Archive for August 2012

RAINBOWS   Leave a comment

DEFINITION: an arc or ring containing the colours of the spectrum in consecutive bands, formed in the sky by the refraction, reflection, and dispersion of light in rain or fog.

When light from a distant source, such as the sun, strikes a collection of water drops such as rain, spray, or fog a rainbow may appear. It appears as a multicoloured arc whose “ends” seem to touch the Earth. Rainbows are seen only when the observer is between the sun and the water drops, so rainbows appear in the part of the sky opposite the sun. The centre of the rainbow’s arc is located on an imaginary line extending from the light source through the observer’s eye to the area of the water drops.

Rainbows are most commonly seen when the sun’s rays strike raindrops falling from distant rain clouds. Generally, this is only in the early morning or late afternoon. When the sun is too far above the horizon no rainbow can be seen.

When the sun is lower in the sky, however, part of the arc becomes visible. In fact, if the sun is low enough and the observer is located in a place that is high enough, such as on a mountain, in an aeroplane or a spaceship, the observer may see a circular rainbow.

The most brilliant and most commonly seen rainbow is called the primary rainbow. The arcs of colour in a rainbow are caused by the refraction, or bending, and internal reflection of light rays that enter the raindrops. A ray of white sunlight is actually composed of all the colours of the spectrum. Inside the drop the ray of white light is separated into the colours that make it up and reflected back toward the observer. In the primary rainbow the colours are, from inside to outside, violet, blue, green, yellow, orange, and red. The red band makes an angle of about 42 degrees with the sun’s rays, and the other coloured bands make successively smaller angles. Sometimes another less intense rainbow may also be seen; this is called the secondary bow. The secondary bow, when visible, is seen outside the primary bow and with its colour sequence reversed. It is produced by light that has been reflected from two different points on the back of the drop before emerging into the air. Higher-order rainbows are very weak and so are rarely seen.

Occasionally, faintly coloured rings are seen just inside the primary bow. These are called spurious, or supernumerary, bows. When raindrops are extremely fine, an almost white bow, called a fog bow, is produced. A fog bow at night, sometimes called a lunar rainbow, is made by sunlight reflected from the moon and appears as a ring around the moon.

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Posted 2012/08/26 by Stelios in Education

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CATERPILLARS   Leave a comment

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DEFINITION: the worm like larva of various insects, esp. of a butterfly or moth [C-] trademark for a tractor equipped on each side with a continuous roller belt over cogged wheels, for moving over rough or muddy ground.

The larvae, or young, of butterflies and moths are called caterpillars, from the Latin catta pilosa, meaning “hairy cat.” Although people usually recognize the hairy kinds, many caterpillars with bare skins are popularly called worms, such as the cabbage worm and army worm

A caterpillar’s body consists of a head followed by 12 or 13 segments. Like all insects, it also has three pairs of permanent or “true” legs one pair on each of the first three segments directly behind the head. These true legs are usually hard, jointed, and tipped with tiny claws, but in a few caterpillars they are not developed. To support the rest of its long body, the caterpillar also has from two to five pairs of soft, thick prolegs that disappear when it changes into a moth or butterfly.

A caterpillar has six eyes like tiny beads on each side of the head, just above the strong upper jaws. It breathes through nine pore like openings, called tracheae, on each side of its body.

When a caterpillar hatches from the egg laid by a female butterfly or moth, it is usually very small. But it grows rapidly and soon gets too large for its skin. Thereupon the old skin splits, and the caterpillar wriggles out of it, revealing a soft new covering. This skin-shedding is called moulting and occurs four or five times. Some caterpillars eat their old skins. The hawk moth caterpillar, one of the largest, may grow 4 inches (10 centimetres) long; the clothes moth caterpillar, one of the smallest, seldom exceeds a quarter of an inch. Some caterpillars may take only a few days before they turn into butterflies or moths, but most last throughout the warm season. A very few may live as long as four years in the caterpillar form before they change.

Cocoon, envelope, often largely of silk, which an insect larva forms around itself.

The change that caterpillars undergo is called metamorphosis. The first step for many moth caterpillars is to build cocoons. They spin them with threads of sticky fluid that flows from an opening in the lower lip and hardens in the air.

Some caterpillars form bags of silk that entirely enclose them. Others roll up a leaf, fastening the edges with the silk. Many of the hairy kinds pad the cocoons with their own hair.

Some caterpillars do not build cocoons. Many of the moth caterpillars take shelter simply by burrowing in the ground or under a stone or fallen leaf. Butterfly caterpillars may suspend themselves from leaves or twigs by their tails, or spin a button of silk on a twig or leaf and hang from it by a silk girdle.

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

Whether protected by a cocoon or not, the caterpillar becomes ready to shed its last skin, and in place of it grows a tough flexible shell or case. When this happens it has become a pupa. The moth pupa is usually dull brown and mummy like The butterfly pupa, sometimes called a chrysalis, is shiny and often brilliantly coloured

Inside the pupal, or chrysalids, case, the rudimentary wings and other organs enlarge to make the moth or butterfly. This transformation from the larval to the adult stage may be completed in a few days or take several months.

To grow and prepare for this period of change, caterpillars eat enormously, causing widespread damage to trees, flowers, and crops. The larva of the Polyphemus moth, a species of the American silkworm, has been estimated to eat as much as 86,000 times its own weight during its 56 days as a caterpillar.

Caterpillars are the prey of many birds and insects, especially parasites. To avoid their attacks, caterpillars have various natural protections. Some are coloured to blend with their surroundings. Others have gaudy dots or stripes to make them look fierce or very large. A few give off unpleasant smells, and a very few grow poisonous nettle like hairs.

Posted 2012/08/19 by Stelios in Education

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