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Classification

Classification is ordering of organisms into groups (or sets) on the basis of their relationships that are of associations by contiguity, similarity or both.

Classification of living things is an attempt to interpret nature. It attempts to bring together the kinds that are alike and closely related and to separate those that are unlike and unrelated.

Animals might be classified in various ways but the classification is based primarily on structural characters. Those animals with certain structures in common are classified into one group, and those with other structures into other group. Thus the animal kingdom is divided into a dozen or so major groups called phyla (singular, phylum); each phylum has a name, and its members have certain structural characters in common. The characters used to distinguish phyla include the number of cells, symmetry, body form and segmentation, the nature of the appendages, and the arrangement of the internal organs.

On the basis of degree of complexity, and probably evolutionary sequence, the animal phyla are usually arranged in a series from the lower phyla to the higher ones.

Nomenclature should not be confused with terminology which deals with terms, like antenna, abdomen, radius etc. Names of animal and plants may be either vernacular or scientific.

Vernacular names or local names of plants and animals are different from region to region, from language to language.

Scientific names or Latin or International names are used universally all over the world. Without them we would not be able to communicate and to use the knowledge accumulated about animals and plants in literature. These international names are under certain rules or nomenclature.

Nomenclature

No-men-cla-ture means a system of names. It is derived from Latin Nomen means name and calor means to call, which means to call by names. Nomenclature is the “language of Zoology” and the rules of nomenclature are its grammar. Since all zoologists work with animals and used their names, it is essential that the general principles of zoological nomenclature be familiar to all zoologists.

Origin of Insects

Insects are highly specialized group of invertebrates belonging to the largest animal Phyla, the ARTHROPODA, which also includes in it, the prawns, crabs, lobsters, centipedes, millipedes, scorpions, ticks, mites and spiders. Practically three fourth of the animal species come under this phylum.

The Arthropods evolved during Cambrian period from primitive annelidan ancestor. The main evolutionary stem of Arthropod is represented by the Crustacia- Insecta- Myriapoda.

The insects originated in the Devonian period of Palaeozoic era. Diplura and Thysanura are the most primitive wingless insects while mayflies, damselflies, dragonflies and stoneflies represent the primitive winged insects.

Evolution

Evolution is a process whereby population are altered over a period of time and may split into separate branches, hybridize together or terminate by extinction.

Arthropods occur up to the height of over 6,080 meters on land and up to the depth of about 59,728 meters in water. Different arthropod species are adapted for living in air, on land, in soil and in fresh, brackish or salt water.

Many insects are economically important for being pests of crops, stored grains, stored food, household goods and clothing etc. Some insects and arachnids are in one way or the other responsible for certain diseases of crops, domestic animals and human beings. Some arthropods, on the other hand, such as crabs, lobsters, prawns and shrimps form a part of human diet while, small sized crustaceans are staple food for fishes. Many parts of the world have witnessed some insects and spiders eaten by many land animals including man.

Insects share many characters with other arthropods, but the other arthropods have not been able to exploit these to the extent insects have evolved. The presence of functional wings is a very important character which insects do not share with other arthropods, that is why insects have distinct advantage over other arthropods in their struggle for existence and dominance. A comparative account of Insecta and other arthropod classes is given in Table 1.

Origin of Systematics

In Vedas (1500 BC), Ramayana (1900 BC), Mahabharata (1400 BC), and Upanishad (350 BC) mention had been made about various animals. The number of known species of animals is much greater than that of plants and has been estimated at about more than one million. When subspecies are included, the number may double to more than two million named forms of animals. The new animals are being described at the rate of about 10,000 per year. Metcalf (1940) estimated that one and a half million species names are already applied while Silvestre (1929) calculates an estimated 3 million probable species of insects.

Systematics

Systematics is a term coined by a Swedish naturalist Linnaeus (Systema Naturae 1735) which is a Latinized Greek word Systema, as applied to the systems of classification. According to Simpson, “Systematics is the scientific study of the kinds and diversity of organisms and any or all relationships among them.” It includes Taxonomy, identification, classification and nomenclature. Now-a-days, both terms are used inter-changeably in the fields of plant and animal classification.

