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MOSQUITOES   Leave a comment

DEFINITION: any of a large family (Culicidae) of two-winged dipteran insects, the females of which have skin-piercing mouth parts used to extract blood from animals, including humans: some varieties are carriers of certain diseases, as malaria and yellow fever.

More than just annoying insects, some mosquitoes are responsible for transmitting diseases that can result in serious illness and even death. Mosquitoes were once viewed merely as a nuisance because of the itching and irritation that resulted from their bites. In the early 1900s, however, they were recognized as carriers of yellow fever, malaria, and other diseases.

The mosquito is in the family Culicidae and belongs to the same order of insects as flies and gnats the order Diptera and has the same anatomical structure. Its soft body is covered by an exoskeleton (an external supportive covering) and divided into three parts: the head, thorax, and abdomen. It has two narrow wings and a pair of knob-like structures, known as halters, that are present in place of a second pair of wings. Unlike other Diptera, the wings of the mosquito have tiny scales on the veins.

The mosquito’s head is rounded and supported by a slender neck. It has large compound eyes, complex mouth parts, and two antennae, usually divided into 15 segments. The antennae of the male are more feathery in appearance than those of the female. The major body segment behind the head is called the thorax, to which the wings and six legs are attached. The legs are long, slender, and segmented. The final segment of the mosquito’s body is the soft, cylindrical abdomen. It has ten segments, the last ones bearing the openings for the anus and reproductive organs.

Proboscis, snout, trunk, or other tubular organ projecting from the head of an animal.

The most dangerous parts of a mosquito’s anatomy are the female’s mouth parts These are modified into a proboscis for piercing and sucking. The proboscis looks like a single thin tube and is straight in most species. It actually consists of a sheath (the labium) that encloses saw-tipped daggers (the mandibles and maxillae), an injection tube (the hypopharynx), and a sucking tube (formed by closing the labium against the hypopharynx). The construction of the proboscis is ideal for removing blood from beneath the skin of animals. The mouth parts of the male mosquito are modified for feeding on plant juices; male mosquitoes do not bite.

Mosquito Bites

Not all species of mosquitoes suck blood. However in some species a blood meal by the female is essential to the reproductive cycle. In most species the females, like the males, suck nectar and other juices from plants for nourishment. The bloodsucking species feed primarily on mammals or birds, though some mosquitoes will feed on reptiles and amphibians. Some species are particular in their choice of host species, whereas others appear to be less selective. The feeding periods of many types of mosquitoes are restricted to particular times of the day or night.

To obtain a blood meal, a female mosquito selects a likely spot on her victim, brings her labium against it, and begins sawing through the skin with her mandibles and maxillae. Through her hypopharynx she injects saliva into the wound to prevent the blood from clotting so that it flows freely into her labro-hypopharyngeal tube. She then sucks up a supply of blood, stores it in her abdomen, and flies away.

The itching of a mosquito bite is caused primarily by the saliva that has been injected. If the mosquito completes her withdrawal of blood before being driven away, much of the saliva will be removed and the itching may be less severe.

Life Cycle and Habitats

A mosquito’s life cycle is one of complete metamorphosis it consists of four distinct stages: egg, larva, pupa, and adult though the pattern of development may vary between species. The female mosquito typically lays her eggs in standing water, where they float on the surface in a tiny cluster. The eggs may also be deposited singly or attached to vegetation, depending upon the species of mosquito. Some mosquitoes lay their eggs in the vicinity of water rather than directly in water, and the eggs develop when the area becomes flooded.

During warm weather the eggs develop into larvae within two or three days. Mosquito larvae are long, transparent, and constantly wriggling as they move up and down in a water column. They feed on organic matter, including small animals, bacteria, dead plant material, and algae. Some species feed on other mosquito larvae.

Pupa, quiescent stage between larva and adult in insect metamorphosis.

