Cave-dwelling insects are among the most widespread and prominent
troglofauna
Troglofauna are small cave-dwelling animals that have adapted to their dark surroundings. Troglofauna and stygofauna are the two types of subterranean fauna (based on life-history). Both are associated with subterranean environments – troglofa ...
(
cave
A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...
-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,
troglophile
Troglofauna are small cave-dwelling animals that have adapted to their dark surroundings. Troglofauna and stygofauna are the two types of subterranean fauna (based on life-history). Both are associated with subterranean environments – troglofau ...
s, 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."Cave Plants and Animals". 9 December Am ...
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, the term habitat summarises the array of resources, physical and 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 ...
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 () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
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,
Collembola
Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects (the other two are the Protura and Diplura). Although the three orders are sometimes grouped together in a class called Ento ...
n, or
Blattodea
Blattodea is an order of insects that contains cockroaches and termites. Formerly, termites were considered a separate order, Isoptera, but genetic and molecular evidence suggests they evolved from within the cockroach lineage, cladistically ...
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 is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...
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 grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
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
planarian
A planarian is one of the many flatworms of the traditional class Turbellaria. It usually describes free-living flatworms of the order Tricladida (triclads), although this common name is also used for a wide number of free-living platyhelmint ...
s,
Oligochaeta
Oligochaeta () is a subclass of 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 megadrile earthworm ...
,
Polychaeta
Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are m ...
,
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 bo ...
,
Mollusca, and
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
es. Troglobitic
Crustaceans include species of
Amphipoda,
Cladocera
The Diplostraca or Cladocera, commonly known as water fleas, are a superorder of small crustaceans that feed on microscopic chunks of organic matter (excluding some predatory forms).
Over 1000 species have been recognised so far, with many more ...
,
Copepoda
Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have p ...
,
Decapoda,
Isopoda, and
Syncarida
Syncarida is a superorder of crustaceans, comprising the two extant orders Anaspidacea and Bathynellacea, and the extinct order Palaeocaridacea. Fifty-nine living genera are known, in six families:
;Anaspidacea Calman, 1904
*Anaspididae Thomson ...
. Many troglobites are predatory, including
Chilopoda
Centipedes (from New Latin , "hundred", and Latin , "foot") are predatory arthropods belonging to the class Chilopoda ( Ancient Greek , ''kheilos'', lip, and New Latin suffix , "foot", describing the forcipules) of the subphylum Myriapoda, a ...
,
Acari
Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear evid ...
,
Opiliones
The Opiliones (formerly Phalangida) are an order of arachnids colloquially known as harvestmen, harvesters, harvest spiders, or daddy longlegs. , over 6,650 species of harvestmen have been discovered worldwide, although the total number of ext ...
,
Chernetidae
The Chernetidae are a family of pseudoscorpion
Pseudoscorpions, also known as false scorpions or book scorpions, are small, scorpion-like arachnids belonging to the order Pseudoscorpiones, also known as Pseudoscorpionida or Chelonethida.
Pseu ...
, and
spider
Spiders ( order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species ...
s.
Troglobitic insects include
Apterygota
The name Apterygota is sometimes applied to a 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 silverfish, the firebrat, and the ...
such as
Campodea and various
Collembola
Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects (the other two are the Protura and Diplura). Although the three orders are sometimes grouped together in a class called Ento ...
. 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 fami ...
,
Curculionidae,
Leiodidae
Leiodidae is a family of beetles with around 3800 described species found worldwide. Members of this family are commonly called round fungus beetles due to the globular shape of many species, although some are more elongated in shape. They are g ...
, and
Silphidae
Silphidae is a family of beetles that are known commonly as large carrion beetles, carrion beetles or burying beetles. There are two subfamilies: Silphinae and Nicrophorinae. Nicrophorines are sometimes known as sexton beetles. The number of ...
. Some
Orthoptera are troglobitic and some are trogloxenic. The order
Blattodea
Blattodea is an order of insects that contains cockroaches and termites. Formerly, termites were considered a separate order, Isoptera, but genetic and molecular evidence suggests they evolved from within the cockroach lineage, cladistically ...
includes troglobites, and so do the
Trichoptera
The caddisflies, or 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 b ...
and
Diptera.
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, some
Stenopelmatidae,
Diptera, 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
Troglofauna are small cave-dwelling animals that have adapted to their dark surroundings. Troglofauna and stygofauna are the two types of subterranean fauna (based on life-history). Both are associated with subterranean environments – troglofau ...
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."Cave Plants and Animals". 9 December Am ...
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 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 suborder Ensifera, ...
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, dew, or fog to be present.
Humidity dep ...
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 biochemical and physiological process by which an organism uses food to support its life. It provides organisms with nutrients, which can be metabolized to create energy and chemical structures. Failure to obtain sufficient ...
.
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,
flies
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 advanced m ...
