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mosquito

Mosquito


INTRODUCTION

Mosquitoes are well known by most people because of their pesky biting habit. Of greater concern, they are very important as vectors of numerous human diseases such as West Nile virus, malaria, yellow fever, filariasis, dengue, and encephalitis. Mosquitoes are distributed throughout the world, including the United States. 

RECOGNITION

Adults about 1/8-3/8" (3-9 mm) long; body and legs usually  covered with scales. Color mostly gray to dark, some marked with white, silver, green, or iridescent blue scales. Head with proboscis/beak long; ocelli absent. Antennae 15- segmented, plumose (feathery) in male, hairy in female. Wings 2 in number, long, narrow, with scales along veins and wing margin; outer part of wing with an unforked vein between 2 forked veins.

The adults of the 4 most common genera of mosquitoes can be distinguished as follows:

  1. Anopheles with palps about as long as proboscis/beak and rests with body and proboscis in one plane or axis which is at an angle to the surface.
  2. Aedes and Ochlerotatus with palps much shorter than proboscis and rests with body and proboscis in 2 different planes (at an obtuse angle to each other) and body resting on the surface.
  3. Culex with palps much shorter than proboscis and rests with body and proboscis in 2 different planes and body up off but parallel to the surface.

Larvae with head and thorax wider than abdomen, thorax of 3 fused segments and widest. Antenna 1-segmented. Mouthparts with labial brush usually consisting of numerous fine hairs, sometimes reduced to about 10 stout curved rods. Abdominal segment 8 usually with a breathing/respiratory tube. Abdominal segment 10 (last segment) with hardened dorsal plate or ring, with long dorsal hairs and prominent ventral brush, and with 2 or 4 membraneous anal papillae (fingerlike projections).

SIMILAR GROUPS

(1) Dixid midges (Dixidae) with wings lacking scales and body bare.

(2) Midges (Chironomidae) with wings lacking scales, front tarsi usually lengthened (very long), male antennae generally plumose (feathery), and proboscis (beak) short.

(3) Crane flies (Tipulidae) with wings lacking scales, body bare, legs very long, and usually much larger (3/8-1"/10-25 mm).

(4) Phantom crane flies (Ptychopteridae) with wings lacking scales.

REPRESENTATIVE SPECIES.

  1. Yellowfever mosquito, Aedes aegypti (Linnaeus). Adult dark brown to black with silvery white markings, identifying marks include dorsum of thorax with 2 outer curved and 2 median parallel marks forming a lyre-shaped pattern, tarsal segments ringed with white only on basal portion of segments, and abdomen with a dorsal narrow white basal band on each segment; wing length about 1/8" (3.5 mm); breed in shaded artificial containers; occurs in southern states where nights are above 68°F/20°C, but not in California.
  2. Asian tiger or forest day mosquito, Aedes albopictus (Skuse). Adult dark to black with silvery white markings, identifying marks include dorsum of thorax with a  single median silver-white stripe, tarsal segments ringed with white, and abdominal segments with a narrow dorsal white basal band; wing length about 1/8" (3.5 mm); breed in artificial containers; found in eastern, midwestern, and southern United States.
  3. Floodwater mosquito, Ochlerotatus [formerly Aedesstictcus (Meigen). Adult dark to black with whitish markings, identifying marks include head and thorax gray, thorax with darker longitudinal stripes, wings black scaled except costa and subcosta with white scales intermixed, legs speckled with white but no white rings, and abdominal segments with a narrow dorsal white basal band; wing length about 1/8" (4 mm); breed in flood pools; distributed throughout the United States.
  4. Eastern saltmarsh or saltmarsh mosquito, Ochlerotatus [formerly Aedessollicitans (Walker). Adults golden brown with golden yellow or white markings, identifying marks include black proboscis/beak with wide white band near middle, black palps tipped with white, dorsum of thorax with darker brown areas, tarsal segments with narrow white basal ring, hind tarsal 1st segment with median yellow ring and 5th/last segment largely white, abdominal segments with dorsal yellowish narrow basal band and wider median longitudinal stripe and with white lateral/side markings; wing length about 3/16" (4.5 mm); breed in salt marshes and saline areas; occurs in eastern and Gulf coastal areas from New Jersey to Texas and in inland saline areas.
  5. Vexans or inland floodwater mosquito, Aedes vexans (Meigen). Adult golden brown to brown with white to light gray markings, identifying marks include short palps with a few white scales at tip, all tarsal segments with narrow white basal ring, abdominal segments with dorsal white basal band notched in center, sometimes separating band into 2 parts, and 7th (terminal/last) abdominal segment with white only at tip if present; wing length about 3/16" (4.5 mm); breed in temporary pools; found throughout the United States.
  6. Common malaria mosquito, Anopheles quadrimaculatus Say. Adults dark brown, legs lack bands and wings with only dark scales, each wing with 4 patches/spots of dark scales; palps as long as proboscis/beak; body and proboscis in one plane/axis when feeding; breed in permanent freshwater sites; found east of Rocky Mountains.
  7. Northern house mosquito, Culex pipiens Linnaeus. Adults brownish with white or gray markings, identifying marks include legs dark or black and lack bands but femora cream with dorsal or outer portions dark and abdominal segments with broad dorsal basal white band widest along midline and broadly jointed to lateral patches; wing length about 1/8" (3.5 mm); breed in artificial containers, ditches, storm sewer catch basins, and polluted water; occurs throughout the northern states and as far south as Oklahoma and Georgia.
  8. Southern house mosquito, Culex quinquefasciatus Say. Adults similar in almost all respects to C. pipiens except abdominal basal white bands narrowly joined or disconnected from lateral patches; breeding sites similar to those of C. pipiens; occurs throughout the southern states and as far north as Nebraska, Iowa, Illinois, and Ohio.
  9. Encephalitis mosquito, Culex tarsalis Coquillett. Adults brown with white or gray markings, identifying marks include proboscis/beak with white ring near middle, tarsal segments with white rings, and femora and tibiae each with a longitudinal white line or row of white spots on outer surface; wing length about 3/16" (4.5 mm); breed in temporary pools; throughout United States except for approximately eastern Michigan to eastern North Carolina and eastward.