As the larvae grow, they periodically shed their skins (called moulting) in order to accommodate their larger bodies. Mosquito larvae normally moult four times. After the final moult the animal emerges as a pupa. The pupa has an enlarged anterior portion, composed of a head and thorax, and a curved, elongate abdomen. The pupa is aquatic but does not feed. Both the larvae and pupae of most species must come to the water’s surface to breathe. After two or three days the pupa develops into an adult, emerges from its pupal case, and flies away.

Mosquitoes vary in their courtship and mating habits. Many species mate while in flight. The males of some congregate in huge swarms, to which the females are then attracted. The humming sound made by mosquitoes is often a signal to attract mates.

In cooler temperate regions, adult mosquitoes hibernate, emerging in the spring to lay eggs. In some species mating occurs before the approach of winter and the males die, leaving only fertilized females. In others, eggs are laid in the fall and survive the winter without harm to hatch in the spring.

Mosquitoes are found almost everywhere in the world except open ocean areas, the most arid deserts, and the polar regions. Because of their dependence on water for development during their first stages of life, mosquitoes are most abundant in wet regions of the world. Nevertheless, of the more than 150 species of mosquitoes that inhabit the United States, many persist in arid regions of the South west Some species thrive in the extremely cold climates of Canada and Alaska, where vast swarms can sometimes be seen around some of the larger lakes and marshes.

Mosquitoes live in a wide variety of aquatic habitats. Besides lakes, ponds, and marshes, some mosquitoes lay their eggs in small depressions where water has collected temporarily. For example, many species use tree holes or fallen leaves, where water has accumulated after rains. In urban areas, common egg-laying sites for mosquitoes are empty containers that have collected water. Furthermore, mosquitoes are not restricted to fresh water for egg laying salt marshes are also a common habitat of many species.

1902: Cure for yellow fever. Walter Reed was a physician and bacteriologist in the service of the United States Army when he proved that yellow fever is transmitted by mosquito bites. Throughout the 19th century the general assumption was that yellow fever was transmitted by contact with such articles as clothing or bedding touched by someone who had the disease.

A Cuban doctor, Carlos Juan Finlay, theorized that the disease was carried by insects, but he had not been able to prove it. In 1896 an Italian scientist, Giuseppe Sanarelli, isolated the organism Bacillus icteroides from yellow fever patients. Reed, along with physicians James Carroll and Aristides Agramonte, was assigned the task of investigating the bacillus. At the same time, a yellow fever outbreak started in the American military garrison in Havana, Cuba. The three travelled there in the summer of 1900 and, by 1902, proved that mosquitoes were the carriers of the disease.

Shortly afterwards an insect extermination program was undertaken, and Havana was freed of yellow fever within 90 days. Colonel William Crawford Gorgas of the U.S. Army Medical Corps later used Reed’s techniques to rid Panama of yellow fever, making way for the construction of the Panama Canal.

Mosquitoes and Disease

Mosquito-transmitted diseases differ in their geographic distribution, specific causes and effects, and in the types of mosquitoes that transmit them. Yellow fever is caused by a virus that is transmitted primarily by the mosquito species Aedes aegypti, found in tropical and warm temperate regions of Africa and the Americas.

The primary mechanism of transfer of the yellow-fever virus (as well as other disease-causing organisms) is the mosquito bite specifically, when a mosquito bites an infected person and then bites a healthy one. The virus is thus passed from one person to another through the fluids from the mosquito’s mouth. The yellow-fever virus can also be present in other mammals, including monkeys, armadillos, and rodents, and a mosquito can transmit the disease to humans after biting an infected animal. Yellow fever attacks the liver, kidneys, and digestive tract, producing high fever and jaundice, a yellow skin colour from which the disease gets its name. More than half of the victims of yellow fever die within a few days. Those who recover are immune thereafter.

Malaria, disease consisting usually of successive chill, fever, and “intermission” or period of normality.

Malaria is another disease transmitted by mosquitoes. It is caused by microscopic protozoan parasites of the genus Plasmodium. The transmission of malaria is more complicated than that of yellow fever because the parasite must spend a portion of its life cycle inside a mosquito and the other part inside a human. (Yellow fever is dependent on the mosquito only as a transmitting agent.) Malaria is transmitted by mosquitoes in the genus Anopheles.

