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''Leptothorax acervorum'' is a small brown to yellow
ant Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from vespoid wasp ancestors in the Cretaceous period. More than 13,800 of an estimated total of ...
in the
subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus Genus ( plural genera ) is a taxonomic rank used in the biological classifica ...
Myrmicinae Myrmicinae is a subfamily of ants, with about 140 extant genera; their distribution is cosmopolitan. The pupae lack cocoons. Some species retain a functional sting. The petioles of Myrmicinae consist of two nodes. The nests are permanent and ...
. It was first described by
Johan Christian Fabricius Johan Christian Fabricius (7 January 1745 – 3 March 1808) was a Danish zoologist, specialising in "Insecta", which at that time included all arthropods: insects, arachnids, crustaceans and others. He was a student of Carl Linnaeus, and is co ...
in 1793. ''L. acervorum'' is vastly distributed across the globe, most commonly found in the
coniferous forest Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All exta ...
s of Central, Western and Northern Europe. The morphology of ''L. acervorum'' is extremely similar to that of other ''
Leptothorax ''Leptothorax'' is a genus of small ants with mainly Holarctic distributions. The genus is notable for its widespread social parasitism, i.e. they are dependent on the help of workers from other ant species during a part or the whole of their li ...
'' ants. The difference arises in the two-toned appearance of ''L. acervorum'', with the head and
metasoma The metasoma is the posterior part of the body, or tagma, of arthropods whose body is composed of three parts, the other two being the prosoma and the mesosoma. In insects, it contains most of the digestive tract, respiratory system, and cir ...
being darker than the mesosoma segment of the body, and hair across its body. Following
Bergmann's rule Bergmann's rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer r ...
—unusually, for ectothermic animals—body size increases with latitude.


Taxonomy

''Leptothorax acervorum'' was first described by
Johan Christian Fabricius Johan Christian Fabricius (7 January 1745 – 3 March 1808) was a Danish zoologist, specialising in "Insecta", which at that time included all arthropods: insects, arachnids, crustaceans and others. He was a student of Carl Linnaeus, and is co ...
in 1793 in his publication ''Entomologia systematica emendata et aucta. Vol 2''. The ant belongs to the family of Formicidae, which include all organisms that contain a metapleural gland. Using DNA analysis, the divergence date estimated for clades within the Formicidae imply that most ant subfamilies originate in the late
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
period. The subfamilies would have diverged around the
Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...
period. This species of ant is usually found in mid to northern Europe, regions in North America such as Alaska and northern Canada and in Japan.Collingwood, C. A. "The Formicidae (Hymenoptera) of Fennoscandia and Denmark. (1979)." Fauna Entomologica Scandinavica 8 (1979): 1-174. Antbase.org. Web. 26 Sept. 2013. ''L. acervorum'' are small myrmicine ants with distinct propodeal spines and have three-segmented antennal clubs. Based on a taxonomy experiment performed by Dekoninck, the entire body of ''L. acervorum'' is light brown in color and is covered with erect hairs. The region on the head and the antennal club are slightly darker in colour. The thorax was described as being light brown in colour and having a rounded shoulder.


Morphology

''Leptothorax acervorum'' is a small red
ant Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from vespoid wasp ancestors in the Cretaceous period. More than 13,800 of an estimated total of ...
Similar to other ants, ''L. acercorum'' displays geniculate (elbowed)
antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...
e, metapleural glands, and a constriction of the second abdominal segment. The exoskeleton provides a protecting casing of the body, which can be divided into 3 segments: the head, mesosoma, and metasoma. The head contains eyes that detect acute movement, three small ocelli to detect light and polarization, and two mandibles. Attached to the head are two antennae. All six legs are attached to the mesosoma. The metasoma houses vital internal organs. "The pedicel of the metasoma is two-segmented," which is unique for the Subfamily Myrmicinae. The head and abdomen are dark, thereby giving the ant a two-toned appearance. Individual ants are small, with workers measuring around in length and
queens Queens is a borough of New York City, coextensive with Queens County, in the U.S. state of New York. Located on Long Island, it is the largest New York City borough by area. It is bordered by the borough of Brooklyn at the western tip of Long ...
being only 10% larger. Colonies are small compared to those of other ants—they have anywhere from a few dozen to a few hundred workers and one to several queens.


