Mosquitoes belong to the kingdom Animalia, in the phylum Arthropoda, in the class Insecta, in the order Diptera, and in the family Culicidae. They go through four life stages. These include the egg stage, larval stage, pupal stage, and adult stage.|
The egg stage is the first in the mosquito's life cycle. The egg is protected by a shell called the chorion. The chorion is constructed of three layers, making it strong and providing safety for the embryo inside the egg. The eggs can be laid in a variety of places. For example, the species Culex nigripalpus, a dominant species in the area that is the vector of St. Louis encephalitis, lays its eggs in groups called rafts on the surface of water. This could be anything, from a permanent pool to an artificial container, that has filled with water. The eggs of Culex must remain on the water or they will dry out and the embryo will die. They hatch within 1-3 days when the embryos are fully developed.
On the other hand, Ochlerotatus taeniorhynhcus and Psorophora columbiae, common biting species in Collier County, lay their eggs individually on moist substrates. The species Oc. taeniorhynchus, known as the "black salt-marsh mosquito," prefers substrates with a high salt content that are well shaded, while Ps. columbiae prefer to lay their eggs in temporary fresh water areas such as grassy rain pools, ditches, inland pastures, rice fields, and citrus groves. These eggs will not hatch unless they are inundated with water. Once the eggs are laid, the chorion becomes water resistant. If the eggs remain in an environment that has more than 70% relative humidity, the embryos become dormant and can survive for months or years waiting to be flooded. However, if the eggs are exposed to dry air for extended periods of time, the eggs will collapse and the embryos will die. Hatching also depends on the amount of oxygen in the water when the eggs are flooded. The more oxygen in the water, the greater the hatch. Hatching usually takes place several minutes after flooding.
Once the eggs hatch, the juvenile mosquito enters the larval stage. This is the most active stage and the larvae are often called wigglers. This is also the feeding and growth stage, during which time the larvae molt several times, shedding their "skin" in the process. They pass through what is known as four instars. Feeding continues until late in the fourth instar at which time final preparations are made to enter the pupal stage. The body of the larva is composed of three sections, the head, thorax, and abdomen. The larva breathes through a tube called a "siphon" that touches the surface of the water and helps the mosquito to receive air.
The pupal stage is the third life stage and the one that prepares the juvenile mosquito to become an adult. The pupa is shaped like a comma and is heavily sclerotized. This means the pupa is covered in sclerites, hard scales made of chitin that protect the body. Chitin is a protein that is chemically related to cellulose found in the cell walls of plants. The "head" of the comma is a region called the cephalothorax. The cephalothorax contains the head and the thorax of the mosquito where the mouthparts, legs, and wings of the adult mosquito form. The pupa breathes through two tubes, called trumpets, that extend from the cephalothorax to the surface of the water. The trumpets also help to stabilize the pupa in the water. The cephalothorax also houses an air bubble that keeps the pupa buoyant and helps the trumpets to stay in contact with the water surface. The air bubble is imperative for survival because without it the pupa sinks to the bottom of the water and drowns.
The "tail" of the comma is the developing abdomen of the mosquito. The abdomen is composed of nine sclerotized segments. Between each abdominal segment are flexible areas that allow the pupa to swim. Active movement is unusual for most insect species during the pupal stage; however, mosquitoes will dive under water when the surface is disturbed or there is a disruption in light penetrating the water. The pupa has two large "paddles" on the eighth segment of the abdomen that propel it through the water as the abdomen flexes up and down. When the abdominal movement stops, the pupa floats back to the top of the water. A float hair, found on the first segment of the abdomen, helps the pupa to stabilize on the water's surface. The area between the paddles on the ninth segment of the abdomen is where the copulatory organs form. The sex of individual pupae can be determined by looking at the ninth segment of the abdomen and by the size of the pupae. The ninth segment on male mosquitoes is more prominent during the pupal stage, while the female pupa is usually larger in size than the male.
Although the pupae do not feed, they must be able to breathe at the water's surface. Therefore, it is important that there is no oil on the surface of the water, because it can cause them to suffocate. The pupae can survive on the surface of water or on a moist substrate. Pupal survival is dependent on water temperature. In Florida, the juveniles can pupate in temperatures of 62° F to 95° F; however, temperatures above or below these cause increased mortality in the population. No pupae will survive in temperatures above 100° F and very few survive in temperatures below 56° F.
After the pupal stage, the mosquito emerges as an adult. The entire body of the adult mosquito is covered by an exoskeleton. The exoskeleton is composed of a cuticle that is secreted by the epidermis. The cuticle is made up of chitin that hardens, or sclerotizes. This hard layer surrounding the mosquito provides the body with strength and support. The body of an adult mosquito has three regions, the head, thorax, and abdomen.
The head contains the eyes, antennae, palpi, and mouthparts of the mosquito. Mosquitoes have compound eyes. This means their eyes are made up of numerous ommatidia, or facets. Each ommatidium is capable of forming a separate image. Therefore, the compound eye allows the mosquito to see movement over a large area, helping it to react quickly.
