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| 13.30 |
Level IV life: Insects
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Insect, is common name given to any
animal of a class belonging to the arthropod category. The insects make up the
largest class in the animal world, outnumbering all other animals. At least
800,000 species have been described, and entomologists believe that as many or
more remain to be discovered. The class is distributed throughout the world
from the polar regions to the Tropics and is found on land, in fresh and salt
water, and in salt lakes and hot springs. The insects reach their greatest
number and variety, however, in the Tropics. In size, the insects also exhibit
great variation. Some small parasitic insects are less than 0.25 mm (0.01 in)
in length when fully grown, whereas at least one fossil species related to the
modern dragonflies is known to have had a wingspan of more than 60 cm (24 in).
The largest insects today are certain stick insects about 30 cm (12 in) long
and certain moths with wingspans of about 30 cm (12 in). |
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Insects are also the most highly
developed class of invertebrate animals, with the exception of some molluscs.
Insects such as the bees, ants, and termites have elaborate social structures
in which the various forms of activity necessary for the feeding, shelter, and
reproduction of the colony are divided among individuals especially adapted for
the various activities. Also, most insects achieve maturity by metamorphosis
rather than by direct growth. In most species, the individual passes through at
least two distinct and dissimilar stages before reaching its adult form. |
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In their living and feeding habits, the
insects exhibit extreme variations. Nowhere is this more apparent than in the
life cycles of different species. Thus the so-called 17-year cicada matures
over a period of 13 to 17 years. The ordinary housefly can reach maturity in
about ten days, and certain parasitic wasps reach their mature form seven days
after the eggs have been laid. In general, the insects are very precisely
adapted to the environments in which they live, and many species depend on a
single variety of plant, usually feeding on one specific portion of the plant
such as the leaves, stem, flowers, or roots. The relationship between insect
and plant is frequently a necessary one for the growth and reproduction of the
plant, as with plants that depend on insects for pollination. A number of
insect species do not feed on living plants but act as scavengers. Some of
these species live on decaying vegetable matter and others on dung or the
carcasses of animals. The activities of the scavenger insects hasten the
decomposition of all kinds of dead organic material. |
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Certain insects also exhibit predation or
parasitism, either feeding on other insects or existing on or within the bodies
of insect or other animal hosts. Insects are sometimes parasitic upon other
parasitic insects, a phenomenon known as hyper parasitism. In a few instances an
insect may be parasitic upon a secondary parasite. A few species of insects,
although not strictly parasitic, live at the expense of other insects, with
which they associate closely. An example of this form of relationship is that
of the wax moth, that lives in the hives of bees and feeds on the comb which
the bees produce. Sometimes the relation between two species is symbiotic. Thus
ant colonies provide food for certain beetles that live with them, and in
return the ants consume fluids that have been secreted by the beetles.
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| 13.30.1
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Social Insects
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One of the most interesting forms of
insect behaviour is exhibited by the social insects, which, unlike the majority
of insect species, live in organized groups. The social insects include about
800 species of wasps, over 1,000 species of bees (including semi-social bees),
and the ants and termites. Characteristically, an insect society is formed of a
parent or parents and a large number of offspring. The individual members of
the society are divided into groups, each having a specialized function and
often exhibiting markedly different bodily structures. For discussion of the
organization of typical insect societies, see the articles on the insects
mentioned above.
