INSECTS (Part 1 of 2).   Leave a comment

The total number of people, plants, and animals in the world is smaller than the total number of insects in the world.

Although over 800,000 insects have been described and named, there are still so many different kinds of insects on earth that scientists have named fewer than half of them. There are so many insects around us because insects are able to stay alive under many different conditions.

The world’s most abundant creatures are the insects, whose known species outnumber all the other animals and the plants combined. Insects have been so successful in their fight for life that they are sometimes described as the human race’s closest rivals for domination of the Earth. Entomologists, the scientists who study insects, have named almost 1,000,000 species perhaps less than one third of the total number.

Many kinds of insects are very adaptable. This means that they can live in almost any kind of weather and eat almost any type of food and still survive.

The most adaptable insects are usually small, require little food, and produce many young in a very short time. It is hard for humans to get rid of these insects. One example of a highly adaptable insect is the common cockroach, which can be found all over the world.

Insects thrive in almost any habitat where life is possible. Some are found only in the Arctic regions, and some live only in deserts. Others thrive only in fresh water or only in brackish water. Many species of insects are able to tolerate both freezing and tropical temperatures. Such hardy species are often found to range widely over the Earth. Few insects, however, inhabit marine environments. Small size, relatively minor food requirements, and rapid reproduction have all aided in perpetuating the many species of insects.

Many insects are parasites. Parasites cannot stay alive by themselves. They must live off the body of another organism, called the host.

The parasite stays alive by using the host’s body for food, water, warmth, and protection. Sooner or later, the parasites take so much from the host that they cause the host to die. An insect parasite may spend all or only part of its life inside a host.

Certain parasitic insects spend much of their lives on or within the body of an animal host, where all the comforts of life food, moisture, warmth, protection from enemies are optimal. Other kinds of insects spend all or some part of their lives securely enclosed in a food plant.

Rain, wind, cold, or human activity can quickly endanger insects. When insects are in trouble, they can:

  • fly, swim, or run away,

  • use their mouths or legs for fighting,

  • squirt poison at their enemies,

  • make themselves look bigger or smaller,

  • blend into the ground or plants around them,

or

  • use protective armour or spines.

Some species have become remarkably versatile in order to meet the changing demands of the environment. Various water bugs and water beetles are able to fly and swim, as well as crawl. Many types of insects, such as the bees, ants, and wasps depend on a complex social structure and defensive behaviour Non-predatory species frequently have special defences, such as an unpleasant taste or odour, venomous spines, or camouflage.

Insect numbers are kept down by sudden weather changes and by the creatures that eat them. Birds, fish, bats, spiders, and many other forms of life depend on insects for their food. In some areas, a very cold winter will kill many insects that would otherwise multiply in the spring.

Although they are adaptable and versatile as a group, insects are often unable to adjust to unusual weather conditions. Excessive rain, an unusually early frost, an extended drought these and other weather extremes can quickly wipe out or drastically reduce insect populations in a region. Because insects are an important item in the diet of many other animals birds, reptiles, amphibians, and fish, as well as other insects the number is constantly held in check.

The total of all factors unfavourable to insect survival is overwhelming; thus, in some species, out of hundreds of eggs laid by a single female, seldom do more than a few individuals reach adulthood. The survival of some species is enhanced by the large numbers of eggs laid.

INSECT STRUCTURE AND FUNCTION

Despite their diversity, all adult insects share some basic external and internal anatomical features. Insects are distinguished from other members of the animal kingdom by having six legs; one pair of antennae; a ringed, or segmented, body; and three well-defined body regions. It is from the joined body rings, or segments, that insects derived their name, for the Latin word insecta means “segmented.”

Many creatures closely resemble insects and are often mistaken for them for instance, spiders and scorpions, which have eight legs; centipedes, which have dozens of legs; and mites and ticks, which have sac-like bodies unbroken by segments. The name bug refers to certain insects with piercing and sucking mouth-parts but is also commonly applied to insects in general.

