Anal Cills

FIGURE 12.3 External anatomy of mosquito larvae, dorsal view, with anal segment and siphon at posterior end rotated to provide better view. (A) Anopheline form (Anopheles maculipennis)-y (B) culicine form (Aedes cinerms). (From Marshall, 1938.)

FIGURE 12.4 Heads of mosquito larvae, postero-ventral view. (A) Anopheline form (Anopheles quadrim&'C^la'tus); (B) culicine form (Ochlerotatus ftdvuspollens); (C) toxorhynchitine form (Toxorhynchites hrevipalpis). The lateral palatal brushes of most larvae are used to generate water currents for filter feeding; in Toxorhynchites they are modified for seizing prey. (From Harbach and Knight, 1980.)

Ochlerotatus Detritus
FIGURE 12.7 External anatomy of mosquito adult. (A) Generalized adult, dorsal view; (B) wing, showing typical venation and vein nomenclature. (From Ross and Horsfall, 1965.)
Anopheles Female Head

Woodbridge A. Foster and Edward D. Walker

Pivjboscis

Labella

Female

FIGURE 12.9 Mouthparts of adult female mosquito, showing labium, splayed stylets, and variations in structure of their tips. (From Mathcson, 1944.)

FIGURE 12.8 Heads of anopheline and culicine mosquitoes, females (left) and males (right); males, typically, with plumose antennae. (A) ylBi^/Wei (anopheline), both males and females with palps about as long as the proboscis; male with plumose antennae and rips of palps broadened; (B)Cwlex (culicine), females typically with short palps and males with long, curved, or brushlike palps. (From Gordon and Lavoipierre, 1962.)

of sensory structures, including those for detecting host odors.

The mosquito proboscis is prominent, projecting anteriorly at least two-thirds the length of the abdomen. It consists of the basic complement of insect mouthparts; the labrum, paired mandibles, hypopharynx, paired maxillae, and labium. The first four structures have evolved into fine stylets, forming a tightly fitting fascicle that in females is used to penetrate host skin (Fig. 12.9). The fascicle is cradled within the groove of the targe and conspicuous labium (Fig. 12.10), which comprises the bulk of the proboscis. The dp of the labium bears two small tastesensitive labellar lobes and a short, pointed ligula (function unknown) between them. Of the fascicle of stylets, the hypopharynx and mandibles are narrowly pointed at their tips, whereas the maxillae end in serrated blades.

The two compound eyes, each represented by 350— 900 ommatidial lenses, wrap around the front and sides of the head. The antennae arise between the eyes, are long and filamentous, and are usually sexually dimorphic. In species in which sound is used to locate females in flight, the flagellum of the male antenna has whorls of much longer fibrillae, giving it a plumose appearance (Fig. 12.8). The pedicel at the base of the antenna is a large globular structure that contains Johnston's organ, a mass of radially arranged mechanoreceptors that respond to vibrations of the flagellum induced by sound. In addition to the long fibrillae, the antenna has a variety

Mosquito Proboscis
FIGURE 12.10 Fascicle of stylets of adult female mosquito. Mouthparts near tip of proboscis, showing natural arrangement of stylets in a single bundle, or fascicle, within a groove in the labium, which forms a sheath, (From Jones, 1978; illustration by Tom Prentiss.)

the base of the proboscis and bear several kinds of sensilla. Although there are many exceptions, palps usually are short in female culicines and toxorhynchitines but ionger than the proboscis in most male culicines and toxorhynchitines and also in both sexes of anophelines (Fig. 12.8).

The mosquito thorax forms a single, relatively rigid muscle-filled locomotor unit with obscured segmentation. The mesothorax and metathorax each have a pair of lateral spiracles. The slender legs are attached close together on the underside of the thorax by elongate, downward-projecting coxae; the tarsi are tipped with two claws and a central pad, the empodium, The wings are narrow, have a distinctive pattern of veins, and bear scales along the veins and the hind margin, the latter forming a fringe. The halteres, tiny modified hind wings used in flight control, are located right behind the insertions of the wings.

The abdomen is clearly segmented and capable of extensive expansion and some movement, owing to the membranous areas between each set of tergites and stern-ites. This allows for expansion of the abdominal wall to accommodate large blood and sugar meals and developing clutches of eggs. Abdominal segments 5—8 are progressively smaller, so that the abdomen tapers toward the posterior end. Segment 9 is quite small and bears the cerciy the postgenital lobe of the female, and the claspers and other genitalic structures, or terminalia, of the male (Fig. 12.12). At emergence, the male genitalia are inverted. During the first hours of adulthood, segments 8

Both mandibles and maxillae puncture the skin and advance the fascicle into the host's tissue. A salivary channel runs the length of the hypopharynx, delivering saliva to the tissue during probing. The labrum is curled laterally to form a food canal for drawing the host's blood or a sugar solution up the proboscis. In males, and in females of non—blood-feeding species, the mandibles and maxillae have atrophied, so they cannot pierce skin. In both sexes of Toxorhyncbites, the nonpiercing proboscis is curved downward (Fig. 12.11). Maxillary palps arise at

FIGURE 12.11 Toxorhynckitessp., adult female; legs not shown. The form of the palps and antennae in females and males is similar to that of culicines; however, the proboscis of both sexes is bent downward at an angle of 90° or more. (From Smart, 1948.)

the genera Aedes, Ochlerotatus, and other Aedini that typically develop in temporary water. Their eggs are laid on solid substrates out of water, and the larvae within them remain quiescent until inundated. The eggs can tolerate periods of cold and desiccation and may remain viable for years. Hatching usually occurs at warm temperatures after the eggs have been submerged and microbial activity has caused the oxygen level in the water to drop.