When an Anopheles mosquito bites a person infected with malaria, it may ingest blood that contains parasites in the sexually reproductive stage, called gametocytes. These gametocytes unite in the mosquito’s digestive tract and produce egg-like cells that burrow into the intestinal wall. They then hatch into free-swimming forms that travel to the mosquito’s salivary glands.

When the mosquito bites an uninfected human, the free-swimming parasites are transmitted to the victim through the mosquito’s saliva. These tiny parasites then enter the victim’s red blood cells and begin to divide to form new parasites. Eventually, the affected blood cells burst, and the parasites are released to enter new blood cells within the host and repeat the process of growth and division.

Within one to two weeks millions of these parasites are being released from burst blood cells, resulting in the characteristic symptoms of malaria: periodic chills and fever. Within ten days to two weeks after the initial infection, a new generation of sexually reproductive parasites develops in the blood of the victim. These parasites produce gametocytes, and victims can then infect any Anopheles mosquito that bites them. In this way the cycle of the disease is perpetuated.

In many areas of the world, including North America, mosquitoes of the genus Culex are transmitters of viral encephalitis (sleeping sickness) and other diseases. Dengue, or “break bone fever,” is a common tropical disease that results in muscular pains and eruptions of the skin. It is transmitted by Aedes and Anopheles mosquitoes.

Filariasis, disease caused by roundworms and transmitted by mosquitoes.

Roundworm, worm of the phylum Aschelminthes and the class Nematoda.

Filariasis, a disease that affects the lymph glands, is caused by parasitic roundworms and is transmitted by several different mosquito species in tropical regions.

Mosquito-transmitted diseases can be controlled through the elimination of mosquitoes or their egg-laying sites, medical treatment of victims, and prevention of mosquito bites through the use of insect repellent or protective clothing. As early as the 1700s South Americans recognized that quinine, an alkaloid obtained from the bark of the cinchona tree, alleviated the symptoms of malaria, though they did not know how the disease was transmitted.

In the late 1800s mosquitoes were implicated in the transmission of yellow fever in Cuba, and in the transmission of malaria in India. The United States Army initiated the first major effort to eradicate a mosquito-transmitted disease when it launched its campaign to quell the Cuban yellow-fever epidemic. Once the relationship between mosquitoes and yellow fever was understood, major projects were undertaken to eliminate the egg-laying sites of the Aedes mosquito. Similar measures were taken in malaria-infested areas of the world.

In addition to eliminating mosquito habitats, large-scale production began of chemical products that would kill mosquitoes or their eggs. Aerial sprays have been developed to kill adult mosquitoes. Toxic chemicals and oil products have been used in aquatic habitats to kill mosquito eggs, larvae, and pupae. Such chemicals must be used with caution because of their potentially damaging environmental effects. Mosquito bites can be prevented effectively with the use of a wide variety of insect repellents.

Assisted by J. Whitfield Gibbons, Senior Research Ecologist and Professor of Zoology, Savannah River Ecology Laboratory, University of Georgia.

Posted 2012/04/29 by Stelios in Education

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BEETLES (Part 1 of 2)   Leave a comment


Giraffe beetle. Also Giraffe weevil.

DEFINITION:1 any of a large order (Coleoptera) of insects, including weevils, with biting mouth parts and hard front wings (elytra ) that cover the membranous hind wings when the hind wings are folded 2 any insect resembling a beetle.

There are more species of beetles than of any other kind of insect. They constitute the largest order of insects Coleoptera which includes almost one third of a million recognized species. About 20 percent of all known species of animals in the world are beetles.

Beetles are found throughout all continents except Antarctica. Although most species are terrestrial, many such as the whirligig, water scavenger, and true water beetles have become adapted to aquatic environments. Some beetles are only about 0.01 inch (0.025 centimetre) long, whereas tropical rhinoceros beetles and Goliath beetles may reach lengths of 4 to 6 inches (10 to 15 centimetres).

