Cave-dwelling insects are among the most widespread and prominent troglofauna (

cave Caves or caverns are natural voids under the Earth's Planetary surface, surface. Caves often form by the weathering of rock and often extend deep underground. Exogene caves are smaller openings that extend a relatively short distance undergrou ...

-dwelling animals), including

troglobite A troglobite (or, formally, troglobiont) is an animal species, or population of a species, strictly bound to underground habitats, such as caves. These are separate from species that mainly live in above-ground habitats but are also able to live u ...

s, troglophiles, and

trogloxene Trogloxenes or subtroglophiles, also called cave guests, are animal species which periodically live in underground habitats such as caves or at the very entrance, but cannot live exclusively in such habitats. Among many scientists, trogloxenes and ...

s. 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 ecology, habitat refers to the array of resources, biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species' habitat can be seen as the physical manifestation of its ...

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 Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

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

Orthoptera Orthoptera () is an order of insects that comprises the grasshoppers, locusts, and crickets, including closely related insects, such as the bush crickets or katydids and wētā. The order is subdivided into two suborders: Caelifera – gras ...

Collembola Springtails (class Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects. Although the three lineages are sometimes grouped together in a class called Entognatha because they have internal m ...

n, or

Blattodea Blattodea is an order (biology), order of insects that contains cockroaches and termites. Formerly, termites were considered a separate order, Isoptera, but genetics, genetic and molecular evidence suggests they evolved from within the cockroach ...

n, 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 Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...

thus represent, at least in part,

relicts A relict is a surviving remnant of a natural phenomenon. Biology A relict (or relic) is an organism that at an earlier time was abundant in a large area but now occurs at only one or a few small areas. Geology and geomorphology In geology, a r ...

. It does not follow that they had been in those particular caves since ancient times though. For example, modern troglobitic

Onychophora Onychophora (from , , "claws"; and , , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, ''Peripatus''), is a phylum of el ...

had not been occupying their current caves since the

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

period but had entered new caves comparatively recently and flourished by

exaptation Exaptation or co-option is a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and be ...

. 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 Oligochaeta () is a subclass of soft-bodied animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, including all of the various earthworms. Specifically, oligochaetes comprise the terrestrial megadril ...

, Polychaeta,

leeches Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bod ...

Mollusca Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

, and

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

es. Troglobitic

Crustacea Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

ns include species of

Amphipoda Amphipoda () is an order (biology), order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphip ...

, Cladocera,

Copepoda Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (living in the water column), some are benthic (living on the sediments), several species have ...

Decapoda The Decapoda or decapods, from Ancient Greek δεκάς (''dekás''), meaning "ten", and πούς (''poús''), meaning "foot", is a large order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfish, shrimp, a ...

Isopoda Isopoda is an order of crustaceans. Members of this group are called isopods and include both aquatic species and terrestrial species such as woodlice. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed l ...

, and

Syncarida Syncarida is a superorder of crustaceans, comprising the two extant order (biology), orders Anaspidacea and Bathynellacea, and the extinct order Palaeocaridacea. Taxonomy Fifty-nine living genera are known, in six families: ;Anaspidacea Calman, ...

. Many troglobites are predatory, including

Chilopoda Centipedes (from Neo-Latin , "hundred", and Latin language, Latin , "foot") are predatory arthropods belonging to the class Chilopoda (Ancient Greek , ''kheilos'', "lip", and Neo-Latin suffix , "foot", describing the forcipules) of the subphyl ...

Acari Mites are small arachnids (eight-legged arthropods) of two large orders, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari. However, most recent genetic analyses do not recover the two as ea ...

Opiliones The Opiliones (formerly Phalangida) are an Order (biology), order of arachnids, Common name, colloquially known as harvestmen, harvesters, harvest spiders, or daddy longlegs (see below). , over 6,650 species of harvestmen have been discovered w ...