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| 13.30.2 |
Structure |
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Insects are divided into two major categories and then
several sub categories |
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| Categories of insects |
Examples |
| Apterygota, wingless insects |
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| Pterygota, including most insects, the majority of which have wings in the imago
form. |
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Apterygota, wingless insects |
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| Sub categories
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Examples |
| Protura |
a group of very tiny, blind insects;
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| Thysanura |
that includes the silverfish |
| Diplura |
a small group that contains the largest of the Apterygota, an insect of the
genus Heterojapyx (about 5 cm/2 in long); and
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| Collembola,
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that includes the springtail. |
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Pterygota, including most insects, the majority of which
have wings in the imago form. |
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| Sub categories |
Examples |
| Ephemeroptera |
that includes the mayflies;
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| Plecoptera |
the stoneflies
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| Odonata |
dragonflies and damselflies;
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| Grylloblattodea |
a small wingless order
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| Orthoptera |
the katydids, crickets, and grasshoppers (locusts);
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| Phasmida |
the stick insects |
| Thysanoptera |
the thrips;
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| Dermaptera |
the earwigs;
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| Mantodea |
the mantis;
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| Blattaria (or Blattodea),
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cockroaches
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| Isoptera,
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the termites;
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| Embioptera,
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a small group of sub-social insects living in tropical and subtropical regions
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| Psocoptera |
the bark lice and book lice
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| Phthiraptera |
bird lice and true lice (sometimes separated into the two orders Mallophaga and
Siphonculata) |
| Zoraptera,
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of which only one termite-like genus (including about 20 species) is known
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| Megaloptera |
the alder flies and dobsonflies
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| Raphidiodea |
snake flies;
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| Neuroptera |
ant-lions and lacewings
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| cMecoptera,
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the scorpion flies;
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| Trichoptera |
the caddis flies
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| Lepidoptera |
the butterflies and moths
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| Diptera,
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the mosquitoes, gnats, and true flies
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| Siphonaptera |
the fleas
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| Coleoptera |
the beetles;
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| Strepsiptera |
a group of tiny insects parasitic on other insects |
| Hymenoptera |
including the ants, bees, wasps, hornets, and ichneumons, and the chalcids;
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| Hemiptera |
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Although the superficial appearance of insects is
extremely varied, certain characteristics of their anatomy are common to the
entire class. All mature insects have bodies composed of three parts: head,
thorax, and abdomen (the abdomen and thorax are not always differentiated in
larvae). Each of these parts is composed of a number of segments. The segments
of the head are usually so fused that they are scarcely differentiated. On the
head are two antennae (feelers); a pair of jaws, or mandibles; a pair of
auxiliary jaws, or maxillae, that in turn bear a pair of palps (usually sensory
feelers); and a fused second pair of accessory jaws, the labium, which also
bears a pair of palps. The antennae, usually attached to the front part of the
head, are segmented. In some insects, the antennae carry organs of smell as
well as organs of touch. The mandibles are large, heavy jaws on each side of
the mouth. They close horizontally and are used for grasping food and crushing
it. The maxillae, or inner jaws, are lighter in structure. The mouths of many
insects are adapted for piercing and sucking rather than for biting. The eyes
of an insect are also situated on the head. |
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All insects have three pairs of legs, each pair
growing from a different part of the thorax. These parts are called, from front
to back, the prothorax, the mesothorax, and the metathorax. Many larvae have,
in addition, several pairs of leg-like appendages called struts, or prolegs.