Beetles display a remarkable array of colours, forms, and habits. Some are plain black or have brownish patterns that help to camouflage the insects against certain types of wood or soil. Some beetles are brilliant orange, red, or yellow; others are iridescent green or blue or have a metallic sheen. The antennae of some beetles are large and ornate. Some stag beetles have enlarged, hooked mandibles, or lower jaws, that are almost as long as the beetle itself. Male rhinoceros beetles have huge horns projecting over their heads. The shapes of beetles’ bodies vary from round to elongate. Some are flattened; others are domed or cylindrical.

Some beetles are of great significance to humans. Members of the family of beetles known as weevils, or snout beetles, are notorious agricultural pests. They have specialized, elongated heads and down-curved snouts with mouth parts at the end. Some beetles feed on plant materials such as wood, paper, and fabrics. The larvae of some dermestid beetles are destructive pests of clothing and carpets and even of plant and animal specimens in museums.

Many beetles are valuable because they prey on destructive insect pests. Ladybugs, for example, destroy untold numbers of aphids each year and so protect a wide variety of flowers and vegetables. Many other beetles play more subtle but equally important roles in various ecosystems. Dung beetles, or tumble bugs, eat vast quantities of dung in livestock areas. Carrion beetles are scavengers whose larvae feed on dead animals. Many beetles pollinate flowers.

Physical Characteristics

Mandible, from Latin mandere, to chew; term applied to: (1) chewing jaws of insects and other arthropods; (2) the lower jawbone of mammals; (3) the upper or lower part of a bird’s beak.

Like other insects, beetles have three major body segments: the head, with a single pair of antennae and a pair of compound eyes; the thorax, which bears two pairs of wings and three pairs of legs; and the abdomen, where the reproductive organs are housed. Beetles have chewing jaws called mandibles and paired structures known as maxillary and labial palpi (singular, palpus) that are used for feeding or handling food. The bodies of beetles and other insects are covered by a usually hard layer known as the cuticle that supports the internal organs and protects the body. The cuticle is hard because it contains a substance called chitin. Each defined, plate like area of the cuticle is called a sclerite.

A distinctive feature of beetles is their front pair of wings, which are thick, hard, and opaque, without the veins characteristic of most other insect wings. These fore wings, called elytra (singular, elytron), serve as protective wing covers for a second pair of functional wings underneath. The hind wings are membranous and translucent. These are ordinarily used for flying, while the heavy elytra are held out of the way. When the beetle is at rest, the elytra fold over the back and form a straight line down the centre where they meet. Some beetles have shortened wings, and a few species are entirely wingless.

Life Cycle and Behaviour

Like other insects, beetles reproduce sexually by means of internal fertilization. The ovaries of the female and testes of the male are enclosed within the abdomen. In some species, such as the stag beetles, males engage in combat with one another for the right to mate with the females. After mating, the females lay the fertilized eggs in a location suitable for development of the larvae.

Beetles undergo a complete metamorphosis: they develop from egg into active larva into inactive pupa and finally into an adult. The larva, or grub, does not resemble the adult in structure. The pupal stage though soft, pale, and immobile does have the body form of an adult. The life spans of beetles range from a few months in some species to more than four years in others.

Feeding habits. Most beetles feed on living or dead plant materials, but some are scavengers of dead animal matter and some prey on other insects. A few are parasites.

The adults and larvae of a number of beetle species feed on various plant roots, stems, fruits, seeds, and foliage. Some beetles feed only on certain plant species and plant parts, whereas others are less particular in their choice of foods. The adults and larvae of many beetles feed on decaying wood and help break down dead trees and other vegetation in forest habitats.

Some beetles, such as tiger beetles, are voracious predators. Adult tiger beetles search for prey that they can subdue with their powerful mandibles. The larvae are sedentary; they live in small tunnels where they wait to capture passing insects. Water scavenger beetles are predators as larvae but are plant eaters as adults. Some species of beetles have highly selective feeding habits: they may eat only mites, ant larvae, aphids, or zoo plankton

Defences Although most beetles are protected by their heavy armour, some species have developed additional methods of defence Blister beetles secrete an oily, blister-causing substance that deters predators. Beetles may also discourage or avoid predators by making a startling noise (see below, “Light and sound production”), secreting or ejecting an obnoxious fluid, biting, hiding (using their natural colouring as camouflage), or simply fleeing on foot or on wing.