, Chernetidae, and

spider Spiders (order (biology), order Araneae) are air-breathing arthropods that have eight limbs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude spider silk, silk. They are the largest order of arachnids and ran ...

s. Troglobitic insects include

Apterygota The name Apterygota is sometimes applied to a former Subclass (biology), subclass of small, agile insects, distinguished from other insects by their lack of wings in the present and in their evolutionary history; notable examples are the silverfis ...

such as Campodea and various

. There are many species of beetles in families such as

Carabidae Ground beetles are a large, cosmopolitan family of beetles, the Carabidae, with more than 40,000 species worldwide, around 2,000 of which are found in North America and 2,700 in Europe. As of 2015, it is one of the 10 most species-rich animal ...

Curculionidae The Curculionidae are a family of weevils, commonly called snout beetles or true weevils. They are one of the largest animal families with 6,800 genera and 83,000 species described worldwide. They are the sister group to the family Brentidae. Th ...

, Leiodidae, and

Silphidae Silphidae is a family of beetles that are known commonly as large carrion beetles, carrion beetles or burying beetles. There are two family (biology), subfamilies: Silphinae and Nicrophorinae. Members of Nicrophorinae are sometimes known as bury ...

. Some

are troglobitic and some are trogloxenic. The order

includes troglobites, and so do the

Trichoptera The caddisflies (order Trichoptera) are a group of insects with aquatic larvae and terrestrial adults. There are approximately 14,500 described species, most of which can be divided into the suborders Integripalpia and Annulipalpia on the basis ...

and

Diptera Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advance ...

Categorization of cave dwellers

The cave dwellers fall under one of the following categories: *

Troglobite A troglobite (or, formally, troglobiont) is an animal species, or population of a species, strictly bound to underground habitats, such as caves. These are separate from species that mainly live in above-ground habitats but are also able to live u ...

species are true cave dwellers, occurring exclusively in caves and unable to survive in the open.
True troglobites among insects include many

Coleoptera Beetles are insects that form the Taxonomic rank, order Coleoptera (), in the superorder Holometabola. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 40 ...

, some Stenopelmatidae,

, and

Zygentoma Zygentoma are an order in the class Insecta, and consist of about 550 known species. The Zygentoma include the so-called silverfish or fishmoths, and the firebrats. A conspicuous feature of the order are the three long caudal filaments. The t ...

. * Troglophile species sometimes occur outside the cave habitat but typically complete their life cycle in caves. *

Trogloxene Trogloxenes or subtroglophiles, also called cave guests, are animal species which periodically live in underground habitats such as caves or at the very entrance, but cannot live exclusively in such habitats. Among many scientists, trogloxenes and ...

cannot live permanently in caves, but may enter caves and spend parts of their life cycles in caves.
Unusual South African

Tettigoniidae Insects in the family (biology), family Tettigoniidae are commonly called katydids (especially in North America) or bush crickets. They have previously been known as "long-horned grasshoppers". More than 8,000 species are known. Part of the subo ...

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 Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...

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 Nutrition is the biochemistry, biochemical and physiology, physiological process by which an organism uses food and water to support its life. The intake of these substances provides organisms with nutrients (divided into Macronutrient, macro- ...

. Trogloxenes are important to cave ecology, because they commonly feed outside and import material that serves as food when they return. Insect species such as some

butterflies Butterflies are winged insects from the lepidopteran superfamily Papilionoidea, characterized by large, often brightly coloured wings that often fold together when at rest, and a conspicuous, fluttering flight. The oldest butterfly fossi ...

flies Flies are insects of the Order (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwin ...

, and beetles over-winter in caves, and casualties remain as food. Cavernicolous

bat Bats are flying mammals of the order Chiroptera (). With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out ...

s, being trogloxenes, are major ecological factors in some caves where they spend their daylight hours, and some species such as Mexican free-tailed bats provide massive deposits of

organic matter Organic matter, organic material or natural organic matter is the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come fro ...