Arthropod life began very early in Palaeozoic region, about 600 million years ago in the sea. The arthropod foundations were laid with the evolution of metamerism in segmented worms, and in the ancestral arthropods it resembled the now extinct Trilobites. They had harder cuticle forming the exoskeleton and a pair of jointed legs with each of its metameric segments.

The Trilobite stock soon diversified into:

a) Chelicerates.

b) Mandibulates.

a) Chelicerates: In Chelicerates, the anterior body segments fused to form the prosoma, bearing a pair of chelicerae, the feeding organs; a pair of pedipalps and four pairs of walking legs. The remaining segments formed the legless opisthosoma. This gave rise to Eurypterids (Extinct) Xiphosurians, Pycnogonids and the highly successful Arachnids, including the scorpions, spiders, ticks and mites.

The name Lepidoptera derived from the Greek words “lepido” meaning Scale and “ptera” meaning wings refers to the flattened hairs (Scales) that cover the body and wings of most adults. Lepidoptera (Moths and Butterflies) is the second largest order (1.10 lakh sp.) in the class insecta, characterized by the body, wings and appendages being generally covered densely with pigmented scales which provide the colour pattern of the characteristic of the species and play an important role in courtship and intraspecific recognition.

Adults

• Head: Hypognathus and relatively small.

• Compound eyes are large and set well apart. Ocelli two in number in moths or reduced to Chaetosoma in butterfly.

• Mouthparts form a coiled tube (Proboscis) beneath the head.

• Antennal type:

• Butterflies: Clubbed, clavate, capitate.

• Moths: Pectinate (Uni/Bi).

• Adults have siphoning type mouthparts except in micropterygidae (Mandibulate mouthparts). Front wings large, triangular, hindwings large, fan shaped.

• Body and wings covered with small, overlapping scale.

• Thoracic segments are fused and metathorax is the largest and most prominent.

• Wings covered with scales. Androconia (Plumules) on upper surface of wings in some males serves as outlet for odouriferous glands. are fringed distally with each tip finely divided. • Wing coupling apparatus variable (generally Frenulum and retinaculum or Amplexiform type)

• The name Zoraptera, derived from the Greek “zor” meaning pure and “aptera” meaning wingless was given to the order before winged forms were discopvered.

• Members of the order Zoraptera are small (less than 4 mm) and usually found in rotting wood, under bark or in piles of old sawdust.

• They live in small aggregations (gregarious) and appear to scavenge on spores and mycelium of fungi, or occasionally, on mites and other small arthropods.

• polymorphism in populations; with blind, apterous individuals dominating during colony life, but populations produce eyed alates (individuals with wings) for dispersal and founding new colonies.

• males with a distinctive “mating hook” and unique mating behaviors.

• unique grooming behaviors

Adults

• Antennae 9-segmented (moniliform).

• Mouthparts mandibulate, hypognathous.

• Soft bodied, small (usually less than 3 mm).

• Wings often absent with reduced venation when present.

• Tarsi 2-segmented

• Cerci 1-segmented

• ovipositor absent

• incomplete development (egg, nymph, adult)

Apterygotes

The Apterygota, which formerly included the other primitively wingless insects currently placed in the class Entognatha, are now restricted to the two orders Archaeognatha and Zygentoma, which in turn were formerly united as the Thysanura. Despite the superficial similarity of the two groups, it is now clear that they are not closely related, mainly because of fundamental differences in the mouthparts. The Archaeognathous mandibles are monocondylic, having a single articulating point with the head so that the mandible can rotate. The Zygentomous mandibles are dicondylic, with two articulating points that restrict the motion to a single plane yet enable the development of a much stronger biting action; this is the type found in all the higher insects, and the Zygentoma may well be the sister group of the Pterygota, or winged insects. Clearly the ‘Apterygota’ is not a monophyletic group and is simply retained for convenience in grouping these two orders that superficially resemble each other and have similar life histories.