Luminescence, emission of light resulting from causes other than high temperature.

Light and sound production. Many beetles are capable of producing light and sound, primarily for the purposes of attracting a mate or for frightening enemies. The familiar fireflies, or lightning bugs, are beetles that have special light organs on the underside of their abdomens. These beetles usually the males flash their lights rhythmically as a signal that they are ready to mate, and the females return the signal. The kind of signal system used by the two fireflies allows males and females of the same species to recognize and locate one another. Some tropical click beetles have large, luminescent eye spots on the back of the thorax that presumably are also used in courtship.

Many species of beetles make sounds by rubbing together hard parts of their bodies a practice called stridulation. The vibration created by the friction of these parts produces a shrill creaking noise. Beetles may stridulate by rubbing the two elytra together, by rubbing a hind leg against an elytron, or by rubbing the head against the front of the thorax. In some species, even the immature grubs can produce sounds. Although stridulation is often used by adult beetles as a mating signal, its purposes in other instances by juveniles, for example are not fully understood.

A wood-boring beetle known as the death-watch beetle strikes its head against the sides of its burrow as a mating signal. The name death-watch is derived from the superstition that the sound was an omen of death. One explanation is that the ticking sound of a death-watch beetle that had made its burrow in an old piece of furniture was most often heard late at night by someone sitting at a sickbed.

When threatened by a predator, bombardier beetles squirt, with a loud popping sound, an unpleasant-smelling liquid from the rear of their abdomens. The noise and the ejection act together to startle and repel the predator and give the beetle time to make its escape. When click beetles fall on their backs, they right themselves by snapping their bodies in such a way that they are tossed into the air with a loud clicking sound that can startle a predator.

Kinds of Beetles

There are many families of beetles about 135, according to some experts. The beetles discussed below represent a sampling of some of the most commonly known as well as some of the most unusual beetle families in the order Coleoptera.

Tiger beetles and ground beetles are the most common beetles in North America. The fierce, long-legged tiger beetles are fast-running, fast-flying, often brightly coloured beetles that capture and eat other insects. Species of tiger beetles occur throughout the world but are especially abundant in the tropics.

Ground beetle, one of a group of the order Coleoptera, family Carabidae; especially the fiery searcher (Calosoma scrutator), one of the largest beetles; if held carelessly will discharge quantities of “fiery” juice.

Ground beetles are also abundant in most parts of the world. Many species are black and shiny; some are iridescent. Like the tiger beetles, ground beetles have long legs. Some have enlarged, pinching mandibles that are used to capture prey. Many are nocturnal.

True water beetles (also known as diving beetles or predaceous diving beetles) are oval-shaped insects that can swim, dive, and fly. They are found in most freshwater habitats worldwide but are most common in northern temperate regions. The hind pair of legs of the true water beetle are long, flattened, and fringed to provide a greater surface area that helps the insect float. The beetle breathes through spiracles openings on the abdomen just under the tips of the elytra. Before diving, the beetle collects an air bubble beneath its elytra and then breathes from the bubble while it is underwater. It is carnivorous, preying on insects and other aquatic organisms, including fish larger than itself. The larvae of the true water beetles are sometimes called water tigers because of their voracious appetites. True water beetles often fly from one aquatic habitat to another and may be seen around outdoor lights at night.

Whirligig beetles, like the true water beetles, are oval-shaped aquatic predators that can swim, dive, and fly. They are known for their gregarious habits they are usually seen in groups, spinning and whirling around on the surfaces of quiet ponds or lakes. They have distinctive, divided eyes a top pair for seeing above the water’s surface and a bottom pair for seeing below.

Posted 2012/01/25 by Stelios in Education

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