Workers

The workers have reddish to brownish yellow body colour with the head, antennal club and dorsal surface being darker. The petiole nodes and femora are frequently infuscate. They have a total of 11 segments in antennae. The head is longitudinally striated, and smooth and the average length is usually 3.7–4.5 mm.


Queen

The queen is similar in appearance to the worker. However, the colouring of the queen is a dark brown, sometimes almost completely black. The average length of the queen is between 3.8–4.8 mm.


Male

The male is brownish black in color and is robust and significantly larger than both the worker and the queen. It has an antenna with 12 segments with a very short scape. The average length is between 4.5–5 mm long.


Size variance

Bergmann's rule Bergmann's rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer r ...
establishes that among
endothermic In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. ...
animals of the same species, body size increases with
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north ...
. Studies have tested whether this rule also applies to
social insects Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping gen ...
. ''L. acervorum'' workers were counted in a sample of colonies from
Erlangen Erlangen (; East Franconian: ''Erlang'', Bavarian: ''Erlanga'') is a Middle Franconian city in Bavaria, Germany. It is the seat of the administrative district Erlangen-Höchstadt (former administrative district Erlangen), and with 116,062 inha ...
and
Karelia Karelia ( Karelian and fi, Karjala, ; rus, Каре́лия, links=y, r=Karélija, p=kɐˈrʲelʲɪjə, historically ''Korjela''; sv, Karelen), the land of the Karelian people, is an area in Northern Europe of historical significance fo ...
. The worker size was significantly larger in the Karelian population, with the average
thorax The thorax or chest is a part of the anatomy of humans, 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 divisions of the c ...
length being 1.15 mm ± 0.07 mm. The average thorax length from the Erlangen population was 1.08 ± 0.05 mm. As evidenced, the workers from Karelia were on average 10% larger than the workers from Erlangen. The results suggest that larger body sizes in ''L. acervorum'' from
boreal Boreal may refer to: Climatology and geography *Boreal (age), the first climatic phase of the Blytt-Sernander sequence of northern Europe, during the Holocene epoch *Boreal climate, a climate characterized by long winters and short, cool to mild ...
habitats might result from selection for increased fasting endurance. Larger workers had more fat than small workers, and would survive longer in colder environments. ''Leptothorax acervorum'' might extend their survival time in areas with long winters and unpredictable climate by storing more reserves. Thus, the body size of workers of this
holarctic The Holarctic realm is a biogeographic realm that comprises the majority of habitats found throughout the continents in the Northern Hemisphere. It corresponds to the floristic Boreal Kingdom. It includes both the Nearctic zoogeographical reg ...
ant increases with latitude.


Distribution


Habitat

''Leptothorax acervorum'' are commonly found in dry
coniferous Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All ext ...
forests, where they nest in small rotting branches, tree stumps, and under bark. However, colonies that inhabit the periphery of its range are patchily distributed. Patchy distribution is positively correlated with an increase in latitude because, in the case that a queen leaves its colony due to a resource deficit, there is a low possibility that it will find and thereby compete with another one. The ideal environment for this species consists of
temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout ...
or
subtropical The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° north a ...
biomes A biome () is a biogeographical unit consisting of a biological community that has formed in response to the physical environment in which they are found and a shared regional climate. Biomes may span more than one continent. Biome is a broader ...
, in which resources are readily available for survival and success of the colony.


