Those pesky skeeters.
On a hot August evening in 1955, Jim Haeger of the Entomological Research Center in Vero Beach, Florida, climbed 25 feet into the top of a black mangrove tree. He had come to oversee a 60-acre salt marsh that had been flooded several days earlier by the rains and high tides of a passing hurricane. The storm had broken a summer-long drought, during which salt- marsh mosquitoes had laid egg after egg on the damp soil surface. With the sudden flooding, the eggs had hatched en masse, producing an estimated 2 billion larvae, and Haeger had been following their development on a daily basis.
Earlier that day, the mosquitoes had reached adult stage and climbed out of the water into nearby vegetation. At precisely 14 minutes past sunset, as if cued by an unseen prompter, the brood took flight. Later, Haeger recorded the event in daunting terms:
"There was a massive movement of ascending mosquitoes, almost darkening the sky. The flight just above my head consisted of a great band of millions of mosquitoes, all moving in unison, probably 2--4 feet thick and several hundred feet wide. I would compare the movement to a huge belt or tread of a caterpillar tractor moving on rollers over the tops of the trees, only missing them by a few inches and dipping down between. There was a tremendous singing hum of millions of frenzied wingbeats."
Within 15 minutes, all was quiet; just a few mosquitoes from an earlier brood buzzed about, biting. Haeger was amazed and impressed. A few days later, the swarm of insects made a different sort of impression on people in their flight path, as the females avidly sought their first blood meals. Today, thanks to regular efforts at mosquito control, we are spared such a nightmarish onslaught from those pesky skeeters.
All over the world
Insects are known for their diversity, and mosquitoes are no exception. There are more than 2,500 species worldwide, of which about 200 occur in the United States. Not all feed on human blood--many specialize in dining only on birds or reptiles. In Florida, where we have found 77 species, just a dozen or so species are pestiferous to humans, mostly in the genera Aedes, Anopheles, Mansonia, Ochlerotatus, and Psorophora.
The adult mosquito's body, like that of other insects, consists of three segments: head, thorax, and abdomen. The head, which includes a pair of compound eyes and mouthparts, carries antennae that can sense certain chemicals. The thorax contains a compound heart, flight muscles, and nerve cells. The creature's two wings and six legs are attached to the thorax. The digestive and excretory organs are located in the abdomen.
Both male and female mosquitoes regularly feed on flower nectar and other plant juices. For most species, the female also seeks blood meals, which provide proteins needed to produce eggs. The female is therefore specially equipped to find and feed on its victims. Its heat sensors allow it to find warm-blooded animals and humans; its chemical sensors detect gases, such as carbon dioxide, produced during normal respiration; and its compound eyes help it identify objects that move and contrast with the background.
The female also has a long mouthpart, called a proboscis, to pierce the victim's skin and suck blood. To prevent the blood from clotting while traveling through the proboscis, the insect injects its saliva, which contains an anticoagulant protein. A person who is bitten feels itchy because of an allergic reaction to this protein. It should be noted that the male mosquito also has a proboscis, but it is structurally different and adapted for feeding on plant sugars only.
Female mosquitoes have an incredible number of egg-laying strategies. Virtually every type of aquatic habitat is utilized, from freshwater ponds to salt marshes, running tropical streams to birdbaths and discarded items that collect water. Some species lay their eggs directly in the water. Others, referred to as floodwater species, deposit their eggs on substrates that will eventually be flooded. The eggs may be laid as single entities or as a group that forms a raft floating on the water.
Among the most notorious of the floodwater species is my familiar nemesis, the black salt-marsh mosquito (Ochlerotatus taeniorhynchus). During times of drought, as many as 10,000 eggs have been found per square foot of salt-marsh sod. Multiply this number by a few thousand acres, and you get an idea of the extent of misery they could inflict upon reaching the adult stage.
Eggs of floodwater species develop within the shell up to a point (called diapause) and remain viable for several months to a year. As soon as the area is flooded, they hatch almost simultaneously, producing larvae--the "wigglers" that many of us are acquainted with.
During their growth, the larvae molt through four stages, or instars. The first instar of a salt-marsh mosquito is barely visible, but the fourth is a robust black creature, a quarter-inch long. In the heat of Florida's summer, the wigglers develop rapidly--roughly one instar per day. They feed on microorganisms and organic detritus by actively grazing over the substrate or by filtering water.
During the third and fourth instars, salt-marsh mosquito larvae have the unique ability to cluster together, sometimes forming huge rafts 20 or more feet in width. Each dark, quivering mass gives the water a gelatinlike texture. Late in the fourth instar, the wigglers stop feeding and molt to the pupal stage. The pupa--or "tumbler"--has a curled abdomen and resembles a comma. It functions as the intermediate in the amazing transformation of the water-dwelling larva to the flying adult.
I have watched the pupa change to an adult hundreds of times, yet the process never ceases to mesmerize me. At the appropriate time, the pupa straightens its abdomen by a flexing muscular action, while simultaneously inhaling through breathing tubes on its head. As a result, the pupal skin breaks open and a flaccid adult emerges, forelegs first. After a brief rest on the pupal skin, the adult "skates" across the water to hide and rest in nearby vegetation. In a matter of hours, it will take wing.
How far can the adults travel? In the 1950s, entomologist Maurice Provost and his research team performed the most extensive series of studies on mosquito flight ever undertaken. Provost's group tagged 6 million larvae with radioactive tracers, raised them to adults, and released them on both coasts of Florida. Of the estimated 3.5 million that took wing, 982 were recaptured and identified by Geiger counters. The most distant traps, placed 28 miles from the release point, collected tagged mosquitoes within three days. These experiments showed that the creatures can travel relatively long distances, probably by a combination of their own flying power and wind currents.
Mosquitoes were first clearly implicated as disease carriers in 1898, when members of the genus Anopheles were found to transmit malaria. The disease is caused by a protozoan carried in the mosquito's salivary glands. Currently, there are about 300 million acute cases of the disease, which kills 1 million people annually. Other tropical horrors include dengue and yellow fever, both of which are viral illnesses transmitted through mosquito saliva.
Closer to home, we have had our own mosquito-borne disease scare in the last several years, with the introduction of West Nile virus into this country [see "A New Disease in the New World," The World & I, February 2002, p. 156]. Starting with 62 cases of sickness and 7 deaths among the people of New York in 1999, the viral infection spread to most parts of the United States by 2002, leading to over 4,000 cases of illness and 274 deaths. In this era of borderless travel, perhaps additional arthropod-borne diseases may come calling. Nonetheless, most mosquito species do not carry diseases.
Given their track record as bloodsuckers and disease carriers, I am often asked about the usefulness of mosquitoes in the natural world. It is worth noting, therefore, that mosquito larvae are important as food for any number of aquatic predators, from fish to other insect larvae. The adults are readily gobbled up by spiders and dragonflies. Yet, contrary to popular opinion, mosquitoes are just a minor component of the diets of bats and purple martins.
If nothing else, mosquitoes are interesting for their collective development, persistence, and single-minded pursuit. Speaking of which, watch out for those that may come after you this summer!n
D. Scott Taylor is a land management specialist with the Brevard County Environmentally Endangered Lands Program in Melbourne, Florida. He also has 14 years of experience as a mosquito control biologist.
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|Author:||Scott Taylor, D.|
|Publication:||World and I|
|Date:||Jun 1, 2003|
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