Pogonomyrmex Occidentalis
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''Pogonomyrmex occidentalis'', or the western harvester ant, is a
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of
ant Ants are Eusociality, eusocial insects of the Family (biology), family Formicidae and, along with the related wasps and bees, belong to the Taxonomy (biology), order Hymenoptera. Ants evolved from Vespoidea, vespoid wasp ancestors in the Cre ...
that inhabits the deserts and arid
grasslands A grassland is an area where the vegetation is dominated by grasses (Poaceae). However, sedge ( Cyperaceae) and rush ( Juncaceae) can also be found along with variable proportions of legumes such as clover, and other herbs. Grasslands occur ...
of the
American West The Western United States (also called the American West, the Western States, the Far West, the Western territories, and the West) is census regions United States Census Bureau As American settlement in the U.S. expanded westward, the mea ...
at or below . Like other harvester ants in the
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
'' Pogonomyrmex'', it is so called because of its habit of collecting edible seeds and other food items. The
specific epithet In Taxonomy (biology), taxonomy, binomial nomenclature ("two-term naming system"), also called binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin gramm ...
"''occidentalis''", meaning "of the west", refers to the fact that it is characteristic of the interior of the Western United States; its mounds of gravel, surrounded by areas denuded of plant life, are a conspicuous feature of
rangeland Rangelands are grasslands, shrublands, woodlands, wetlands, and deserts that are grazed by domestic livestock or wild animals. Types of rangelands include tallgrass and shortgrass prairies, desert grasslands and shrublands, woodlands, savanna ...
. When numerous, they may cause such loss of grazing plants and seeds, as to constitute both a severe ecological and economic burden. They have a painful and venomous sting.Capinera, John L. Encyclopedia of Entomology. Publisher: Springer 2008.


Colonies

Mature
colonies A colony is a territory subject to a form of foreign rule, which rules the territory and its indigenous peoples separated from the foreign rulers, the colonizer, and their '' metropole'' (or "mother country"). This separated rule was often or ...
consist of up to 20,000 workers and one
queen Queen most commonly refers to: * Queen regnant, a female monarch of a kingdom * Queen consort, the wife of a reigning king * Queen (band), a British rock band Queen or QUEEN may also refer to: Monarchy * Queen dowager, the widow of a king * Q ...
. A queen can live up to 30 years, and many colonies survive for 20 years. A colony inhabits a nest that is up to deep.Cole, 1994 The queen stays at the bottom of the nest, and workers usually relocate themselves and brood within the nest, capturing safe levels of heat.Cole,1932 A colony's nest is topped by an irregularly conical nest mound that can be more than in diameter. The composition, shape, and size of the mound differ across plant environments. The mounds of most colonies are surrounded by an area devoid of vegetation, and so do not burn during fires. The soil in the mound is drier than that in the surrounding denuded area. Bigger ''P. occidentalis'' colonies (in number of workers) have bigger mounds. Individual queens found colonies on their own, without workers or other queens. Survivorship of colonies in the first year is negatively correlated with increasing density of foundresses. Foraging workers kill queens that they encounter aboveground and occasionally excavate queens. Factors independent of colony density are responsible for >90% of foundress mortality. Queens in some populations found colonies claustrally and in others, semi-claustrally. Many of the eggs laid in the first batch die or are unembryonated. During colony founding, larvae may eat eggs. The first workers produced, known as nanitics, are stunted. They measure 2 mm long (33% shorter than typical workers of established colonies). 2/188 founding queens survived from July to March.


Workers

Workers' bodies are usually dark red and those in a mature colony are on average 6 mm long. Workers vary in size, but are not subdivided into groups of different-sized individuals with special roles. A worker lives for an average of six months, and as it ages, it usually progresses through different roles within the colony. For example, workers forage towards the ends of their lives. In the field, workers are active when the temperature at the surface of the colony's mound is .