Depending on the species and particular conditions of the water, most mosquito larvae spend most of their time either at the water surface or at the bottom of the water column, coming to the surface for air only occasionally or not at all. At ideal conditions of food and temperature (26-28°C), the entire larval phase of Ae. aegypti, a tropical and subtropical mosquito, may last as few as 5-6 days. The first three instars are completed in about 1 day each and the fourth lasts about 3 days. In males these periods are slightly shorter, so the males pupate about 1 day earlier than females. Larvae of many species grow even faster, as when the water is heated by direct sunlight, whereas others develop slowly. Toxorhyn-chites and Wyeomyia species usually take 2—3 weeks even under ideal conditions. At cooler temperatures, or when food is scarce, growth becomes slower and can practically cease, with larvae remaining alive for months. Larvae of some species that inhabit high latitudes or high altitudes, or that develop in the early spring in temperate regions, have growth thresholds close to freezing and can tolerate even temporary entrapment in solid ice. This is typical of the snowpool Ochlerotatus species and of mosquitoes that overwinter as larvae, such as Wyeomyia smithii in pitcher plants and Orthopodomyia alba in tree holes.

The pupa spends nearly all of its time at the water surface. By the time it has molted to form a pharate adult within the pupal cuticle, it is very dark. In warm water the entire pupal stage typically lasts about 2 days in both sexes. In some mosquitoes, such as Toxorhyncbites and Wyeomyia species, the shortest pupal periods may be 5—6 days. In all species the pupal period lasts longer at lower temperatures.

Adult males tend to emerge earlier than females, because of their shorter larval growth periods. As adult emergence approaches, the pupa remains stationary at the water surface, and the abdomen gradually straightens over 10—15 min. The adult emerges from the pupal cuticle by ingesting air, causing the cephalothorax to split and the adult to rise up out of the cuticle and stand on the water surface. The entire process takes only a few minutes. The newly emerged adult is capable of short flights a few minutes later but cannot sustain long flights for many hours until after the cuticle becomes fully sclero-tized. Lipids and glycogen, carried over from larval reserves, provide sufficient energy for a few days of flight and survival.

It is typically during the first 3—5 days of adult life that both sexes obtain sugar from plant nectar or honeydew, become sexually mature, and then mate. In some species (e.g., Culiseta inornata, Wy. smithii, and Deinoceritescancer) sexual maturation is complete at the time of emergence or only a few hours later, and mating occurs almost immediately. Mosquitoes typically first feed on sugar to obtain enough energy for sexual maturation and for the flight necessary for mating, dispersal, and finding vertebrate blood. Natural sugar is taken repeatedly throughout adult life by both sexes of most species. Females typically mate only once. Males can inseminate several females before their supplies of mature sperm and accessory gland secretion become depleted. The semen supply is replenished in a few days.

Amorphous masses of fat body line the inner walls of the abdomen. The fat body synthesizes and stores both glycogen for flight and lipids for maintenance, using the digestive products of sugar and blood meals. Glycogen also is stored in the fibrillar flight muscles of the thorax, serving as a source of energy for immediate flight if the sugars in the crop and hemolymph have been exhausted.

Only females feed on vertebrate blood. In most mosquitoes, ingestion and digestion of a blood meal initiates egg development by stimulating a cascade of hormones from the brain and ovaries. The large amount of protein contained in hemoglobin and the blood serum provides the amino acids for synthesizing vitellogenin, the proteinaceous precursor of egg yolk. The protein also serves as the substrate for building lipid and glycogen, which contribute both to the egg yolk and to the maternal energy reserves used for survival and flight. A blood meal will stimulate egg development only if it is sufficiently large and if the female's ovarian follicles have reached the resting stage, at which point they are considered to be gonoactive. If a female has had poor larval nutrition, the follicles may not have reached the resting stage, and she will be unable to develop any eggs until having ingested sugar or a preliminary blood meal. Such agonoin-active female, needing food to bring the ovarian follicles to the resting stage, is sometimes said to be "pre-gravid." Details of the hormonal control of these processes are discussed by Brown and Lea (1990), Hagedorn (1994, 1996), and Klowden (1996).

