categorization of cave dwellers
The cave dwellers fall under one of the following categories: Troglobite species are true cave dwellers, occurring exclusively in caves and unable to survive in the open. True troglobites among insects include many Coleoptera, some Stenopelmatidae, Diptera, and Zygentoma. Troglophile species sometimes occur outside the cave habitat but typically complete their life cycle in caves. Trogloxene cannot live permanently in caves, but may enter caves and spend parts of their life cycles in caves. Unusual South African Tettigoniidae in a few genera such as Cederbergeniana are trogloxenes that feed on plants by night and shelter in caves by day.
The cave environment
The climate in deep caves typically is without distinction of day and night, But insects have a sleeping pattern and not many are affected even by the passage of the seasons. Violent winds and storms are unknown, though there may be steady air currents under some circumstances. Humidity is roughly stable. Communications with the outside world only occur under special conditions such as floods and exceptional droughts. Where streams pass through caves or water seeps in, they commonly are important sources of nutrition.