, mainly in the form of

faeces Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the ...

and carcasses. Other trogloxenic animals include vertebrates such as

bear Bears are carnivoran mammals of the family (biology), family Ursidae (). They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats ...

hyena Hyenas or hyaenas ( ; from Ancient Greek , ) are feliform carnivoran mammals belonging to the family Hyaenidae (). With just four extant species (each in its own genus), it is the fifth-smallest family in the order Carnivora and one of the sma ...

s, other predators, reptiles, oilbirds, cave swiftlets and even humans, that enter for short term shelter or for hibernation. Most of them contribute organic matter rather than consuming it, and are important resources for troglobitic insects, many of which actually specialise in reliance on particular species that are long-term regular visitors. The cave environment thus is characterised by absence, restriction, or attenuation of certain factors such as light,

circadian A circadian rhythm (), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., endogenous) and responds to the environment (is entrai ...

or seasonal stimuli, living space, freedom of movement, or abrupt contrasts in temperature and humidity. Other items may depend on local conditions; for example, most caves provide little available food and some provide little water, whereas some provide perennial water or quantities of dung so great as to support ecological stratification, with organisms preying on other organisms that live in turn on different stages of the original product. The ultimate sources of nearly all food in caves are outside the cave. Running water and air currents carry in carcasses and other organic

detritus In biology, detritus ( or ) is organic matter made up of the decomposition, decomposing remains of organisms and plants, and also of feces. Detritus usually hosts communities of microorganisms that colonize and decomposition, decompose (Reminera ...

Fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

and bacteria that develop on this material provide food for many cave dwellers. Bat

guano Guano (Spanish from ) is the accumulated excrement of seabirds or bats. Guano is a highly effective fertiliser due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant growth. Guano was also, to a le ...

represents another source.

Lepidoptera Lepidoptera ( ) or lepidopterans is an order (biology), order of winged insects which includes butterflies and moths. About 180,000 species of the Lepidoptera have been described, representing 10% of the total described species of living organ ...

that enter caves for sleeping are preyed upon by troglobitic

, largely

and

Gryllidae The family Gryllidae contains the subfamilies and genera which entomologists now term true crickets. Having long, whip-like antennae, they belong to the Orthopteran suborder Ensifera, which has been greatly reduced in the last 100 years (''e.g ...

. The cavernicolous Collembola feed on

colloidal A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exten ...

matter in the water or dust borne on the

surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension (physics), tension is what allows objects with a higher density than water such as razor blades and insects (e.g. Ge ...

. The insects and similarly sized invertebrates are food for spiders and

Myriapoda Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial. Although molecular evidence and similar fossils suggests a diversifi ...

. Most such activities go on in darkness, except close to the outside, or where certain microbes or insects such as

Arachnocampa ''Arachnocampa'' is a genus of nine fungus gnat species which have a bioluminescent larval stage, akin to the larval stage of glowworm beetles. The species of ''Arachnocampa'' are endemic to Australia and New Zealand, dwelling in caves and grotto ...

provide

bioluminescence Bioluminescence is the emission of light during a chemiluminescence reaction by living organisms. Bioluminescence occurs in multifarious organisms ranging from marine vertebrates and invertebrates, as well as in some Fungus, fungi, microorgani ...

, even if only to attract prey.

Evolutionary characteristics

In individual caves, the most conspicuous and perhaps most ubiquitous peculiarity of insects, as with other troglobites, is the reduction of body pigmentation. This does not apply to all cavernicolous insects. It is particularly marked in

. The reduction or total loss of body pigmentation is correlated with the absence of sunlight. A second peculiarity is the reduction of eyes in all cavernicolous species. This contrasts with most

nocturnal Nocturnality is a ethology, behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnality, diurnal meaning the opposite. Nocturnal creatur ...

crepuscular In zoology, a crepuscular animal is one that is active primarily during the twilight period, being matutinal (active during dawn), vespertine (biology), vespertine/vespertinal (active during dusk), or both. This is distinguished from diurnalit ...

species, in many of which there is a strong tendency to adapt to low light levels by responding to selection for large, highly sensitive eyes. A form of adaptation common to many cave insects as well as some external predatory species, is elongation of

appendage An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

s, especially the antennae, palps and forelegs that assist in precise location of prey before striking. Many also bear elongated sensory organs, typically

setae In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae ...