Geographical range

''Leptothorax acervorum'' vastly populate Central, Western, and Northern Europe, ranging from central Spain and Italy (40° N) to the tundra/taiga ecotone habitats of northern Scandinavia and Siberia (40° N). This species typically lives in facultatively
polygynous Polygyny (; from Neoclassical Greek πολυγυνία (); ) is the most common and accepted form of polygamy around the world, entailing the marriage of a man with several women. Incidence Polygyny is more widespread in Africa than in any ...
colonies. They can, however, exist in
monogynous This is a glossary of terms used in the descriptions of ants. A B D E F G H M N O P Q R S T U W See also ...
colonies at the periphery of its geographic range. When this species is found at the margins, where resources for survival may not be as readily available, areas for colony development and nesting are less frequently found. For instance, according to Trettin et al., in the northern mountain ranges of Spain, colonies were found to be functionally monogynous; here, the survival of the colonies were presumed to be at risk, unlike those that preferably exist at “low-skew” population of Boreal Eurasia.”. Heinze et al. identified another relationship relating to the ant's geographical range. As the latitude of the colonies' expanded outward, the mean body size of each individual worker ant increased as well. The authors point out that ants living near the
Polar Circle A polar circle is a geographic term for a conditional circular line (arc) referring either to the Arctic Circle or the Antarctic Circle. These are two of the keynote circles of latitude (parallels). On Earth, the Arctic Circle is currently d ...
were 10% larger than those living in central Europe. They attribute this relationship to a "Bergmann's rule-like pattern" for the
ectothermic An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life ...
ant. Bergmann's rule states populations and species of larger size tend to be found in colder environments, while smaller organisms are found in warmer regions. In accordance with this principle, Heinze et al. suggest that larger body size in ''L. acervorum'' from boreal habitats could be a result of selection for increased fasting endurance. In other words, in colder environments, the ants evolved larger body size in response to the adaptation of increased fasting endurance under starvation conditions, or peripheral habitats with a lack of resources.


Ecology and behavior


Queen behavior

''Leptothorax acervorum'' is a model organism to investigate the social structure of multiple-queen colonies. ''Leptothorax acervorum'' is a facultatively
polygynous Polygyny (; from Neoclassical Greek πολυγυνία (); ) is the most common and accepted form of polygamy around the world, entailing the marriage of a man with several women. Incidence Polygyny is more widespread in Africa than in any ...
ant, meaning that colonies with one or more than one queen occur, and these colonies acquire extra queens by adoption—thus polygyny is secondary. Electrophoretic
allozyme Alloenzymes (or also called allozymes) are variant forms of an enzyme which differ structurally but not functionally from other allozymes coded for by different alleles at the same locus. These are opposed to isozymes, which are enzymes that pe ...
analysis showed that cohabiting queens are close relatives. This reinforces the assumption that the queens in L. acervorum colonies form mother-daughter-sister groups, which arise from adopting newly mated queens into their natal nests. Newly eclosed queens mate with unrelated males near the natal nest and then return to it, where they are readopted. Other queens disperse to mating aggregations, mate, and then leave the aggregations to establish new colonies elsewhere. Matings near the nest may occur because ''L. acervorum'' queens 'call' males through the use of
pheromones A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
.


Oophagy

An important behavior noticed in ''L. acervorum'' was the eating of reproductive eggs by queens. On average, approximately 69% of eggs eaten were intact. Also in observed colonies, the proportions of eggs eaten out of all eggs laid were 25%, 93%, 125% (i.e. more eggs were eaten than laid in that period) and 64%. This oophagy had a major impact on the colony's output of eggs. The queens appeared to exhibit no discrimination when targeting eggs. It was actually observed that one queen interrupted an egg-eating queen and removed the egg to eat it herself. Feeding rate is positively correlated with
fecundity Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...
. In the four colonies where intact eggs were eaten, one of the two most fecund queens was among the top two egg eaters. A ''L. acervorum'' queen eats eggs by picking up the egg with her mandibles and manipulating it against her mouthparts with her forefeet. She pierces the egg's membranous skin and laps the egg's fluid through the hole. When the contents of the egg are emptied, typically after a few minutes, the queen will then discard the remaining skin by either dropping it to the floor or placing it on the mouthparts of a
larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...
(which then eats the skin). A possible explanation for this phenomenon is reproductive competition between queens. However, the overall lack of egg defense and overt aggression seem to provide contrary evidence. It is possible that direct confrontation would increase risk of injury for the egg-laying queen, thereby making egg defense too costly.