In most species, females are anautogenous; the egg follicles remain in the resting stage until a blood meal is taken. Following each blood meal, the female develops one mature clutch of eggs, exhibiting what is known as gonotrophic concordance. However, females of autogenous species or populations can develop eggs without a blood meal; among these there are obligate and facultative types. A facultatively autogenous female typically develops only the first clutch of eggs without blood; she does so only if she emerges with sufficient reserves and cannot readily find blood. Thereafter, a blood meal is

ovariole where a follicle has degenerated after a blood meal, instead of developing into an egg (Fig. 12.15). Thus, a count of the maximum numbers, per ovariole, of dilatations in the stalk and zones of granules in the calyx yields an estimate of the number of gonotrophic cycles completed. This physiological age ¿railing can provide the medical entomologist with valuable information on the age of individuals and the age structure of a mosquito population. Details of these processes and their interpretation and application are given by Detinova (1962), Sokolova (1994), Fox and Brust (1994), and Hoc (1996).

Univoltine mosquito species complete only one generation per year. This occurs either if the developmental time is slow in relation to the season favorable for development or if the life cycle includes an obligate form of diapause, a compulsory phase of arrested development. Bivoltine and multivoltine species can complete two or more generations, respectively, during each breeding season, but the number actually completed may depend on temperature, available larval habitats, or available hosts. Mosquitoes pass through the winter or dry season as eggs, larvae, or adults, depending on the species and the climate. In cold climates, overwintering takes place in a state of diapause.

Eggs that are laid on or in water generally are not resistant to desiccation and hatch shortly after embryogenesis, provided that they are wet and not too cold. This is typical of Anopheles, Culex, Culiseta, and Toxorhynchites species. Aed.es, Ochlerotatus, Psorophora, and Haemagogus eggs, on the other hand, typically are laid on damp substrates, display great resistance to desiccation, and remain quiescent for months or years after embryogenesis until they receive a hatching stimulus. Sometimes moisture by itself is sufficient to induce hatching. Usually, however, the requisite stimulus is a reduction of dissolved oxygen in the water caused by microbial activity and decomposition of organic matter. Among quiescent eggs that are eventually submerged, only a portion of a single egg clutch may hatch during any one inundation, resulting in installment hatching. This apparently is the combined result of intrinsic variations among eggs in their hatching-stimulus thresholds and of local variations in microbial activity, causing differences in oxygen tension around the eggs. Even during a single inundation, hatching may not occur all at once but over a period of many days.

When an egg is ready to hatch, the first-instar larva uses a dorsal hatching spine on its head, the egg breaker or egg burster, to apply pressure to a preformed weakness in the chorion. This causes the chorion to pop open at one end, and the larva wriggles free. Because the eggs of Culex, Culiseta, and Coquillettidia usually stand vertically on the water surface in rafts, the larvae develop inside them with their anterior end oriented downward and hatch directly into the water.

Mosquito larvae are not buoyant and must, at rest, be suspended at the surface by special hairs and spiracular structures that cling to the surface tension while obtaining oxygen directly from the air. Culicinae typically migrate up and down in the water column, so they occur both at the surface and at the bottom of a body of water, depending on the availability of food. At the surface the tip of the siphon opens above the surface film, and the larvae hang diagonally downward most of the time (Fig. 12.16B). Mansonia, Coquillettidia, and some Mi-momyia species are unusual in remaining submerged throughout larval and pupal development, with their siphons embedded in the tissues of aquatic plants from which they derive some oxygen (Fig. 12.16C). Mosquitoes that live in water-filled leaf axils (e.g., Wyeomyia spp.) are adept at flattening themselves against vertical surfaces and maneuvering in narrow spaces. Anopheline larvae spend most of their time at the water surface, often close to vegetation or floating material. They are able to remain suspended horizontally at the surface (Fig. 12.16A) due to pairs of dorsal palmate setae (float hairs) on several abdominal segments (Figs. 12.3A).

Larvae propel themselves by a back-and-forth lashing movement of the abdomen. Anopheline larvae usually swim horizontally at the surface film. When larvae of typical culicine mosquitoes are feeding below the surface, they periodically swim actively back to the surface to obtain oxygen. However, in many microenvironments dissolved oxygen also is absorbed from the water through the cuticle, requiring infrequent trips to the surface by some species.

Mosquito larvae feed on a variety of organic detritus, suspended material, and small organisms in their aquatic habitats. The organisms include bacteria, protists, fungi, algae, microinvertebrates, and small macroinvertebrates; the organic detritus usually consists of dead plant material and dead macroinvertebrates. They collect these food items in five basic ways: filtering, gathering, scraping, shredding, and preying. Filterers generate water currents with their lateral palatal brushes on the labrum, drawing suspended particles though fine combs, where they are collected and directed to the mouth. Gatherers use their mouthparts in a similar manner, but only after stirring up the particles from solid surfaces. Scrapers obtain food by scraping it off solid surfaces, whereas shredders gnaw, chew, and bite off pieces of organic matter. Predators grasp insects and other small, mobile prey in their large and sharp mandibles or maxillae (e.g., some Psorophora spp.) or with long, curved palatal brushes (e.g., Toxorhynchites) (Figs. 12.4C and 12.17). Most species use more

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