External Anatomy

The three main sections of an insect body are the head; the middle section, or thorax; and the hind section, or abdomen. The body is covered with a horny substance containing chitin. The protective armour plate also serves as an external skeleton, or exoskeleton, for the support of the internal organs.

The head bears the antennae, the mouth-parts, and the eyes. The thorax has three segments; on each is a pair of legs. In winged insects the thorax also bears one or two pairs of wings. The abdomen typically has 11 segments, though no more than 10 are visible; it contains a large part of the digestive system. In females the ovipositor, or egg-laying organ, is located at the tip of the abdomen.

Internal Organs

The nervous system of the insect includes a brain and a pair of parallel nerve cords, which extend along the length of the underside of the body. Along the nerve cords are a series of nerve masses, called ganglia. Each ganglion controls certain activities and is more or less independent of the others.

Insect blood is usually green, yellow, or colourless Few insects have red blood. The fluid is not enclosed in a system of arteries, veins, and capillaries but fills the body cavity. It is circulated by a tube that extends down the length of the body along the centre of the back. The tube has valved intake openings along its sides and is open at the anterior, or front, end. By means of muscles, it draws the blood through the side openings and pumps it forward into the head cavity and out again into the body. The pulsations of the tube can be easily seen in light-coloured caterpillars.

Air enters the body through breathing pores, called spiracles. A pair of spiracles is usually found on each of two thoracic segments and on several abdominal segments. From the spiracles, large air tubes called tracheae and smaller ones known as tracheoles carry air to all parts of the body. Some water insects breathe by means of gills. Other aquatic insects have a snorkel-like tube that leads to the water’s surface. Certain internal parasites and very primitive insects breathe directly through the body wall.

Mouth-parts

The mouth-parts of an insect can tell us about the kinds of food an insect eats.

Moths, cicadas, and butterflies have long slender tubes that they use to suck nectar out of flowers and into their mouths.

Caterpillars, grasshoppers,beetles, and crickets have chewing mouth-parts that may seem more complex than even a human mouth. The lower mouth-parts hold the food,and the upper mouth-parts chew the food.

Mouth-parts vary with feeding habits. For example, the mouth of a chewing insect, such as the grasshopper, has several parts. There is an upper lip, the labrum, and a lower lip, the labium. Between these are two pairs of jaws, which work sideways. The upper jaws, or mandibles, are for crushing; the lower pair, the maxillae, manipulate the food. On the maxillae and on the labium are two pairs of sensory structures called palpi. On the floor of the mouth is the tongue-like hypopharynx, which secretes digestive juices.

The sucking type of mouth is a modification of the chewing type. The butterfly’s coiled proboscis, or sucking tube, is a modification of the maxillae.

Sense Organs

The sense organs of insects are as varied as they are intricate. In some of these creatures the visual organs are capable of nothing more than distinguishing night from day. Others have eyes as efficient and sensitive as those of the vertebrates.

Insects have two different kinds of eyes: simple eyes and compound eyes.

Simple eyes are not paired, are very small, and act mainly as light detectors. They often act as helpers to the compound eyes, so the insect can react more quickly to any changes in the amount of light that is present. This is very important because many insects use light to tell them where they are. If you covered the simple eyes of a honeybee, it would still see with its compound eyes, but it would not be able to react as quickly to changes in light.

Insect eyes are of two general types simple and compound. Simple eyes, also called ocelli, are usually located in small clusters on the sides of the head or on the frons, or forehead. Although small, they may easily be seen by means of a magnifying glass. Ocelli are found in both immature and mature insects, but they appear to be more important in the mature forms. Individually these organs can do no more than detect light; however, the sensations received by several ocelli can together produce in the insect’s brain an image of the surrounding area as the creature turns its head from side to side.

A compound eye is actually a group of many eyes clustered together. These tiny eyes look like lenses or facets of the compound eye. Each one points in a slightly different direction and sees only a very small part of the world. The insect brain is able to put all of these tiny pictures together like a puzzle and form one overall picture. The more facets an insect has on its compound eye, the better it sees. Compound eyes are better than our eyes at seeing movement, which is very important since this may warn the insect that an enemy is near.

