Micromalthus Debilis
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The telephone-pole beetle (''Micromalthus debilis'') is a
beetle 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 ...
native to the eastern
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
and the only known living representative of the otherwise extinct
family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
Micromalthidae.
Larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
e of the beetle live in decaying wood and can be pests to wooden structures, lending them their common name, the 'telephone-pole beetle.' The
larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
e of ''Micromalthus debilis'' start as tiny white creatures with well-developed legs, resembling
carabid 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 fa ...
larvae. Larvae bore into moist, decaying chestnut and oak logs, creating galleries as they consume wood fibers. Adult beetles are dark brown to blackish with
vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...
reproductive organs. Mating behavior includes sex-role reversal, with females exhibiting more aggression and competition for mates. ''Micromalthus''s evolutionary history dates back millions of years, with fossil records found in various
amber Amber is fossilized tree resin. Examples of it have been appreciated for its color and natural beauty since the Neolithic times, and worked as a gemstone since antiquity."Amber" (2004). In Maxine N. Lurie and Marc Mappen (eds.) ''Encyclopedia ...
s. Their larvae infest timber, weakening structures and attracting fungi, as seen in South African gold mines in the 1930s. The telephone-pole beetle used to have reproducing adults, but has evolved to become obligately paedogenetic. The rare adult is sometimes referred to as a 'ghost adult' due to its vestigial existence. They have an unusual reproductive system involving asexually reproducing (
parthenogenetic Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...
) female larvae. Being one of few parthenogenetic
haplodiploid Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid. Haplodiploidy is sometimes called arrhenotoky. Haplodiploidy determines the se ...
species, the telephone-pole beetle is an interesting subject of
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 ...
n reproductive behavior and physiology.


Distribution

Reports of the species are infrequent and it is unknown whether they are rare, or common and unrecognized. A recent study by Bertone ''et al.'' (2016) found telephone-pole beetles in a survey of the indoor arthropod fauna in 50 houses located in and around
Raleigh, North Carolina Raleigh ( ) is the List of capitals in the United States, capital city of the U.S. state of North Carolina. It is the List of municipalities in North Carolina, second-most populous city in the state (after Charlotte, North Carolina, Charlotte) ...
. A recent survey found that the species had spread to every continent except Australia. With finds in South Africa, Hong Kong, Belize, Cuba, Brazil, Japan, Hawaii, Italy and Austria, the dispersal is likely connected to the timber trade.


Taxonomy

Classification of ''M. debilis'' was historically controversial and unsettled. The species, first reported by
John Lawrence LeConte John Lawrence LeConte MD (May 13, 1825 – November 15, 1883) was an American entomology, entomologist, responsible for naming and describing approximately half of the insect taxon, taxa known in the United States during his lifetime,
in 1878, was long considered one of the
Polyphaga Polyphaga is the largest and most diverse suborder of beetles. It comprises 144 families in 16 superfamilies, and displays an enormous variety of specialization and adaptation, with over 350,000 described species, or approximately 90% of the b ...
, and placed in the Lymexylidae or Telegeusidae, or as a family within the Cantharoidea. However, characteristics of larvae, wings, and male genitalia show that it is in the suborder
Archostemata The Archostemata are the smallest suborder of beetles, consisting of 50 living species in five families and over 200 described fossil species. They are an ancient lineage with a number of primitive characteristics. Antenna (biology), Antennae may ...
, where it has been placed since 1999.


Description


Larvae

''Female-producing ( thelytokous) female larvae'' The thelytokous female larva resembles a
carabid 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 fa ...
larva when first hatched. The first
instar An instar (, from the Latin '' īnstar'' 'form, likeness') is a developmental stage of arthropods, such as insects, which occurs between each moult (''ecdysis'') until sexual maturity is reached. Arthropods must shed the exoskeleton in order to ...
larva is the smallest in length, measuring about 1.26mm. The body is white, distinct lateral bulges are present, and the legs are well-developed. The second instar and subsequent instars have similar morphologies, with the head and body size and width increasing with each subsequent instar. The larvae measure between 2.7 and 3.3 millimeters in length. The head displays a light brown hue and notable
sclerotization Sclerotization is a biochemical process that produces the rigid shell of sclerotin that comprises an insect's chitinous exoskeleton. It is prominent in the thicker, armored parts of insects and arachnid Arachnids are arthropods in the Class (b ...
. In almost all segments, the
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
covering the
thorax The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main di ...
and abdomen lacks pigmentation. The body is characterized by its slender, elongated form, maintaining parallel sides and cylindrical shape, featuring dorsal and ventral ampullae along with lateral bulges on numerous segments. The thorax is shorter than the combined length of abdominal segments I to III. In later instars, the body is slightly flatter, broader, and shorter. Legs are not present in second and older instars which is considered a trait unique to ''M. debilis''. ''Male-producing ( arrhenotokous) female larvae'' The penultimate instar of the male-producing larva is vaguely described as having a dense white color due to much fat. The body is cylindrically shaped and marked by segmented constrictions. ''Male larvae'' Unlike female larvae, which resemble the carabid type when first hatched, male larvae resemble that of a
weevil Weevils are beetles belonging to the superfamily Curculionoidea, known for their elongated snouts. They are usually small – less than in length – and herbivorous. Approximately 97,000 species of weevils are known. They belong to several fa ...
. The body is short and stubby with stump-like legs.