, as for example, in the beetle ''Scotoplanetes arenstorffianus'', in which there are well developed setae, including on the elytra and also supraorbitally, but the eyes themselves are absent. from

Herzegovina Herzegovina ( or ; sh-Latn-Cyrl, Hercegovina, separator=" / ", Херцеговина, ) is the southern and smaller of two main geographical Regions of Bosnia and Herzegovina, regions of Bosnia and Herzegovina, the other being Bosnia (reg ...

. In contrast, none of the free-living related carabids have such sensory setae on the elytra. Troglobitic insects commonly have no functional wings, and many have no wings at all. One exception is Troglocladius hajdi (family

Chironomidae Chironomidae , commonly known as non-biting midges or chironomids , are a family of Nematoceran flies with a global distribution. They are closely related to the families Ceratopogonidae, Simuliidae, and Thaumaleidae. Although many chironomid ...

), which has strongly reduced eyes consisting of only 0–4 ommatidia, yet have well-developed wings and is able to fly in total darkness. Among the cave beetles the elytra may be retained as sclerotised bodily protection, but the hind wings, that are used for flight in most beetles, are non-functional or absent.

Geographical locations

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 ''Leptodirus'' is a cave beetle in the family Leiodidae. The genus contains only the single species ''Leptodirus hochenwartii''. It is a true troglobite, endemic to Slovenian, Croatian and, partly, Italy, Italian caves. Biology and ecology '' ...

'', found in the Postojna cave system in

Slovenia Slovenia, officially the Republic of Slovenia, is a country in Central Europe. It borders Italy to the west, Austria to the north, Hungary to the northeast, Croatia to the south and southeast, and a short (46.6 km) coastline within the Adriati ...

, was the first animal to be recognized as a true cave dweller. The cave insects found in the

Atlas Mountains The Atlas Mountains are a mountain range in the Maghreb in North Africa. They separate the Sahara Desert from the Mediterranean Sea and the Atlantic Ocean; the name "Atlantic" is derived from the mountain range, which stretches around through M ...

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 Kentucky (, ), officially the Commonwealth of Kentucky, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Illinois, Indiana, and Ohio to the north, West Virginia to the ...

. The American stenopelmatid '' Hadenoecus subterraneus'' is recorded from Kentucky caves. The remarkable carabid '' Comstockia subterranea'' is a true cave species found in

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

. The exclusively cave-dwelling silphid '' Adelops hirtus'' occurs in Kentucky caves and has very minute, unpigmented, atrophied eyes. The

Hawaiian Islands The Hawaiian Islands () are an archipelago of eight major volcanic islands, several atolls, and numerous smaller islets in the Pacific Ocean, North Pacific Ocean, extending some from the Hawaii (island), island of Hawaii in the south to nort ...

have a significant number of endemic cave insects that inhabit

lava tube A lava tube, more rarely called a pyroduct, is a 'roofed conduit through which molten lava travels away from its vent'. If lava in the tube drains out, it will leave an empty cave. Lava tubes are common in low-viscosity volcanic systems. La ...

s, including several blind planthoppers that feed on ʻōhiʻa roots ('' Oliarus polyphemus'', '' Oliarus priola'', '' Oliarus kalaupapae''), a lava tube water treader ('' Cavaticovelia aaa''), and several species of '' Caconemobius'' cave crickets.

Introduction

Categorization of cave dwellers

The cave environment

Evolutionary characteristics

Geographical locations

References

Further reading