Compound eyes, like the sight organs of higher animals, are present in pairs, with one eye on each side of the head. They are most common in adult insects. Some certain mayflies, for example have two pairs of compound eyes.

The eyes are called compound because each one is composed of many lens-like facets. Each of these facets of which there are, for example, some 25,000 in a single dragonfly eye receives a separate image. The total effect of these images is a composite picture in the insect’s brain. The eyes of many insects bees, for example are sensitive to ultraviolet light, but insect eyes are generally less sensitive to colours at the red end of the spectrum.

The antennae are vital structures, because organs of taste, touch, smell, and hearing may be located in them. The loss of the pair of antennae usually leaves the insect in a shocked and helpless state. Their appearance and structure may vary greatly, even between insects of the same order.

The hearing organs of insects are well developed in many species and are found on various parts of the body. The ears of katydids and crickets are located on the tibiae of the forelegs. The typical grasshopper’s ear is clearly visible as an oval plate on the first abdominal segment.

The “Voices” of Insects

Insect sounds are produced by specialized structures to attract the opposite sex, to communicate with other members of a group, or to frighten enemies. Wings or mouth-parts may be rubbed together. Legs may be scraped against wings or bodies.

The grubs of certain wood-boring beetles produce sound by rubbing their legs together. The male cicada vibrates miniature “drum-heads” on the lower surface of its abdomen. The song of the female mosquito comes from the vibration of special bands stretched across its breathing organs.

Growth and Development

As they grow, the bodies of some insects go through major changes. Insect bodies are very different from human bodies. Whereas humans have an internal skeleton, the insect’s skeleton is a thick, hard, outer layer called an exoskeleton. Since this exoskeleton does not stretch, the insect must replace it with a larger covering when its body needs to grow.

There are different kinds of growth and development in insects. Some insects go through many changes from egg to adult, and some go through very few. Below are the types of development and examples of insects that grow that way.

DEVELOPMENT

CHANGES

EXAMPLES

No metamorphosis

Little change in appearance from birth to adult

Silverfish

Cockroaches

Incomplete metamorphosis

Young look like adults, but body parts do not work as they will in the adult

Grasshoppers

Crickets

Cicadas

Complete metamorphosis

Insect goes through many very different changes before becoming an adult

Butterflies

Ants

Bees

 

The development from egg to adult is most interesting, especially in those insects that go through the complex changes called complete metamorphosis. The growth of insects is quite different from that of vertebrates because the insect skeleton is an external covering rather than an internal framework. Except for the pliable fold between the plates of chitinous cuticle making up the exoskeleton, there is no place where expansion can occur; thus the growing insect must periodically shed, or moult, its covering. The new skin, already formed, then expands and begins to harden.

The offspring of all insects undergo a varying number of such growth intervals before maturity. Adult insects do not grow at all. With the exception of thesubimago (subadult) stage of the mayfly, only adults have functional wings. Primitive species such as silverfish mature with little change in appearance except their size. These kinds of insects are known as ametabolous insects. The immature insects of such species are called simply the “young.”

Immature grasshoppers, cicadas, the true bugs, and a number of other types resemble the adults in many respects but lack functional wings. Such young, called nymphs, are hemimetabolous or are said to exhibit incomplete metamorphosis. A variation of such development occurs in dragonflies, mayflies, and caddis flies. The nymphs of these forms are aquatic and have a way of life quite unlike that of the adults.

Bees, beetles, butterflies, and moths are some of the insects that go through all the changes of complete metamorphosis. They are said to be holometabolous. The young are called larvae (singular, larva). In the inactive stage immediately preceding adulthood they are called pupae (singular, pupa).

