Education India, Colleges, Universities, Courses, Exams, Schools
New to Target Study? Sign Up to get started.
Forgot password?
Submit your email address and we'll send you a link to reset your password.
Insects are Class of Animals within the Arthropods (a type of invertebrate, animals that lack a backbone) which have an External Skeleton and whose body is composed of three sections. Insects are mostly solitary, but some insects, such as certain bees, ants, and termites are social and live in large, well-organized colonies. Insects evolved during the Silurian Period, 438 to 408 mya (long before the dinosaurs appeared). The biggest Insect that ever lived was the ancient Dragonfly called 'Meganeura'. This predatory flying Insect lived about 250 million years ago and had a wingspan of about 2 feet.
Insects are among the most diverse group of animals on the planet and include more than a million described Species and represent more than half of all known living organisms. The number of extant Species is estimated at between 6 and 10 million and potentially represent over 90% of the differing metazoan life forms on Earth. Insects are not only the most abundant they are unique in the sense that they aren't even an Animal Phylum, they're just a Class within Phylum Arthropoda.
Scientific Classification
Kingdom - Animalia.
Phylum - Arthropoda.
Subphylum - Mandibulata.
Superclass - Hexapoda.
Class - Insecta.
1. Head - The Head supports:
a) A pair of 'Sensory Antennae' - Antennae are used to perceive sounds, vibrations, and other environmental factors.
b) A pair of 'Compound Eyes' or one to three 'Simple Eyes' or Ocelli - Compound Eyes are usually large with many lenses, giving the Insect a complex image of its surroundings. A simple eye contains just a single lens. Some Insects have both kinds of eyes.
c) Three sets of variously 'Modified Appendages' - These form the mouth-parts. Some Insects drink nectar, and have mouth-parts modified into a tube called a 'Proboscis' to suck up liquid. Other Insects have chewing mouth-parts and eat leaves or other plant matter. Some Insects bite or pinch, and others pierce and suck blood or plant fluids.
2. Thorax - It is the middle region of an Insect’s body. The thorax has 'Six Segmented Legs' - one pair each for the Prothorax, Mesothorax and the Metathorax segments making up the thorax and, if present in the Species, two or four wings. Wings come in different shapes and sizes. Butterflies and Moths have wings made of overlapping scales, often in brilliant colors. When at rest, Insects like Beetles keep their wings folded flat against their bodies. Other Insects hold their wings vertically, like Butterflies.
3. Abdomen - The abdomen consists of 'Eleven Segments', though in a few Species of Insects these segments may be fused together or reduced in size. The abdomen also contains most of the digestive, respiratory, excretory and reproductive internal structures.
The above mentioned are the basic parts of Insects, there are variations and many adaptations in the body parts, especially of wings, legs, antenna, mouth-parts etc.
Exoskeleton also known as the 'Cuticle' is composed of two layers:
i) The Epicuticle - The outer layer is called 'Epicuticle'. It is a thin and waxy water resistant layer which contains no Chitin.
ii) The Procuticle - The lower layer is called 'Procuticle'. It is 'Chitinous' and much thicker than the Epicuticle.
The Procuticle has two layers:
a) Exocuticle - The outer layers is known as the 'Exocuticle'. The Exocuticle is greatly reduced in many soft-bodied Insects especially during their larval stages. It is often hard and dark in colour
b) Endocuticle - The inner layer known as the 'Endocuticle'. It is made up of numerous layers of fibrous Chitin and proteins, criss-crossing each others in a sandwich pattern. This pattern makes the Endocuticle tough and flexible. the exocuticle is rigid and hardened.
Insects undergo either 'Complete or Incomplete Metamorphosis'.
i) Complete Metamorphosis - This involves four stages: egg, larvae, pupae and adult. The larvae looks like a worm and hatches from the egg. As it grows, it also sheds it skin. It then grows into a pupa. While the pupa doesn't move around like the larva, it does eventually at this stage change from an immature form to adult form. Insects like the butterfly, bee, ant, beetle and fly undergo Complete Metamorphosis.
ii) Incomplete Metamorphosis - In this process there are three stages called egg, nymph and adult. When the nymph hatches from the egg, it looks like the adult Insect except it is smaller, has no wings and cannot reproduce yet. As the nymph grows it outgrows its outside skeleton and it is shed. It then regrows a new one. This happens several times before it finally becomes an adult. The Dragonfly, Termite, Grasshopper and true bug undergo Incomplete Metamorphosis.
Weight - The 'Largest Insect' weighs around 115 g (4.1 oz) and the 'Smallest Insect' weighs few milligrams.
Size - The 'Largest Insect' reaches a length of 11.5 cm (4.5 in) and the 'Smallest Insect' reaches 139 micrometre or micron long.