History

• Aristotle (384-322 BC): A good beginning of taxonomy was made by Aristotle (Historia Animalium). He erected entomon. He could distinguish mandibulates and haustellates; winged and wingless forms. The insect orders like coleopteran, dipteral, psyche (Lepidoptera) were created by him. Hence, he was called as Father of Biological Classification.

• Linnaeus (1707-1778): Father of Taxonomy and Binomial Nomenclature. He coined the term Insecta. He used the term Systematics. His greatest contribution is Systema Naturae.

• Brawer (1885): Laid foundations for modern system of classification and responsible to divide class Insecta into Apterygota and Pterygota.

• Michael Anderson (1727-1806): Contemporary and rival of Linnaeus. Introduced Numerical Taxonomy and hence is called Father of Numerical Taxonomy. 

• Johan Christian Fabricius (1745–1808), the first systematist considered to specialize on insects, published in 1778 his Philosophia Entomologica (40), the first textbook of entomology.

• Fabricius had already set forth his system of insects in 1775 (Systema Entomologiae), his system of genera in 1776 (Genera Insectorum), and then numerous works on species, culminating in individual accounts of the species of each order

The sense organs in an insect body are distributed on different parts and respond to a given stimulus such as light, sound, touch, chemicals etc. The sense organs may be classified as • Visual organs (or) photoreceptors • Auditory organs (or) organs of hearing

• Chemoreceptors which respond to chemicals

• Tacticle receptors which respond to touch

• Gustatory receptors which respond to taste.

Photoreceptors

Light is perceived by insects through a number of different sense organs and these are the organs that react with light and useful for vision. These are of different types, but most important are the compound eyes.

Compound eyes

Alimentary canal of an insect is a tubular structure passing through the body that provides the special internal environment for the food to be mechanically and chemically disintegrated and brought to the vicinity of absorbent cells. Hence, it is involved in the transport of nutrients to the individual cells. It is concerned with ingestion, trituration (chewing), digestion, absorption of nutrient material into haemolymph and egestion of unused food materials. It is tubular structure, which extends from mouth to the anus. It is divided into three distinct regions, namely fore-gut (stomodaeum), mid-gut (mesenteron) and hind-gut (proctodaeum). The fore- and hind-guts are ectodermal in origin and the mid-gut is endodermal.

Longitudinal and circular (intrinsic) muscles are usually associated with each of these three regions and by means peristaltic contractions move the food along the alimentary canal. The functional stomach is the mesenteron usually a circular valve-like fold separates the cavities between stomodaeum and mesenteron is known as stomodael or cardiac valve, and the one closing the entrance to the proctodaeum as the proctodaeum or pyloric valve.

Fore-gut

An insect’s mouth, located centrally at the base of the mouthparts, is a muscular valve (sphincter) that marks the “front” of the foregut. Food in the buccal cavity is sucked through the mouth opening and into the pharynx by contractile action of cibarial muscles. These muscles, located between the head capsule and the anterior wall of the pharynx, create suction by enlarging the volume of the pharynx (like opening a bellows). This “suction pump” mechanism is called the cibarial pump. It is especially well-developed in insects with piercing/sucking mouthparts.

• In most insects, life begins as an independent egg (ovipary).

• Majority of insects are oviparous. Egg stage is inconspicuous, inexpensive and inactive. Yolk contained in the egg supports the embryonic development.

• Eggs are laid under conditions where the food is available for feeding of the future young ones.

• Production of eggs by the female’s body is called oogenesis and the egg laying process is known as oviposition.

• The outer protective shell of the egg is called chorion. Near the anterior end of the egg, there is a small opening called micropyle which allows the sperm entry for fertilization. Chorion may have a variety of textures.

• Size and shape of the insect eggs vary widely.

Insects development

Once an insect hatches from the egg it is usually able to survive on its own, but it is small, wingless, and sexually immature. Its primary role in life is to eat and grow. If it survives, it will periodically outgrow and replace its exoskeleton (a process known as molting). In many species, there are other physical changes that also occur as the insect gets older (growth of wings and development of external genitalia, for example). Collectively, all changes that involve growth, molting, and maturation are known as morphogenesis.

