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| Sunday, 3 April 2005 |
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Junior Observer |  |
| News Business Features Editorial
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Creepy-crawlies that dominate the world
They were among the first land-dwelling animals, making their appearance over 400 million years ago; even today, they dominate the world because no other living creature matches them in number. They are found everywhere in abundance, except in the polar regions. Know who they are? Insects and arachnids (spiders, scorpions and mites). Even though we may not like many of them, it is hard to exterminate (destroy) them because they multiply rapidly. Their numerous activities benefit man in various ways; they pollinate flowers, control weeds and help remove dead animals. Insects and arachnids have many common characters but also several basic differences. They have an external skeleton and their bodies are made up of a number of segments. But the two groups have different ways of getting air into the body for respiration. Most insects have a network of tracheoles (narrow tubes running through the body and opening to the outside through spiracles). Many arachnids have cavities that contain gills which, from their resemblance to the pages of a book are called 'lung-books'. Generally insects have three pairs of legs while arachnids have four pairs.
Arachnids have two short, paired structures called pedipalps and insects have antennae, long 'feelers' which are covered with microscopic sense organs. Any small arthropod (animal with jointed limbs and segmented body) with wings is an insect. However not all insects have wings while no arachnids come with them either. Let's look at some of these small but fascinating creatures that are found in abundance not only in number but also in variety, dominating the world... Springtails Unquestionably the oldest group of six-legged animals, the majority of springtails have a built-in 'pogo-stick' at the tip of the abdomen. Distributed worldwide, they live in leaf litter, soil dwelling, under barks and also on the surface of ponds, glaciers and snow. They have a body length of 1-5mm. These small creatures which are everywhere are generally harmless. A few such as the Lucerne Flea, are crop pests. The Glacier Springtail or Snow Flea is active at low temperature. Would you believe that in temperatures below freezing point, when most insects are inactive, the snow flea is very active, jumping and moving around? Most springtails multiply rapidly under suitable conditions, with huge numbers suddenly appearing even on the surface of snow! Dragonflies
They have six legs but yet cannot walk! Amazing isn't it? Dragonflies, which are distributed worldwide and are most numerous in the tropics, usually near fresh water, are an ancient group of insects. Giant dragonfly - like insects with wingspans of about 30 inches were in existence over 300 million years ago and the 'true' dragonflies were flying over the earth 200 million years ago. The largest living dragonfly, from South America, has a wingspan of around 19 cm (7.5 in). These large dragonflies cannot fold their wings. So, they hold their wings at right angles to the body. The dragonfly's forewings and hindwings beat alternately, not together as in most insects. This gives them even more flight control and they can hover, moving backwards or dart forward at high speed. Coming back to the six legs, while larger dragonflies cannot walk with them, and hardly move on the ground, the smaller ones, the damselflies, use their legs to move up plant stems when they perch. Rarely found far from water except during migration, adult dragonflies feed on flying insects; aquatic larvae prey on other aquatic insects, small fish and tadpoles. Dragonflies are useful and totally harmless insects. Praying mantises
There are many species of the praying mantis and their habit of sitting motionless with their 'arms' outstretched in supplication has given rise to their name. They are sometimes called holy terrors. The females attack and eat the males. The praying mantis's front legs are specifically modified for grasping prey, usually other insects, and then carrying them up to their strong jaws. Their legs are like the blades of a penknife, snapping back to catch their prey. They either sit quietly, waiting for their prey to land near or stalk stealthily and suddenly grab them. Because of their need to be camouflaged, mantises are generally green or a mix of green and brown to match the plants they sit on. Some even take the colours of the plant's flowers.
A few species have taken the deception further. On each forewing, they have two rounded patches, usually with lighter centres which resemble eyes. They are called eyespots. A predator is suddenly confronted with these 'two eyes' when the mantis spreads its wings a little. Most abundant in the tropics and subtropics, several species are also found in southern Europe. Giant wetas The world's most formidable cricket, it is not an insect one would normally pick up! With a length of 10 cm (4 in) and spiny hindlegs twice as long, this ugly creature can easily inflict a nasty wound on anybody who picks it up. Found in New Zealand, these insects are generally harmless and live on trees where they are remarkably well camouflaged. The females are larger than the males and lay around 600 eggs! These insects have formidable jaws with which they chew fleshy leaves, but they are omnivorous and will eat any insect they manage to catch. The strong spiny hindlegs can be moved up and down to make a distinctive rasping sound. If anyone is bold enough to catch one of them, these spines could scratch the person.
There are several species of weta in New Zealand. Some of them live in caves, while the other ground-dwelling ones are called 'invertebrate mice'. Like the dragonflies, wetas too are an ancient group. Fossils are known from over 180 million years. Several species including the Giant Weta are classed as threatened species. They are protected by law. Honey ants There are a number of species of honey ants throughout the world. They are also known as desert or honeypot ants and are considered a delicacy. The ant pictured here is found in Southern United States and in Mexico. Other species of honey ants are found in Australia and New Guinea. They live on small insects but some are fed by worker ants on liquid honeydew gathered from greenfly and sap from plants. This honey is stored in their abdomen which swells up and stretches, turning golden-yellow with the honey. These living 'honeypots' cannot move and usually attach themselves to the roof of the nest, hanging down like golden pendants. When the ants' nest is in short supply of honey, these honey pots release their honey, drop by drop, to the workers. But no one knows what happens to them once they empty their pots - whether the same honey pots are used to refill or whether they die off. When consumed as a delicacy, the abdomens are nipped off and eaten! Black widow There are a number of spiders called the 'black widows' whose very name creates fear in man, but the most widespread and feared of group is the American species. The female is the killer because the smaller male cannot inject sufficient venom to kill humans. Fortunately the Black Widow is not an aggressive species and tries to avoid man. Its habit of hiding makes it difficult for us to spot it, and so, when we come across it unexpectedly, the spider tends to bite out of fear of being attacked. The venom of a Black Widow spider is said to be 15 times more toxic than that of the rattlesnake in USA. The Black Widows live in meadows and pastureland. In the wild they capture other insects which they paralyse with their venom before sucking out the body fluids. Like many spiders, the Black Widows cause their webs to vibrate rapidly to make it difficult for them to be spotted in the centre of the pulsating web. These are just a few of the fascinating creatures you have learned about today. Read more on the world of insects and discover more about the various types of insects and arachnids that inhabit our Earth! |
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