The larva hatches from an egg. Often larvae are mistaken for worms. They may be smooth-bodied, like the maggots of the fly, or hairy, like some caterpillars (literally, “hairy cat”), or they may be vicious-looking, like the grub of the tiger beetle. Larvae are classified into five forms, based on their shape: eruciform (caterpillar-like), scarabaeiform (grub-like), campodeiform (elongated, flattened, and active), elateriform (wire worm-like), and vermiform (maggot-like). Larvae differ from adults in many respects. The mouth-parts may be completely different. The mouth is always well developed, for this stage is the hungriest period of the insect’s life. Eyes, if present, are usually simple rather than compound. Certain structures found in the larva may be absent in the adult. Caterpillars, for example, have several additional legs, called prolegs, along the abdomen.

Near the end of its larval stage, the insect must find a place in which to pupate, or turn into a pupa. Beetle larvae, as well as certain caterpillars, may hollow out cells in the soil. Some caterpillars may spin silken cocoons about their bodies; some may spin bands to hold themselves against twigs or leaves. Some caterpillars hang upside down from silken pads. Hairy caterpillars pluck out their hairs to line the walls of their cocoons.

The pupal stage is a time of tissue transformation. During this period different kinds of mouth-parts, legs, eyes, and, perhaps, breathing organs must replace those of the larva. When the changes are completed, the creature bursts out of its old skin to become a fully developed insect. In this final, sexually mature state, it is also known as an imago.

HABITS AND BEHAVIOR

Each species of insect, in its struggle for survival, has developed complex behaviour mechanisms and habits. These involve every activity of daily life including egg laying, nest building, self-defence, and the search for food.

Egg Laying and Care of the Young

Most species of insects are of two sexes, but in some the white-fringed beetle, for example males are unknown. In certain insects the sex of the offspring depends upon whether or not the egg has been fertilized. The un-mated females of some parasitic wasps produce males only, while mated ones produce the two sexes in about equal numbers. The queen honeybee can lay either fertilized or un-fertilized eggs, according to the needs of the hive. Un-fertilized eggs produce drones, while fertilized eggs produce females.

The adult female instinctively places her eggs in a place suitable for their hatching and for proper development of the young. Parasitic wasps and flies place their eggs directly on the host. The horse botfly glues her eggs to the hairs of the horse where they can be licked off and thus be transferred to the horse’s stomach; the larvae live on the lining of the stomach and intestines. If a caterpillar feeds on only one species of plant, then the egg from which it will hatch is unerringly placed upon that plant.

Often insects’ eggs are hidden in special protective materials. They may be encased in frothy secretions which dry to form a hard covering, or clusters of eggs may be coated with hairs or scales from the adult insect’s body. The eggs of many species are inserted directly into plant tissues by means of saw-like or spear-like ovipositors.

The young of some insects are born alive. Such insects are called viviparous (from Latin vivus, “alive,” and parere, “bring forth”), to distinguish them from egg-laying insects, which are calledoviparous (from Latin ovum, “egg”). Aphids sometimes lay eggs and sometimes produce live young; female aphids also bear young for many generations without mating. This is calledparthenogenesis (from the Greek words meaning “virgin birth”). A few insects reproduce in the larval or pupal stages. This is known as paedogenesis (from the Greek words meaning “birth from young”).

Nests

Nest building as an adult activity is peculiar to ants, wasps, and bees. Carpenter ants live in galleries, which they chew out of tree trunks, logs, and fence posts. Mound-building ants construct cities in the soil, with thousands of chambers and passageways. The great paper apartment houses of the paper wasps and the honeycombs of the bees are considered to be marvels of engineering.

Nesting species must feed their larvae. Ants forage for food for their young. Some species raise fungus gardens and cultivate aphid “cows,” whose liquid excrement, or honeydew, they eat. The mud dauber wasp lays its eggs in tubes of mud. It then stocks the tubes with paralysed spiders and seals the tubes. After the larvae hatch, a sufficient food supply is at hand until they pupate.

How Insects Spend the Winter

Each species of insect usually passes the winter in one particular phase of development. Some butterflies winter as pupae, caterpillars, or eggs. The monarch butterfly migrates long distances southward in the fall; some survive for a return flight in the spring.