Adults

The adult beetle is elongated and a dark brown to blackish color, with brownish-yellow legs and antennae. The head is larger than the
thorax The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main di ...
, with large eyes protruding from either side. According to H.S. Barber, the adult female is responsible for


Feeding

The
larvae A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect developmental biology, development such as insects, some arachnids, amphibians, or cnidarians typical ...
are wood-borers that feed on moist and decaying
chestnut The chestnuts are the deciduous trees and shrubs in the genus ''Castanea'', in the beech family Fagaceae. The name also refers to the edible nuts they produce. They are native to temperate regions of the Northern Hemisphere. Description ...
and
oak An oak is a hardwood tree or shrub in the genus ''Quercus'' of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisp ...
logs. Telephone-pole beetle larvae infest timber by burrowing into the wood, where they feed and develop. They tunnel through the timber, creating galleries as they consume the wood fibers.


Life cycle

The life cycle of ''M. debilis'' is one of the most complicated life cycles among beetles because it involves
viviparity In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juv ...
,
hypermetamorphosis Hypermetamorphosis, or heteromorphosis, is a term used mainly in entomology; it refers to a class of variants of holometabolism, that is to say, complete insect metamorphosis. Hypermetamorphosis is exceptional in that some instars, usually larva ...
,
parthenogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...
, and paedogenesis. The life cycle is also highly unusual because the larvae rarely mature into adults in both sexes. Haplodiploidy is another notable feature of this species; haploid males are hatched from eggs within the mother's body via haploid parthenogenesis, and diploid females are birthed live via diploid parthenogenesis.


Females

Upon being birthed live, the larva emerges as a minute white creature with long, slender legs that resemble the carabid type. This stage is primarily focused on migration, with the young larvae crawling away from the location of their birth to find new areas within their habitat. During this phase, they may consume the remains of their mother before migration. After a period of wandering, the larvae begin to burrow into wood or other suitable substrates again. They feed minimally during this time and undergo their first molt, transforming into a legless form that resembles the larva of cerambycid beetles. This stage is characterized by the development of an inconspicuous anal armature. Additional molts occur in this form, likely to accommodate head growth and overall body development. The larvae continue to bore through wood, packing their galleries with dust as they progress. The color of their bodies may darken due to the accumulation of food in their
alimentary tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...
. As the larvae near maturity, the eggs in the ovaries of what will become the paedogenetic form become visible. At this stage, the larvae reverse their position in the gallery, construct a cell, and enter an
aestivation Aestivation ( (summer); also spelled estivation in American English) is a state of animal dormancy, similar to hibernation, although taking place in the summer rather than the winter. Aestivation is characterized by inactivity and a lowered m ...
phase. During aestivation, their bodies gradually turn white as they consume all available food in their system. Depending on circumstances, the larvae may either pupate (though this is described as rare) or undergo another molt, ultimately revealing the paedogenetic form, in which they can produce young. Paedogenesis is the process by which larvae reproduce by giving birth to more larvae without the production of adults and is a process exclusive to females. Once the paedogenetic form emerges, it typically takes around two weeks for the new generation to be born. The young larvae are born tail-first and begin the cycle anew, continuing the species' life cycle.


Males

Male larvae are hatched from a single, large egg that adheres to the male-producing larva's body for 8–10 days. The larvae feed on the mother's body and will grow rapidly. The male rarely
pupa A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...
tes and transforms into the
imago In biology, the imago (Latin for "image") is the last stage an insect attains during its metamorphosis, its process of growth and development; it is also called the ''imaginal'' stage ("imaginal" being "imago" in adjective form), the stage in wh ...
, and will often die as a larva. While male adults only develop from uni-oviparous paedogenetic females, adult females only develop from cerambycoid paedogenetic females.