BIOLOGY

Mosquitoes are commonly separated into 3 groups based on where and how their eggs are laid. After a blood meal, the female will lay her eggs:

(1) singly on water, eggs with floats, usually hatch within a few days, e.g. Anopheles;

(2) in rafts on water with up to 100+ eggs per raft, usually hatch within a few days, e.g. Culex; and

(3) singly in semi-dry places such as moist soil near water, do not hatch until water has risen and inundated them, can lie dormant for 3-5 years, e.g. Aedes and Ochlerotatus.

With water present, eggs hatch in a few days into larvae which are commonly called wig- glers because of their jerky movements. All larvae live in water and go through 4 instars and 4 molts. Larvae of most species (e.g. Aedes, Culex, Ochlerotatus, etc.) take in air through a breathing tube (siphon) located on the 8th abdominal segment which penetrates the water surface while they float at an angle just below the surface. Other species (e.g. Anopheles) have a spiracular plate on the 8th abdominal segment which penetrates the surface while they float parallel to and just below the surface, their buoyancy enhanced by clusters of float hairs (palmate hairs) on some abdominal segments.

With their 4th molt, the larvae become pupae which are commonly called tumblers. The pupae live in water and are very active. The pupae of most species breathe through a pair of respiratory trumpets located on the dorsal thoracic surface which penetrate the water surface while they float just below the surface. At the end of the pupal stage, while at the water’s surface, the pupal skin splits open and the adult works its way out and onto the surface of the water, briefly dries and flies away. Developmental time (egg to adult) is usually about 10-14 days; eggs hatch in 1-3 days to 5 years, larval stage lasts 7 days to weeks, and pupal stage takes 2-4 days to a few weeks. Adult females may live up to 1-2 months (males about 6-7 days) in the summer or for 6+ months if they overwinter.

Mosquitoes serve as vectors of many important diseases affecting humans including West Nile virus, malaria, yellow fever, dengue fever, filariasis, and encephalitis. Refer to the chart entitled “Biological Data on Medically Important Mosquitoes in the U.S.” (page 3.18.5) for a summary of such diseases. Also of interest is the dog heartworm (Dirofilaria immitis (Leidy)) which is a common mosquito-transmitted filarial parasite of dogs and is a serious problem from Minnesota to Illinois to Texas and eastward.

HABITS

Mosquitoes have adapted to almost every kind of aquatic situation such as permanent ponds and marshes, temporary flood waters or woodland pools, drainage ditches, and water contained in tree holes, leaves of plants, or artificial containers. The exceptions are flowing streams and the open waters of large streams, rivers, lakes, seas, and oceans. The number of generations per year ranges from 1 where the eggs require cold before hatching (e.g. some Anopheles), to many in warm climates where most breed continuously.

The larvae of most species feed on small aquatic organisms and organic debris which they strain out of the water with a series of oral brushes. Although quite active, the pupae do not feed. The adult males feed on nectar. Although the adult female also feeds on nectar, females of most species require a blood meal before they can lay fertile eggs. Females require 2+ days to digest a blood meal, lay a batch of eggs, and then seek another blood meal.

The flight range of mosquitoes varies with the species, temperature, wind direction, time of year, and distance to blood meal sources. For various species of Aedes and Ochlerotatus (for 7 species formerly in Aedes, see table page 3.18.5), the range for recaptured marked females was from 18 mi (29 km) along coastal Georgia to 110 mi (177 km) at sea off the coast of North Carolina. These records probably represent females that got caught up in wind currents because their normal flight ranges are much shorter, about 5-10 mi (8-16/km).

The time of day in which biting occurs varies with the species. Most medically important species bite at dusk and dawn (crepuscular) and also during the night (nocturnal), e.g. the vexans mosquito (Aedes vexans), the southern house mosquito (Culex quinquefasciatus), and the encephalitis mosquito (Culex tarsalis), whereas others bite only at dusk and dawn such as the eastern saltmarsh mosquito (Ochlerotatus sollicitans). Several species of medical importance bite only during the daytime (diurnal) such as the Asian tiger mosquito (Aedes albopictus), and one species, the yellowfever mosquito (Aedes aegypti), bites during the day but also at dawn and dusk. Some species which normally do not bite during daytime will do so if disturbed, for instance by someone walking through high grass on which they are harboring.