Colony structure

Trivers and Hare (1976) proposed that the population-level sex-investment ratio equals the relatedness asymmetry, so there can be conflict between workers and queens over sex allocation. Thus, the prediction is that sex-investment ratios are 1:1 females:males if queens control sex allocation and 3:1 females:males if there is worker control. This is because the queen is equally related to her sons and daughters (r=0.5 in each case), so she should produce equal numbers of male and female reproductive offspring. However, because of
haplodiploidy 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 sex ...
, full sisters are more closely related to one another because half of their genome is always identical, and the other half has a 50% chance of being shared. Their total relatedness is 0.5+(0.5 x 0.5)=0.75. This means sisters would prefer to skew the population sex-investment ratio to 3:1 females:males. A female is related to her brother by only 0.25, because 50% of her genes that come from her father have no chance of being shared with a brother. This results in 0.5 x 0.5=0.25. It was found that the population sex-investment ratio for "L. acervorum" changed from significantly female biased to significantly male biased with increasing polygyny. In polygynous colonies where multiple queens reproduce, there is a lack of worker aggression towards queens. This is likely a benefit for multiple queens that reproduce in polygynous populations as a result of dilution of relatedness. Workers simply favor the previous reproductive queen because she is their mother, and would thereby rear full sisters. Thus, multiple reproductive queens would decrease this worker regulation because relatedness is lower. The relatedness estimate for nest mate workers in polygynous colonies (0.46 ± 0.040) was significantly lower than that for nest mate workers in monogynous colonies (0.55 ± 0.089). However, this relatedness estimate for nest mate workers in monogynous colonies was distinctly lower than the expected 0.75 value for full siblings.
Season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and ...
al fluctuations of queen numbers may explain why relatedness estimates for workers in
monogamous Monogamy ( ) is a form of dyadic relationship in which an individual has only one partner during their lifetime. Alternately, only one partner at any one time ( serial monogamy) — as compared to the various forms of non-monogamy (e.g., pol ...
colonies are lower than expected. The seasons shape the composition of the colony—young queens are regularly adopted in their natal colonies after mating in late summer. By seeking adoption in established colonies, young queens might avoid long solitary hibernation—winter mortality was found to be lower in polygynous than in monogynous colonies. Some emigrate from the colony after hibernation in the spring. This may be an attempt to found their own colony solitarily or by budding, leaving the natal colony with their own workers and brood to start a new colony. Some monogynous colonies could have recently been polygynous. Thus, colonies of ''L. acervorum'' may easily switch from monogamy to polygyny as a result of adopting young queens and budding, or queen emigration.


Mating behavior

In a study conducted in Spain, ''L. acervorum'' became active in the incubators about one or two hours after the morning rise in temperature. At that time, mating behavior could be studied under natural daylight. When the temperature reached 25 °C, the winged females left the nest chambers and climbed the walls of the flight cage to perform a stationary sexual calling behavior. Other females exhibited a sexual display at very short distances from the nest entrance. Flying before the sexual calling was never observed. The males were always highly aroused when put into a flight cage with calling females, and they immediately tried to mount a calling female and to insert the genitals. During the first contact both partners antennate each other intensively. After the insertion of the genitals the male tilts backwards and remains immobile in this position. The female usually sits still during the copulation and the male sometimes grooms its antennae. After 30 to 90 seconds the female turns round and bites into the male's gaster, which typically ends with separation 10 to 20 seconds later. Copulations could be observed until six or seven hours after the morning rise in temperature, though most copulations took place between one and two hours after the first females began to exhibit sexual calling.


References


External links

* {{Taxonbar , from=Q13395744 acervorum Insects described in 1793 Hymenoptera of Europe Articles containing video clips