Larval sex ratio

In naturally occurring paedogenetic larvae, the sex ratio is strongly biased toward females. None of the three canonical explanations for biased sex ratios, local mate competition, local resource competition, and local resource enhancement, are likely explanations for the biased sex ratio in telephone-pole beetle larvae. Local mate competition selects for female-biased sex ratios when male siblings compete to fertilize their female siblings, but this is unlikely in this species which females tend to avoid mating with siblings. Local resource competition selects for biased sex ratios but typically involves competition between females for resources and thus selects for male-biased ratios. Lastly, local resource enhancement can select for biased sex ratios if the offspring of one sex increases the fitness of parents. However, because female offspring feed on the mother, there is more likely competition between female larvae, contradicting this explanation. As such, the cause of sex ratio deviation remains unclear.


Adulthood

Adult telephone-pole beetles are unable to copulate, and adult females do not have the physiological mechanisms to reproduce because they are unable to lay eggs or produce live progeny, either sexually or by parthenogenesis. Since adults do not have a role in reproduction, they are not a physiological part of the life cycle. Thus, the rarity of adult development in the natural world may be an evolutionary response to the lack of their reproductive role. In laboratory settings, development into adults can be induced by high temperatures, but this also results in high mortality because only one out of hundreds of heat-treated larvae will survive and pupate into an adult. The adult females live for about six days and males only live for around 12 hours, with a strongly biased
sex ratio A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. However, many species deviate from an even sex ratio, ei ...
towards females. The adults of both sexes are sterile and are
vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...
remnants of a time when the life cycle involved sexual reproduction. The loss of sexual reproduction is likely associated with its infection by ''
Wolbachia ''Wolbachia'' is a genus of gram-negative bacteria infecting many species of arthropods and filarial nematodes. The symbiotic relationship ranges from parasitism to obligate mutualism. It is one of the most common parasitic microbes of arthrop ...
'' bacteria.


Literature

Limited observations and experiments on ''M. debilis'' have resulted in conflicting observational conclusions, particularly regarding the beetle's reproduction, in the existing literature. Pollock & Normack reported the existence of reproductive adult males, but this was based on the incorrect conclusions by Barber. However, all existing experimental literature states that adults are sterile.


Mating behavior

A 2016 experimental study used heating to generate substantial numbers of adults to simulate the now non-functional adult reproductive behavior. Due to parthenogenetic reproduction in telephone-pole beetles, information on the sexual mating system of this beetle was previously lost, but recent research is working to uncover this information. Both adult female and adult male modern telephone-pole beetles are sterile, but they still exhibit mating behavior.


Sex role reversal

The study revealed sex-role reversal, meaning that females face more competition for mates compared to males. This was demonstrated by greater initiative by females to mate and increased fighting between females in the presence of unrelated males. Female also display more aggressive mating behavior, as they may grasp the male genitalia with their own genitalia.


Rejection behavior

Adult female beetles exhibit rejection behavior to avoid inbreeding with related males, which are adult male beetles that are located on the same natal log. Right after pupating, males expose their reproductive organs as a mate-seeking gesture. Despite tending to compete for male mates, females will ignore these males because they originate from the same log. The males would need to take a short flight to a neighboring log for female mates. Females also perform a 'kin dance' involving shaking of their abdomens and beating of their wing, which is thought to be a deterring signal to related males. Such rejection behaviors demonstrate that sex roles are not fixed because females can also display choosy behavior. The rejection behavior also supports that the female-biased sex ratio is not due to local mate competition.


Mounting

Females initiated more interactions by actively mounting males, further supporting that female compete for male mates. In cases where several females pile on top of a male, a female may try to dislodge the others with her mandible. Female-female mounting can also occur, and the frequency of this does not change depending on the presence or absence of males.