The forms of the legs vary, depending on their uses, but all insect legs are
made up of five parts. In winged insects, the wings, usually four in number,
grow from the thorax between the mesothorax and the metathorax. The upper and
lower membranes of large wings cover a network of hardened tubes, called veins,
that stiffen the wing. The pattern of veins of the wings is characteristic of
most insect species and is extensively used by entomologists as a basis for
classification. |
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Insect abdomens usually have 10 or 11 clearly defined
segments. In all cases the anal opening is located on the last segment; in some
species, such as the mayflies, a pair of feelers, called cerci, is also present
on this segment. The abdomen does not carry legs. In female insects it contains
the egg-laying organ, or ovipositor, which may be modified into a sting, saw,
or drill for depositing the eggs in the bodies of plants or animals. Insect
sexual organs arise from the eighth and ninth segments of the abdomen. |
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Insects have an external rather than an internal
skeleton; this exoskeleton is a rough integument formed by the hardening of the
outer layer of the body through impregnation with pigments and polymerization
of proteins, a process known as sclerotization. At the joints the exoskeleton
does not become sclerotized and therefore remains flexible. |
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Respiration |
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Certain species of insects breathe through the body
wall, by diffusion, but in general the respiratory system of members of this
class consists of a network of tubes, or tracheae, that carry air throughout
the body to smaller tubelets or tracheoles through which all the organs of the
body are supplied. In the tracheoles the oxygen from the air diffuses into the
bloodstream, and carbon dioxide from the blood diffuses into the air. The
exterior openings of the tracheae are called spiracles. The spiracles are
situated on the sides of the insect and are usually 20 in number (10 pairs), 4
on the thorax and 16 on the abdomen. Some water-breathing insects have
gill-like structures. |
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Circulation |
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The circulatory system of insects is simple. The
entire body cavity is filled with blood that is kept in circulation by means of
a simple heart. This heart is a tube, open at both ends, that runs the entire
length of the body under the exoskeleton along the back of the insect. The
walls of the heart can contract to force the blood forwards through the heart
and out into the body cavity. |
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Digestion |
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The digestive tract of most insects is divided into
the foregut, the midgut (or stomach), and the hindgut. In the foregut, a food
passage, or gullet, from the mouth is followed by a crop and a proventriculus.
The crop serves as a storage space for food. Salivary glands open into the
gullet, and their secretions are mixed with the food during mastication
(chewing). Digestion takes place primarily in the midgut, and the products are
absorbed in the midgut and the hindgut. The food waste passes to the hindgut,
or intestine, for elimination. Connected to the forepart of the hindgut are a
large number of small tubes, called the Malpighian tubules, that float in the
blood of the body cavity. Waste matter in the blood passes through the walls of
these tubes and into the hindgut, from which it is eliminated from the body of
the insect. |
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Neural System
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The nervous system of an insect centres on a nerve
cord that runs from the head to the abdomen along the underside of the body.
Typically the cord is equipped with a pair of ganglia, or nerve centres, for
each segment of the body. The brain, which is located just above the gullet, is
made up of three ganglia fused into one. The brain receives stimuli from the
antennae and from the eyes. |
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The sense organs of insects consist of eyes, auditory
organs, organs of touch, organs of smell, and organs of taste. Insect eyes are
of two types, compound and simple. Each of the two compound eyes, which are
usually situated directly behind the antennae, contains from 6 to 28,000 or
more light-sensitive structures, called ommatidia, grouped under a lens or
cornea that is composed of an equal number of hexagonal prism-shaped facets.
These structures permit only light that is parallel to their axes to reach the
nerve endings, and thus build up an optical image. Many species have in
addition simple eyes, or ocelli, which are usually located between the compound
eyes. Entomologists believe that the compound eyes are adapted to seeing
swiftly moving objects, whereas the simple eyes are adapted to seeing nearby
objects and fluctuations in light intensity. Each ocellus has a simple lens
overlying a series of light-sensitive nerve elements, all of which are
connected by a single nerve to the brain. |
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The auditory organs of insects vary widely in
structure and in some species are quite complex. In some grasshoppers, large
auditory membranes are situated on each side of the first segment of the
abdomen. Behind these membranes are fluid-filled spaces that transmit the sound
impulses to nerve endings that project into the fluid. Other types of
grasshoppers and crickets have auditory organs on their legs below the knee
joints. These organs consist of membranes with air chambers beneath them that
communicate with the outside air through slits in their walls and are supplied
with nerve endings. The organs of touch in insects resemble hairs and are
located on various parts of the body and on the antennae. Most insects have a
keen sense of smell, which they use to distinguish insects of the same species,
especially mates, and to find food. The smell organs are usually found in the
antennae. |
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| 13.30.3 |
Reproduction |
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The various species of insects exhibit extreme variety
in their modes of reproduction. In some insects, such as the honey bee, the
reproductive female, or queen, produces thousands of fertilized eggs over a
period of several years, although the male, or drone, dies shortly after
mating. In other species, such as the mayflies, both male and female insects
have only a short span of life after mating. In a number of species of beetles,
both males and females mate repeatedly. In addition, various species of insects
reproduce parthenogenetically, developing from unfertilized eggs. This form of
reproduction occurs regularly in certain species, and occasionally or in
alternate generations in others. In some gall wasps and saw flies, all
reproduction is apparently by parthenogenesis, and no sexual reproduction is
known. In the social bees and other related insects, male insects are produced
from unfertilized eggs. In certain moths, which exhibit sporadic
parthenogenesis, both sexes may be produced from unfertilized eggs. Among the
aphids several successive generations of females may be produced
parthenogenetically before the production of a generation of male and female
insects that reproduces sexually. |
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Certain flies occasionally reproduce by means of
paedogenesis: the production of eggs by immature forms, either larvae or pupae.