Insects can be divided into two groups historically treated as Subclasses: 'Wingless Insects', known as 'Apterygota', and 'Winged Insects', known as 'Pterygota'.
1. Subclass - Apterygota.
2. Subclass - Pterygota.
Insects may be found in nearly all environments, although only a small number of Species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans.
Diet - Insects eat plants, fruit, seeds, manure, small Reptiles, other Insects etc.
Predators - All the Class of Animals eat Insects because of their huge numbers.
The evolutionary relationships of Insects to other animal groups remain unclear. Although more traditionally grouped with millipedes and centipedes, evidence has emerged favouring closer evolutionary ties with crustaceans. In the Pancrustacea theory, Insects, together with Remipedia and Malacostraca, make up a natural clade. The oldest definitive Insect fossil is the Devonian Rhyniognatha hirsti, from the 396 million year old Rhynie chert. It may have superficially resembled a modern-day silverfish Insect. This Species already possessed dicondylic mandibles (two articulations in the mandible), a feature associated with winged Insects, suggesting that wings may already have evolved at this time. Thus, the first Insects probably appeared earlier, in the Silurian Period. Late Carboniferous and Early Permian Insect orders include both extant groups and a number of Paleozoic Species, now extinct. During this era, some giant dragonfly-like forms reached wingspans of 55 to 70 cm (22 to 28 in) making them far larger than any living Insect. This gigantism may have been due to higher atmospheric oxygen levels that allowed increased respiratory efficiency relative to today. The lack of flying vertebrates could have been another factor. Most extinct orders of Insects developed during the Permian era that began around 270 million years ago. Many of the early groups became extinct during the Permian-Triassic extinction event, the largest mass extinction in the history of the Earth, around 252 million years ago. The remarkably successful Hymenopterans appeared as long as 146 million years ago in the Cretaceous era, but achieved their wide diversity more recently in the Cenozoic era, which began 66 million years ago. A number of highly successful Insect groups evolved in conjunction with flowering plants, a powerful illustration of coevolution. Many modern Insect genera developed during the Cenozoic. Insects from this period on are often found preserved in amber, often in perfect condition. The body plan, or 'Morphology', of such specimens is thus easily compared with modern Species.
Reproduction in Insects is usually sexual with certain exceptions. The reproductive organs of Insects are similar in structure and function to those of vertebrates - a Male's testes produce sperm and a Female's ovaries produce eggs (ova). During copulation, bundles of spermatozoa are sometimes introduced directly into the Female vagina by means of the Male copulatory organ, or 'Aedeagus'. Secretions from the accessory glands of the Female activate the sperm, the sperm bundles disperse, and the free spermatozoa make their way up to the receptaculum seminis, or spermatheca, where they are stored, ready to fertilize the eggs. In most Insects, the Male accessory glands secrete materials that form a tough capsule, or spermatophore; spermatozoa are encased in this spermatophore, which is inserted into the entrance of the vagina. The spermatophore walls commonly contain a gelatinous substance that swells upon exposure to secretions of the Female and forces out the spermatozoa. The vagina serves both for receiving sperm and for laying eggs. The terminal segments of the abdomen of Females sometimes are modified to form an ovipositor used for depositing eggs. In butterflies and moths (Lepidoptera) a second copulatory canal independent of the vagina has been evolved, so that the sperm enter by one route, and the eggs are deposited by another. In certain Beetles like Fireflies and Glow Worms Females have certain parts of the body fat become modified to form a luminous organ that attracts the Male. Male Crickets and Grasshoppers attract Females by their chirping songs and the Male Mosquito is lured by the sound emitted by the Female in flight. Most Female Insects secrete odorous substances called 'Pheromones' that attract and excite Males for reproduction. The Male likewise may produce scents that excite the Female. In many Insect groups eggs sometimes develop without fertilization by sperm which is known as 'Parthenogenesis'. In some Insects, such as bees, unfertilized eggs become Males and fertilized eggs Females. In others, such as aphids, all-Female generations are produced by parthenogenesis. Eggs are usually laid in a sheltered place; in a few Insects they are retained and hatched internally. Some Species of Insects, like the cockroach 'Blaptica dubia', are ovoviviparous. It's eggs develop entirely inside the Female and then hatch immediately upon being laid. Some other Species, such as those in the genus of Cockroaches known as Diploptera, are viviparous, and thus gestate inside the mother and are born alive. Some Insects, like Parasitic Wasps, show 'Polyembryony', where a single fertilized egg divides into many and in some cases thousands of separate embryos. Other developmental and reproductive variations include Haplodiploidy, Polymorphism, Paedomorphosis or Peramorphosis, Sexual Dimorphism, Parthenogenesis and more rarely Hermaphroditism.