The antennae are important sensory organs for the adult mosquito. They house a very significant sensory structure called the Johnston organ that helps to transmit movements of the antennae to the brain. The antennae can be used to determine the sex of a mosquito. Males have many hairs on their antennae, making them look fury, while females have very few hairs.
The palpi are also found on the head and are sensory organs that help the mosquito to smell and feel. The palpi protrude just above the mouth of the mosquito. They can also be helpful in identifying the species of a mosquito, as well as the sex.
The mouthparts of a mosquito are called the proboscis. The outside of the proboscis consists of an upper lip (labium) and a lower lip (labrum) that house the mouthparts that help the mosquito to feed. Inside the lips are the hypopharynx, maxillae, and the mandibles. The hypopharynx is a canal that is connected to the salivary glands of the insect. During blood feeding, the hypoharynx pumps saliva into the wound to prevent the blood from coagulating. This is also what causes a mosquito bite to itch. There are two mandibles and two maxillae that act as saw teeth and are used to penetrate the skin so the mosquito can blood feed. Once these penetrate the skin, the labrum (upper lip) becomes a tube, enters the wound, and blood is pumped up through it by the pharyngeal pump. The pharyngeal pump is an enlargement of the pharynx that is located in the esophagus. It pumps blood by expanding and contracting.
The thorax is the section of the mosquito that accommodates the legs and the wings. This section of the mosquito is heavily sclerotized. The mosquito has two sets of wings, the fore wings and the hind wings. The fore wings are used to propel the mosquito through the air, while the hind wings (halteres) are highly modified structures that help to balance the mosquito during flight. The legs of the mosquito are composed of three segments, the femur, tibia, and tarsi. The tarsi of the insect have chemical sensory organs that assist the mosquito with the sense of smell and taste.
The third section of an adult mosquito is the abdomen. The abdomen is composed of ten segments. Sclerotized shields called the tergite and the sternite protect the top and bottom of the abdomen. Between each segment of the abdomen is a flexible membrane that allows the mosquito some added mobility.
The digestive system of the mosquito is called the alimentary canal. This structure is composed of a foregut, midgut, and hindgut. The foregut contains the pharynx and esophagus and extends from the mouth, through the head, into the thorax. At the end of the esophagus are three tubes called diverticula that lead to food reservoirs. Two of the diverticula extend to the front of the thorax, and one continues to the abdomen. Behind the diverticular tubes, the esophagus joins with the midgut where there is a valve. When the mosquito takes a blood meal, the valve opens and the blood enters the midgut. However, when the mosquito feeds on nectar, the valve closes and the nectar is sent through the diverticula. The midgut is considered the stomach of the mosquito. It contains Malpighian tubules that absorb waste materials from blood and excrete them to the hindgut. The hindgut is the area where water is extracted from the waste and stored before the waste is excreted from the rectum at the end of the hindgut.
Mosquitoes, and other arthropods, have an open circulatory system. This means that a pool of blood is circulated by the action of the heart in open cavities rather than in blood vessels. However, insects do not have blood, they have hemolymph. Their blood is called hemolymph because it does not carry oxygen as does human blood.
The mosquito's respiratory system is very simple compared to a human. Mosquitoes do not have lungs. Instead, they receive oxygen through holes in their exoskeleton called spiracles. These spiracles open and close according to the amount of oxygen the insect needs to function. The oxygen exchange takes place by diffusion. The gas travels through the mosquito through tubules called tracheae.
The central nervous system of a mosquito is also very simple. It is composed of a brain and neurons. The brain is connected to the visceral organs by groups of neurons called ganglia.
The reproductive cycle of the mosquito is similar to other insects. The female has two ovaries that produce eggs. Once the eggs are produced they move to the oviduct, which leads to a genital chamber that is connected to the spermathecae. The spermathecae is a chamber that stores sperm. During copulation, the male inserts sperm into the genital chamber of the female. The sperm is stored in the spermathecae until the female is ready to lay the eggs. When the eggs are laid (oviposition) they move through the oviduct into the genital chamber. Simultaneously, the sperm move from the spermathecae and fertilize the eggs as they pass through the genital chamber. This system of reproduction allows the mosquito to determine the best time for oviposition. That is, if the insect is under stress, and laying eggs would be detrimental to the survival of the embryos, the female can wait for a better opportunity to fertilize and lay the eggs.
All species of mosquitoes engage in searching flights that are the principal part of the insect's life. These flights occur in conjunction with the change from light to dark during the day. Search flights are necessary for mosquitoes to find nectar, blood, a place to lay their eggs, or a place to rest. However, the mosquito Oc. taeniorhynchus not only participates in search flights, but it also migrates. Upon emergence from pupa to adult, this species will migrate long distances in accordance with the change of light. This migration only takes place after the mosquito has emerged as an adult. Beyond that point, all other flights are considered searching flights. Due to the fact that Oc. taeniorhynchus migrates long distances, it makes control very difficult. For example, many of the Oc. taeniorhynchus that are found in Collier County have migrated from the Everglades. The Collier Mosquito Control District is unable to larvicide in that area of the county; therefore, more adult mosquitoes are able to develop and emerge. The most effective method for the District to control Oc. taeniorhynchus is through aerial spraying at night when the adults are on search flights. Click here to view the mosquito migration map.