Metamorphosis

• Metamorphosis actually means change in form Metamorphosis is derived from Greek word ‘Meta’ = Change, ‘morph’ = form or structure.

• The changes in the development from egg to adult, collectively called Metamorphosis (life cycle).

• Post-embryonic changes in form are collectively called as metamorphosis.

• Metamorphosis refers to the way that insects develop, grow, and change form. Metamorphosis include three developmental processes namely growth, differentiation and reproduction which takes place in larval, pupal and adult stages, respectively.

• Majority of insects pass through profound metamorphosis. The growth of an immature insect is accompanied by a series of molts or ecdysis, in which the cuticle is shed off and renewed. The number of molts varies in most insects from 4-8. However some undergo as many as 20 or more molts. A few insects like Bristle tail silver fish (apterygota) may continue molting even after reaching the adult stage. But most of the insects neither molt nor increase in size once then adult stage is reached. The intervals between the ecdysis are called Stadia. The term instar is applied to form of an insect during stadium. In numbering the instar, the form assumed by the insect between hatching from the egg and the first post embryonic molt is termed “First Instar”.

• It is the posterior most and largest region of the insect-body. It is The abdomen is the metabolic and reproductive centre, where digestion, excretion, and the sexual functions take place.

• All the definitive number of segments differentiate in embryo (Epimorphic condition) except protura (Anamorphosis)

• In general more segments are visible in more generalised hemimetabolus insects than in more specialised holometabolus insects. For example in Acridids all 11 segments are visible whereas in muscidae only 2-5 segments are visible

• The basic number of abdominal segments in insect is eleven plus a telson/ tail (Protura) which bears anus. Abdominal segments are called uromeres.

• The undivided tergel plates overlap the sternal ones and the membranous pleura are enclosed in between them.

• The posterior part of each segment overlaps the anterior part of the segment behind it and the two segments are joined by the inter-segmental membrane.

• There are eight pairs of abdominal spiracles. The first pair lies just in front of each tympanum and the other lie dorso-laterally on each of the tergites of the segment to eighth abdominal segments.

• The terga and sterna bear antecostal ridges. The internal dorsal and ventral longitudinal muscles run one antecostal ridge to the other and act as retractors. The contraction of these muscles helps the segments to be retracted. The external muscles originate from the posterior part of the segment and attach to the anterior part of the acrotergite of the next segment. Thus, protraction of these muscles help in the extension of abdomen. Hence, musculature running longitudinally between helps in the contraction and extension of the abdomen.

• The abdominal segments are conveniently divided into three subregions i.e. pregenital, genital and postgenital segments.

• The thorax is the organ of locomotion due to articulation of wings and legs and is connected with the by neck or cervix.

• Thoracic segments are made up of three sclerites namely, dorsal body plate tergum or nota, ventral body plate sternum and lateral plate pleuron.

• Second and middle tagma which is three segmented, namely prothorax, mesothorax and metathorax. Meso and metathorax which bear wings are called as Pterothorax.

• In the apterygote insects the three thoracic segments are similar in size and structure. In the pterygote insects the thorax is highly modified.

• The prothorax is quite large and is somewhat independent of the other segments. The large shield that covers the dorsal and lateral sides of the prothorax is the pronotum.

• The pronotum is the dorsal sclerite of the prothorax, which can be highly modified in various orders such as the Hemiptera, Blattaria, and Coleoptera. E.g undivided and Saddle shaped in grass hopper, Shield like in cockroach.

• Cockroaches have pronotums that extend forward over the head.

• Scarab beetles and other beetles may also have unusual pronotums.

• Treehoppers have some of the most bizarre pronotums of all insects.