Nutrition

Workers harvest
seed In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...
s and
pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...
directly from plants and gather fallen seeds. Some seeds are sometimes stored in chambers within the nest and are depleted during winter. Workers also gather newly dead insects. Workers generally forage April–September. Workers generally forage throughout the day during cooler months and only 5–11 am and 3–9 pm during the summer. Genetically diverse colonies forage for more hours daily. Foraging times within a day and foraging temperature range vary consistently among colonies. Given a choice, workers select seeds containing more energy. Given a choice, workers select a diversity of seeds or seeds that are new to the colony. Workers usually forage one kind of item each day, but change their specialty daily. Workers usually forage in one direction over and over, even across days. Colonies recruit more workers (from the total worker pool) to forage at a good food source. Colonies lose foragers in encounters with neighboring colonies. Workers defend foraging territories against neighboring colonies. Different plant environments support different densities of colonies. Foragers produce more period
mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...
during darkness, the timing of which varies seasonally.


Association with ''Myrmecocystus mexicanus''

Cole et al. surveyed the distribution of ''
Myrmecocystus mexicanus ''Myrmecocystus mexicanus'' is a species of ant in the genus '' Myrmecocystus'', which is one of the six genera that bear the common name " honey ant" or "honeypot ant", due to curious behavior where some of the workers will swell with liquid foo ...
'' nests, (a species of North American honey ant) relative to the distribution of ''P. occidentalis'' nests at a site in western Colorado and found that there was a definite spatial association between the two species. They also observed that ''M. mexicanus'' feeds on dead or close-to-death ''P. occidentalis'' workers, and believed that ''P. occidentalis'' provides the main source of food for the ''M. mexicanus'' colonies at that site. Thus, the ''M. mexicanus'' situates nests nearby ''P. occidentalis'' colonies to collect this source of nutrition. There was no substantial evidence that ''M. mexicanus'' actively predates healthy ''P. occidentalis'' workers.


Biomass and brood production

Different developmental stages of ants within a colony process different kinds of food;
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 ...
ingest solids, while adults ingest liquids, including larval excretions. Immature individuals cannot pass from one larval stage to another or to adulthood without the help of adults; adults help immature individuals remove their old larval and/or
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 ...
l skins during
ecdysis Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remnant ...
(molting). As larvae are relatively immobile, they only interact with nutrients as adults bring the nutrients to the larvae or the larvae to the nutrients. Bigger colonies do not necessarily produce a greater total reproductive biomass. Colonies stop producing brood before they overwinter.


Mating

Colonies release alates synchronously. Alates mate in hilltop leks in swarms. Major mating swarms are about apart, and queens can fly no more than . Gynes mate with 2–11 (an average of 6.3) genetically distinct males. Females always mate multipally. Queens that mate only a few times are less successful. The colonies of queens that mate with more males grow faster. Males sometimes mate multiple times. Females mate nonrandomly. Larger males are more successful at mating (i.e. they are overrepresented among collected maters), but small males can still mate. Certain shape characteristics improve male chances of mating success. ''P. occidentalis'' populations are effectively small and inbred. Queens pick bare and bright areas to land and then dig where they land. Mating swarms that are consistently present and large determine much of the spatial variability in colony density and emerge over the long term. New colonies are founded in a clumped pattern, around the mating sites. The population may self-thin through direct interference competition resulting in a uniformly overdispersed distribution pattern. Long-term colony survival is mediated by proximity to older colonies.Ryti, R.T. and T.J. Case. 1988. The regeneration niche of desert ants: effects of established colonies. Oecologia 75:303-306. Smaller colonies have closer nearest neighbors. Smaller nests are more likely to die. The further a colony is from its nearest neighbor (especially for small colonies), the higher the colony's survival probability. Colony age and size are correlated, especially in young colonies.


References


External links

* {{DEFAULTSORT:Pogonomyrmex Occidentalis occidentalis Endemic insects of the United States Hymenoptera of North America Insects described in 1865 Taxa named by Ezra Townsend Cresson