The larvae of some midges produce several generations of larval females before
producing male and female larvae that develop into adult insects and reproduce
sexually. |
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The method of development of eggs also varies widely
among the insects. Some insects, such as certain species of cockroaches, flies,
and beetles, are viviparous, giving birth to live young. In other species, the
entire larval stage of development takes place within the body of the female,
and the insect becomes a pupa at birth. Most insect eggs, however, are
deposited and hatch outside the body of the parent. The egg-laying habits of
the different species vary. Many insects deposit single eggs or masses of eggs
on the plants on which the larvae will feed. A number of insects lay their eggs
within the tissues of the food plant, and the eggs may give rise to swellings,
or galls, on the leaves or stems of the plant. |
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Certain insects show a unique form of embryonic
development in which more than one embryo is formed by a single egg. This
process is known as polyembryony, and in some species more than 100 larvae are
formed from a single egg by division within the egg. |
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| 13.30.4 |
Metamorphosis |
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One of the characteristics of the development of
insects from birth to maturity is metamorphosis, the change through one or more
distinctive immature body forms to the imago, or adult body form. Metamorphosis
of some kind occurs in most insects, although in a few species, such as the
bristletails, the newborn insect is essentially similar in form to the imago. |
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Entomologists recognize two basic forms of
metamorphosis: complete and incomplete. In complete metamorphosis, the insect
egg hatches to produce a larva, an active immature form typified by the
caterpillar; then changes to a pupa, a more or less dormant form, often
enclosed in a cocoon; and finally emerges as the adult insect, or imago. A form
of complete metamorphosis in which the insect larva undergoes one or more
changes in form (usually to adapt it to a change in food supply) before
becoming a pupa is called hypermetamorphosis. Hypermetamorphosis takes place in
certain beetles and flies, and in certain parasitic insects of the order
Hymenoptera. |
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In incomplete metamorphosis, the insect is born in a
relatively mature form called a nymph, which resembles the imago but lacks or
has only partly developed wings and reproductive apparatus. The nymph changes
to the imago by a gradual process, and no pupal stage occurs. The nymphal
stages are separated by moulting, or ecdysis, of the inelastic exoskeleton,
each successive stage being more like the adult. In the simplest insects, the
changes between successive nymphal stages are slight, but generally the stages
are distinctly different. |
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In a typical example of complete metamorphosis, the
larva is a caterpillar that can crawl in search of food and that has mouth
parts adapted for feeding on leaves or grasses. As the larva grows, it sheds
its skin from three to nine times. At the end of the larval period, the insect
spins a cocoon about itself or, in the case of most cutworms and certain other
insects, forms an underground earthen cell and enters the pupal stage. During
the pupal stage the insect is quiescent and does not eat, but its body
gradually assumes the imago form. At this time the wings and other body
structures of the mature insect begin to develop. When the pupa is fully
developed, it breaks out of its cocoon or earthen cell and pupal exoskeleton,
and emerges as a complete adult insect, such as a moth or butterfly. |
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