Light spectrum holds rich information about the world. The spectrum of natural illumination changes with daily cycle and objects differ in spectral reflection or emission properties. Like many other animals, insects use chromatic information to f ind favorable habitat, to efficiently locate food sources and to identify conspecific mates using the spectral information. Pollinating insects such as many species of bees and butterflies use their color vision to maximize success in foraging. They detect flowers, memorize the colors and patterns of rewarding flowers, and preferentially collect nectars from the flowers in their later visits. Visual guidance of behaviour is challenging when photons, the elementary particles of light, are scarce. To produce a reliable representation of the surroundings, the visual system must (i) ensure absorption of a sufficient number of photons into a photoreceptor, (ii) house photoreceptors that efficiently convert each photon absorption into a neural signal, and (iii) process these signals appropriately. Insects are a numerous and diverse class of arthropods that have evolved to occupy ecological niches from the brightest to the darkest.

Antennae are paired, highly mobile, segmented appendages present between or below or behind the compound eyes. The antennae arise from the 2nd or antennal segment of the head possessing nerves coming from deutocerebrum of the brain. Antennae are absent in order protura and class Arachnida whereas 2 pairs of antenna (antennules) are present in class Crustacea.

Antennae are set in a socket of the cranium called antennal socket (anetnnifer) and connected by an articulatory membrane which allows free movement of antenna. Antennal socket is provided with an antennal suture. Although antennae vary widely in shape and function, all of them can be divided into three basic parts:

• Scape

• Basal segment, larger than other segments and articulates with the head capsule. It is provided with intrinsic muscles.

• Pedicel

• Second segment of antennae. In this, mass of cells called Johnston organ (auditory) is present (eg. houseflies). The above two segments have intrinsic muscles ie., their own muscles. In honey bee, wasps pedicel forms the pivot between scape and flagellum.

• Flagellum/Clavola

• The remaining segments or annuli or flagellomeres are called flagellum which lack individual muscles. Flagellum is supplied with many sensory receptors that are connected by nerves to brain. Flagellum is further divided into three parts

Endoskeleton of the Head (Tentorium)

• The tentorium forms during development when 2-4 pairs of apophysis/ apodemes (finger-like invaginations of exoskeleton) fuse internally to create a “bridge”.

• The major components of the tentorium are the anterior tentorial arms, the posterior tentorial arms, the dorsal tentorial arms, and the tentorial bridge.

• The anterior tentorial arms originate as invaginations at the lateral limits of the frontoclypeal suture. The coalescence of the anterior tentorial arms forms a more or less solid structure, the tentorial bridge.

• The posterior tentorial arms arise from pits at the ventral ends of the post occipital suture and they unite to form a bridge running across the head from one side to the other.

• In many insects, e.g. Orhtoptera, the central part of the composite tentorium enlarged to form a broad plate referred to as the Corporotentorium.

• The tentorium serves as an internal “truss” that reinforces the head capsule, cradles the brain, provides attachment for muscles and provides a rigid origin for muscles of the mandibles and other mouthparts. The antennal muscles also arise from the tentorium.

Body Segmentation

In general, insect body is divided in to a series of segments, which in primitive arthropods are known as “somites” or “metameres”. During the process of evolution, these somites gets fused with each other in different ways forming the body parts of the existing arthropods. The type of arrangement of these body segments in embryonic stage is known as primary segmentation while in adult insects is known as the secondary segmentation which differ from primary in having a sclerotized membranous intersegmental region.

In adult and nymphal insects, and hexapods in general, one of the most striking external features is the amalgamation of segments into functional units. This process of tagmosis has given rise to the familiar tagmata (regions) of head, thorax, and abdomen. In this process the 20 original segments have been divided into an embryologically detectable six-segmented head, three-segmented thorax, and 11-segmented abdomen (plus primitively the telson), although varying degrees of fusion mean that the full complement is never visible.

Types of body segmentation.

(a) Primary segmentation, as seen in soft-bodied larvae of some insects. (b) Simple secondary segmentation. (c) More derived secondary segmentation. (d) Longitudinal section of dorsum of the thorax of winged insects, in which the acrotergites of the second and third segments have enlarged to become the postnota. (After Snodgrass 1935)