2012-06-22 / Columnists

Drawing On Science

The Mosquito And You
Commentary By Stephen S. Yaeger

There is probably no other insect more vilified than the mosquito. It is well known for its incessant, nocturnal buzzing and most annoying, itching punctures. The insect first appeared some 175 million years ago during the Jurassic Epoch long before we humans entered the picture. Fossilized specimens have been found having a length of 5.0 centimeters (about 2.0 inches) as opposed to the present day insect reaching a maximum of about 0.5 centimeters (about 0.25 inches). Mosquitoes can be highly adaptable to the environment. A recent article in Science News reports that scientists subjected mosquitoes to falling drops of water to test their ability to survive raindrops having a mass 50 times that of the mosquito. Rather than resist the collision with the raindrop, the mosquito moves with it basically “surfing” with the falling drop. Its ride subjects the insect to a force of gravity 100 to 300 times greater than that of Earth’s gravity.

Some species of mosquitoes host viruses which are responsible for a number of diseases including malaria, yellow fever, dengue fever, St. Louis and Western Equine encephalitis, and West Nile disease. Only an adult, newly mated female mosquito can transmit a disease. It is when a mated female picks up a pathogen from a host and then bites another host will a disease be transmitted. When the adults emerge from the water they take flight. They mate and soon thereafter the female seeks a blood meal to sustain the developing eggs within her body. It is the protein in the blood that supplies the necessary nourishment for the embryos. The male feeds on plant nectar and will live a very short time after mating. The fertilized female will seek stagnant water whether it is permanent such as a pond or transient such as a rain pool. Depending on the species of mosquito, she deposits her eggs as a raft on the surface of the water or on dry leaves near water. In the latter case rain will immerse the eggs stimulating them to hatch. The mosquito’s life cycle is called complete metamorphosis: egg to larva to pupa to adult. (In complete metamorphosis the larva does not look at all like the adult – it must go through a pupa stage to do so.) Within a few days the eggs hatch releasing the larvae. This stage requires air to breathe so the larva remains just below the water’s surface extending a breathing tube out of the water. The larvae are filter feeders filtering organic matter from the water. They shed their exoskeleton four times before becoming a pupa. The entire life cycle takes about eight to ten days to complete.

After a female deposits her very first raft of eggs she again seeks blood to nourish a second batch. This process may continue for many times depending on weather conditions and the ability of the female to continue producing fertilized eggs; a process which requires no additional mating. Some spring species of mosquitoes produce only one generation per year. Many species will overwinter in the egg stage until warming spring water stimulates the eggs to release their larvae. Some species can overwinter as larvae and a few species’ egg-carrying females can overwinter in sewers, cellars or other well-hidden locations. The arrival of spring activates these females to seek a blood meal to begin the cycle once more.

The mosquito’s head is well-equipped to make you miserable. Mosquitoes have very poor eyesight due to the construction of their eyes. Their compound eyes consist of many individual lenses which are separated by blind spots. This arrangement of the lenses prevents the insect from seeing an object (like your arm) until it is some 30 feet away. But though the female has no idea what she is looking at she can detect movement. And if it moves, it may be a blood source. At some 10 feet from your arm the female’s sensitive thermal receptor at the tip of its antennae kicks in. If heat is detected then the target might be a mammal or bird or your arm. Chemical sensors, also located on the antennae, are used to detect carbon dioxide, uric acid and lactic acid. If you are a good producer of these chemicals you become the mosquito’s choice for blood. Then there’s the mosquito’s piercing parts (mosquitoes don’t bite): the palpus and the proboscis. The former detects chemical substances and the latter, found only in females, is the mosquito’s weapon of choice for piercing your skin. The female’s proboscis consists of paired mandibles and maxillae forming a needle-like structure. When the mosquito settles ever so carefully on your arm she locates a blood vessel, and pierces the skin. The maxillae are used to anchor the mouthparts so that the other straw-like mouth structures can be inserted into the wound. Saliva, containing an anticoagulant, is injected to prevent clotting. This allows the mosquito to draw out and enjoy feeding on your blood. When she is satiated she flies off with a bloated body.

In the meantime you are now left with a small puncture loaded with the female mosquito’s saliva containing digestive juices and anticoagulants. Your body detects these substances as foreign material and produces histamine to prevent any infection. The histamine finds the wound and causes blood vessels to swell leaving a red bump called a wheal. The expanding blood vessels cause nearby nerves to become irritated giving you the classic itching sensation which temporarily makes your life miserable. Questions/ comments? E-mailSteve: Drawingonscience.

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