In the winter some insects may come out of hibernation during brief periods of mild weather. Snow scorpion flies and spring-tails are often found on snow or ice. Honeybees in well-protected hives use their body heat to maintain a hive temperature that permits them to remain somewhat active and to feed on stored sweets. They leave the hive when the temperature rises to about 55° F (13° C).

Extreme heat or drought brings about a period of inactivity called estivation. The eggs of mosquitoes do not hatch and the nymphs and adults of many aquatic insects become dormant when the breeding ponds and marshes dry up.

Protection from Enemies

Insects have developed many methods of self-defence to avoid being devoured by their enemies. Flight, concealment, motion, armour and weapons, and even grotesqueness are some of these methods. Certain insects are specially adapted for hiding. Vast numbers hide beneath stones or the bark of trees. The flattened bodies of cockroaches and bedbugs enable them to disappear into narrow cracks.

The most interesting means of concealment are mimicry and protective colouration. The walking-stick looks like a twig. Certain moths blend so well into the bark of the tree on which they rest that they cannot be distinguished from the tree. Some harmless insects resemble stinging species in shape or colour and so are avoided by predators. Certain moths and flies mimic bees.

Armour and weapons are well developed in many insects. The tough, horny covering of the beetles amounts to a solid shell of armour Sharp jaws and beaks, poisoned stingers, and spines serve as effective weapons. The extreme hairiness of some caterpillars makes birds and other predators avoid them, and in some caterpillars the hairs have venomous spines.

Stink glands in some insects repel attackers in the same way as those in a skunk. When disturbed, the bombardier beetle ejects an irritating gas from its tail. The gas may be fired repeatedly and audibly. Grasshoppers exude a fluid popularly known as “tobacco juice.” The flavour of some insects is so bitter or sour that would-be predators avoid eating them.

Response to Environment

The reactions that insects have to different stimuli are called tropisms. Here are some different types of tropisms and the reactions given by insects.

TYPE OF TROPISM

INSECT IS ATTRACTED TO OR REPELLED BY…

Chemotropism

Certain chemicals, usually related to a smell made by the insect’s food or mate

Phototropism

Light, either natural or man-made

Geotropism

Gravity

Thigmotropism

Touch, usually from a similar insect

Thermotropism

Heat

Hydrotropism

Water

Rheotropism

Currents, or flow, of water

Anemotropism

Currents, or flow, of air

Insects are not able to reason. They are guided by instinct and by physiological reactions to their environment. Such reactions are called tropisms, from the Greek word tropos, meaning “turn.” All tropisms involve turning toward or away from a stimulus.

Through chemotropism, chemical stimuli help insects find places to lay their eggs. The carrion beetle, for example, deposits eggs on decayed meat drawn to it by odour Butterflies and bees are attracted to flowers by odour as well as colour

The scent glands of various insects help them attract a mate. Insects also avoid certain substances by chemotropic reactions. Clothes chests made of cedar or camphor wood have long been used for storing woollens and furs because these woods contain substances repellent to clothes moths.

Moths are attracted to artificial light and moonlight but avoid sunlight. This is called phototropism.

Some moths use the sun as a point by which to guide their flight, always keeping the light source at an 80 degree angle from its eye. When a light bulb is on, a moth will use the light as its guide, but when it flies at an 80-degree angle to the light bulb, its circular path will lead it directly into the bulb.

Many insects seem to be attracted to or repelled by light (phototropism). Moths are attracted to artificial light and moonlight but avoid sunlight. Butterflies react in the opposite way. Cockroaches in a dark room hide when a light is turned on.

Response to gravity (geotropism) may govern the way various boring insects react. Thermotropism, or attraction to heat, may draw parasites to their warm-blooded hosts. Thigmotropism is reaction to touch. Some insects avoid all contact with others; some thrive in close contact. The swarming of bees may be due to an attraction to one another’s bodies. Attraction to water (hydrotropism), adjustment to currents of streams (rheotropism), and adjustment to air currents (anemotropism) may explain the behaviour of a wide variety of insects. However, no single stimulus governs all of their complex activities.

Posted 2011/11/06 by Stelios in Education

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