Evolutionary history

Genetic studies have placed ''Micromalthus'' as more closely related to
Ommatidae The Ommatidae are a family of beetles in the suborder Archostemata. The Ommatidae are considered the extant beetle family that has most ancestral characteristics. There are only seven extant species, confined to Australia and South America. Howev ...
than to
Cupedidae The Cupedidae are a small family of beetles, notable for the square pattern of "windows" on their elytra (hard forewings), which give the family their common name of reticulated beetles. The family consists of about 30 species in 9 genera, with ...
within
Archostemata The Archostemata are the smallest suborder of beetles, consisting of 50 living species in five families and over 200 described fossil species. They are an ancient lineage with a number of primitive characteristics. Antenna (biology), Antennae may ...
. A close relationship between Ommatidae and Micromalthidae is supported by several morphological characters, including those of the mandibles and male genitalia. A close relationship to the enigmatic family Crowsoniellidae has been suggested by some sources. The oldest record of Micromalthidae is '' Archaeomalthus'' from the
Upper Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the sixth and last period o ...
of Russia around 252 million years old, which is morphologically similar in many respects to ''Micromalthus'' including an only weakly sclerotised body. Several other fossil genera of the family are known including '' Cretomalthus'', known from a larva found in Early Cretaceous (
Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 125.77 Ma (million years ago) and 121.4 ± 1.0 Ma (Historically, this stage was placed at 129.4 million to approximately 125 million years ago) It is a ...
)
Lebanese amber Lebanese amber is fossilized resin found in Lebanon and its surroundings. It dates back approximately 130-125 million years to the Barremian of the Early Cretaceous. It formed on what was then the northern coast of Gondwana, believed to be a tropic ...
, as well as '' Protomalthus'' from the mid-Cretaceous (
Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...
-
Cenomanian The Cenomanian is, in the International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...
)
Burmese amber Burmese amber, also known as Burmite or Kachin amber, is amber from the Hukawng Valley in northern Myanmar. The amber is dated to around 100 million years ago, during the latest Albian to earliest Cenomanian ages of the mid-Cretaceous period. Th ...
of Myanmar. Fossils of ''Micromalthus'' are known from the
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
aged
Dominican amber Dominican amber is amber from the Dominican Republic derived from resin of the extinct tree '' Hymenaea protera''. Dominican amber differentiates itself from Baltic amber by being nearly always transparent, and it has a higher number of fossil in ...
(adults and larvae, which were found to not be distinguishable from the living species ) and Mexican amber (larvae), the late
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
aged
Rovno amber Rovno amber, occasionally called Ukrainian amber, is amber found in the Rivne Oblast and surrounding regions of Ukraine and Belarus. The amber is dated between Late Eocene and Early Oligocene, and suggested to be contemporaneous to Baltic amber ...
of Ukraine (''Micromalthus priabonicus''), and the early Eocene (
Ypresian In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...
) aged
Oise amber Oise amber () is a type of amber found near the Oise river near Creil in northern France. Oise amber is around 53 million years old, dating to the Early Eocene (Ypresian). Oise amber is softer than Baltic amber, although Oise amber is older ...
of France (''Micromalthus eocenicus''). A possible specimen of ''Micromalthus'' is known from Burmese amber, but the poor preservation of the specimen makes the assignment tentative.


Wood pests

Telephone-pole beetle larvae infest timber by burrowing into the wood, where they feed and develop. They tunnel through the timber, creating galleries as they consume the wood fibers. This activity weakens the structural integrity of the timber, leading to decay and potential collapse. Additionally, their presence can attract fungi, further contributing to the degradation of the timber. In the 1930s, telephone-pole beetle larvae were reported as the perpetrators of a gold mine infestation in the
Witwatersrand Basin The Witwatersrand (, ; ; locally the Rand or, less commonly, the Reef) is a , north-facing scarp in South Africa. It consists of a hard, erosion-resistant quartzite metamorphic rock, over which several north-flowing rivers form waterfalls, wh ...
in South Africa. The primary timbers employed in the mines, including ''
Acacia ''Acacia'', commonly known as wattles or acacias, is a genus of about of shrubs and trees in the subfamily Mimosoideae of the pea family Fabaceae. Initially, it comprised a group of plant species native to Africa, South America, and Austral ...
'', ''
Eucalyptus ''Eucalyptus'' () is a genus of more than 700 species of flowering plants in the family Myrtaceae. Most species of ''Eucalyptus'' are trees, often Mallee (habit), mallees, and a few are shrubs. Along with several other genera in the tribe Eucalyp ...
'', and ''
Pinus A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. ''World Flora Online'' accepts 134 species-rank taxa (119 species and 15 nothospecies) of pines as c ...
'' species, are all susceptible to infestation, particularly in conditions with ample moisture. Heavy infestations were noted in aged timbers within poorly ventilated shafts, especially in environments with temperatures ranging from 88 to 93 °F. The presence of stagnant or running water may cause even further decay, sometimes resulting in complete pulverization.


References


External links

* *
Tree of Life page, with pictures and video

The life cycle of Micromalthus debilis
{{Authority control Archostemata Beetles of North America Beetles described in 1878 Taxa named by John Lawrence LeConte