The pharaoh ant (Monomorium pharaonis) is a small (2 mm) yellow or light brown, almost transparent ant notorious for being a major indoor nuisance pest, especially in hospitals. A cryptogenic species, it has now been introduced to virtually every area of the world, including Europe, the Americas, Australasia and Southeast Asia. It is a major pest in the United States, Australia, and Europe.
This species is polygynous—each colony contains many queens—leading to unique caste interactions and colony dynamics. This also allows the colony to fragment into bud colonies quickly.
Pharaoh ants are a tropical species, but they also thrive in buildings almost anywhere, even in temperate regions provided central heating is present.
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DiurnalDiurnal animals are active during the daytime, with a period of sleeping or other inactivity at night. The timing of activity by an animal depends ...
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OmnivoreAn omnivore is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and ani...
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CosmopolitanAnimals with cosmopolitan distribution are those whose range extends across all or most of the world in appropriate habitats. Another aspect of cos...
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TerrestrialTerrestrial animals are animals that live predominantly or entirely on land (e.g., cats, ants, snails), as compared with aquatic animals, which liv...
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OviparousOviparous animals are female animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive...
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starts withPharaoh workers are about 1.5–.mw-parser-output.frac{white-space:nowrap}.mw-parser-output.frac.num,.mw-parser-output.frac.den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output.frac.den{vertical-align:sub}.mw-parser-output.sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}2.0 millimetres (1⁄16 in) long. They are light yellow to reddish brown in color with a darker abdomen. Pharaoh ant workers have a non-functional stinger used to generate pheromones. The petiole (narrow waist between the thorax and abdomen) has two nodes and the thorax has no spines. Pharaoh ant eyesight is poor and they possess on average 32 ommatidia. The antennal segments end in a distinct club with three progressively longer segments.
Males are about 3 millimetres (1⁄8 in) long, black, winged (but do not fly). Queens are dark red and 3.6–5.0 millimetres (1⁄8–3⁄16 in) long. They initially have wings that are lost soon after mating, but do not fly.
Pharaoh ants use a positive feedback system of foraging. Each morning, scouts will search for food. When one finds it, it will immediately return to the nest. This causes several ants to follow the successful scout's trail back to the food source. Soon, a large group will be upon the food. Scouts are thought to use both chemical and visual cues to remain aware of the nest location and find their way. If the colony is exploring a new region, they employ a land rush tactic, in which a large number of foragers randomly search, constantly releasing pheromones.
Even though M. pharaonis is most often thought an indoor pest, foraging has been found to be more prevalent outside. Even inside colonies were found to forage close to windows, indicating a propensity for outdoor environment.
Upon scouts' return with food, the queens will attempt to beg for their share. Depending on food availability and each individual's condition, a scout may refuse the queen's entreaties and even run away from her. The decision of an individual to give up food to the queen may be beneficial in situations of plentiful food, as a healthy queen can reproduce and propagate the colony's genes. However, when food is highly scarce, an individual's own survival can outweigh this potential benefit. She will therefore refuse to give up food.
A queen may also feed on secretions from larvae. This creates a positive feedback loop in which more larvae will provide more food to queens who can in turn produce more larvae.
If a large amount of larvae results in a surplus of secretions, pharaoh ants will store the excess in the gasters of a unique caste, the replete workers. Members of this group have enormous gasters and can regurgitate their stored food when needed. In this way, the colony has a cushion against food shortages.
Pharaoh ants have a sophisticated strategy for food preference. They implement two related behaviors. The first is known as satiation. The workers will at first show a strong preference for a particular food type. However, if this food is offered alone, with no other options, for several weeks, workers will afterward show a distinct preference for a different type of food. In this way, the ants become satiated on a certain food group and will change their decision. The second behavior is called alternation. If given the continuous choice between food groups, pharaoh ants will tend to alternate between carbohydrate-rich foods and protein-rich foods. These satiation and alternation behaviors are evolutionarily adaptive. The decision to vary the type of food consumed ensures that the colony maintains a balanced diet. Edwards & Abraham 1990's result is appropriate for highly competitive environments, and consistent with a high intake:expenditure ratio.
The pharaoh ant queen can lay hundreds of eggs in her lifetime. Most lay 10 to 12 eggs per batch in the early days of egg production and only four to seven eggs per batch later. At 27 °C (80 °F) and 80 percent relative humidity, eggs hatch in five to seven days. The larval period is 18 to 19 days, pre-pupal period three days and pupal period nine days. About four more days are required to produce sexual female and male forms. From egg to sexual maturity, it takes the pharaoh ant about 38 to 45 days, depending on temperature and relative humidity. They breed continuously throughout the year in heated buildings and mating occurs in the nest. Mature colonies contain several queens, winged males, workers, eggs, larvae, pre-pupae and pupae.
Mating for pharaoh ants occurs within the nests with males that are usually not from the colony which ensures genetic diversity. The queen can typically produce eggs in batches of 10 to 12 at once, but can lay up to 400 eggs every time she mates. The eggs that are produced take up to 42 days to mature from an egg to an adult. Each queen within the nest lives between 4 and 12 months.
During copulation, sperm is transferred from male to female inside a spermatophore. There are several theories regarding the adaptive value of using a spermatophore. It contains certain chemicals that may inhibit the female's sex drive. Alternatively, it may physically plug the female's gonophore. In either explanation, the spermatophore prevents the female from reproducing with another male. In essence, the use of a spermatophore is evolutionarily favorable because it increases the probability of the male's genetic code being transferred to subsequent generations by lessening potential competition from other males.
Pharaoh ant copulation, like that of many social insects, is harmful to the female. The penis valve contains sharp teeth, which latch onto a thick, soft cuticular layer in the female. This method of copulation too has an evolutionary basis. The teeth ensure sex lasts long enough for sufficient sperm transfer. Also, the pain caused to the female may, in some ways, lessen her desire to mate again.