, and beetles over-winter in caves, and casualties remain as food. Cavernicolous
bat
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". 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 ...
s, being trogloxenes, are major ecological factors in some caves where they spend their daylight hours, and some species such as
Mexican free-tailed bat
The Mexican free-tailed bat or Brazilian free-tailed bat (''Tadarida brasiliensis'') is a medium-sized bat native to the Americas, so named because its tail can be almost half its total length and is not attached to its uropatagium. It has been ...
s provide massive deposits of
organic matter, mainly in the form of
faeces
Feces ( or faeces), known colloquially and in slang as poo and poop, 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 large intestine. Feces contain a relati ...
and
carcasses.
Other trogloxenic animals include vertebrates such as
bear
Bears are carnivoran mammals of the 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 throughout the Nor ...
s,
hyenas, other predators, reptiles,
oilbird
The oilbird (''Steatornis caripensis''), locally known as the , is a bird species found in the northern areas of South America including the Caribbean island of Trinidad. It is the only species in the genus ''Steatornis'', the family Steatornith ...
s,
cave swiftlet
The cave swiftlet or linchi swiftlet (''Collocalia linchi'') is a species of swift in the family Apodidae. It is found on the Indonesia islands of Sumatra, Java and Bali. It is a woodland species and nests in caves. The Bornean swiftlet was c ...
s 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 or seasonal
stimuli
A stimulus is something that causes a physiological response. It may refer to:
* Stimulation
** Stimulus (physiology), something external that influences an activity
** Stimulus (psychology), a concept in behaviorism and perception
* Stimulus (eco ...
, 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.
Fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
and bacteria that develop on this material provide food for many cave dwellers. Bat
guano represents another source.
Lepidoptera that enter caves for sleeping are preyed upon by troglobitic
Orthoptera, largely
Tettigoniidae
Insects in the 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 suborder Ensifera, ...
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 ...
. 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 extend ...
matter in the water or dust borne on the
surface tension. 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.
The fossil record of myriapods reaches back into the late Silurian ...
. 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 grot ...
provide
bioluminescence
Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...
, 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
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compoun ...
. This does not apply to all cavernicolous insects. It is particularly marked in
Coleoptera. 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 or
crepuscular 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
appendages, 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 (singular seta ; from the Latin word for " bristle") are any of a number of different bristle- or hair-like structures on living organisms.
Animal setae
Protostomes
Annelid setae are stiff bristles present on the body. ...
, as for example, in the beetle ''Scotoplanetes arenstorffianus'', in which there are well developed setae, including on the
elytra
An elytron (; ; , ) is a modified, hardened forewing of beetles (Coleoptera), though a few of the true bugs (Hemiptera) such as the family Schizopteridae are extremely similar; in true bugs, the forewings are called hemelytra (sometimes alterna ...
and also supraorbitally, but the eyes themselves are absent. from
Herzegovina. In contrast, none of the free-living related carabids have such sensory setae on the
elytra
An elytron (; ; , ) is a modified, hardened forewing of beetles (Coleoptera), though a few of the true bugs (Hemiptera) such as the family Schizopteridae are extremely similar; in true bugs, the forewings are called hemelytra (sometimes alterna ...
. Troglobitic insects commonly have no functional wings, and many have no wings at all. One exception is Troglocladius hajdi (family
Chironomidae
The Chironomidae (informally known as chironomids, nonbiting midges, or lake flies) comprise a family of nematoceran flies with a global distribution. They are closely related to the Ceratopogonidae, Simuliidae, and Thaumaleidae. Many specie ...
), which has strongly reduced eyes consisting of only 0–4
ommatidia
The compound eyes of arthropods like insects, crustaceans and millipedes are composed of units called ommatidia (singular: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The ...
, 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, Italian caves.
Biology and ecology
''Leptodi ...
'', found in the
Postojna cave
Postojna Cave ( sl, Postojnska jama; german: Adelsberger Grotte; it, Grotte di Postumia) is a long karst cave system near Postojna, southwestern Slovenia. It is the second-longest cave system in the country (following the Migovec System) as wel ...
system in
Slovenia
Slovenia ( ; sl, Slovenija ), officially the Republic of Slovenia (Slovene: , abbr.: ''RS''), is a country in Central Europe. It is bordered by Italy to the west, Austria to the north, Hungary to the northeast, Croatia to the southeast, an ...
, 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
Kentucky ( , ), officially the Commonwealth of Kentucky, is a state in the Southeastern region of the United States and one of the states of the Upper South. It borders Illinois, Indiana, and Ohio to the north; West Virginia and Virginia to ...
. The American stenopelmatid ''Hadenoecus subterraneus'' is recorded from Kentucky caves. The remarkable carabid ''Comstockia subterranea'' is a true cave species found in
Texas
Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...
. The exclusively cave-dwelling silphid ''Adelops hirtus'' occurs in Kentucky caves and has very minute, unpigmented, atrophied eyes.
References
Further reading
*Chapman, R.N. 1931. ''Animal Ecology''. London and New York
*
*{{cite journal , last1 = Hubbard , first1 = H.G. , year = 1898 , title = Insect life in Florida caves , journal = Proc. Entomol. Soc. Wash. , volume = 4 , page = 394
*Mani, M.S., 1968. ''General Entomology'', Oxford & IBH Publishing Co. New Delhi, chp. XIII, pp 308–312