DIVISION OF ENTOMOLOGYSKUAST-J

Topic: EVOLUTIONARY HISTORY OF INSECTS

Name: Bhumika KapoorCourse name: Classification Of Insects

Course title: ENTO-503

EVOLUTIONARY BEGINNING

PRESENT DAYS

Human species(less than 2 million years ago)

Birds & mammals(about 200 million years ago)

Dinosaurs (about 230 million years ago)

First land plant( about 425 million years ago)

First Arthropod(about 600 million years ago)

This era belongs to the CAMBRIAN PERIOD OF THE PALEOZOIC ERA, when bacteria and marine algae were

dominant form of plant life, small invertebrates animals were abundant in warm, shallow seas, and

land masses were still largely devoid of life.

ARTHROPOD EVOLUTION

Evolutionary change may be viewed as occurring in three successively longer time frames;

MICRO-EVOLUTION, the changes that occur in population in response to changes in selection pressure.

SPECIATION, the changes that typically occur over much longer time spans than microevolution and results in reproductively isolated, new species.

MACRO-EVOLUTION, the major Phylogenetic patterns that develop over wide spans of geological time.

Theories Of Arthropods EvolutionA classical controversy that still rages among

invertebrate zoologists including the entomologists, relates to evolutionary pathways of adaptative radiations that arthropods followed as they gradually diverged from primitive ancestors. To support this evidence, the two theories were given:

§ MONOPHYLETIC THEORY; based on observation that many features such as the exoskeleton, open circulatory system, hemocoel, etc., are shared by nearly all taxa within the group and appears to be homologous(i.e. Have the same evolutionary origin).

§ The theory supports the strong evidence that all groups like Myriapods, Crustaceans, And Chelicerates must have evolved from a common ancestors.

§ The monophyletic classification scheme, proposed by BOUDREAUX, recognizes three evolutionary lineages within the Phylum Arthropod: Trilobita, Chelicerata, And Mandibulata. See clad. 2.

§ Boudreaux’s classification scheme excludes onychophorans because they lack a true exoskeleton, but other workers disagree. Megalitsch And Schram(1991) regard the Onychophora As Closely Related To Myriapods. See clad.3.

In their classification, Cruatacea And Chelicerata are the most primitive groups. Insects, Myriapods And Onychophora are grouped together in the Super Class Uniramia because of similarities in leg structures and locomotion.

§ THE POLYPHYLETIC THEORY; if we embrace the idea f polyphyletic origins, then arthropods are represented by as many as four phyla- each of which is presumed to have evolved separately from primitive annelid ancestors:

Trilobita- 4,000 sps.+ - including all trilobites (extinct marine organisms that were abundant during the Palaeozoic era.)

Chelicerata- 70,000 sps. – including spiders, scorpions, mites , ticks, horseshoe crabs, and sea spiders.

Crustacea -30,000 sps. – including shrimps, crabs, lobsters, woodlice, barnacles, amphipods, branchiopods, and copepods.

Uniramia – 1.2 million sps. – including onychophora, millipedes, centipedes, pauropods, symphylans, and insects.

§ Embryological development of head and mouthparts has also been offered as evidences to support the polyphyletic hypothesis. In myriapods and insects, the head is a separate functional region. But in the crustacea and chelicerata, the head and thorax develop together as a single body region, the cephalothorax. Furthermore, within the myriapods and insects there is evidence that additional segments are added to form mouthparts, suggesting that mouthparts of chelicerates, crustaceans and other arthopods are not homologous.

THE HEXAPODS♠ Judging from the fossil records , hexapods must have diverged

from Myriapodan Stock during the the Late Ordovician or Early Siliurian Periods( 400-500 Millions Years Ago).

♠ The Oldest Hexapods Fossils are found in rocks of the late Devonian period i.e. Devonian Rhyniognatha hirsti, from the 396 million year old from the Rhynie Chert Of Scotland.

♠ At one time, all six- legged arthopods were considered “INSECTS”. But it become difficult to justify grouping all “six – leggers” in a single class. There are major differences in external structures, organ systems and three other closely allied groups: Protura, Diplura and Collembola.

♠ These three taxa considered as primitive and classified them as order entognathous, aterm that is derived from Greek “ento”- meaning inner and “gnathos”- jaw, i.e. Their mouth parts are enclosed within a cavity.

♠ All of “true” insects are ectognathous: their mouthparts are extend outside the head capusle.

APTERYGOTE INSECTS The insects probably evolved from some primitive members of

the Super Class Myriapods during the Silurian Period (400-440 million yrs. Ago).

At the same time, Vascular Plants were just beginning to evolve and invade the dry land.

The most primitive insects that we recognize as Bristletails are the members of the Sub Class Aptergyota from the Greek “a”- without and “pterygo”- wing. They are completely wingless and have ametabolous development.

This sub class apterygote has been divided into three orders: Archaeognathna, Thysanura and Monura. Of these, only Archaeognathna and Thysanura have survived to the present time.

Subclass Apteroygote share several primitive characteristics with the Myriapods such as: first, they continue to molt even as adults (example: Thysanura); second, these having short , segmented appendages along the sides of abdomen.; third, they possesses external fertilization.

PTERYGOTE INSECTS About the same time primitive insects began to

diversify and exploit terrestrial habitats, other types of types of arthropods (especially spiders and scorpions) were also invading the land.

Most members of the Apterygote are CRYPTOZOIC i.e. Adapted for life in concealed places – in leaf litter, under bark etc.

Predation also may have been the driving force behind an event that is probably the single most significant land mark in the evolutionary history of insects development of wings.

Insects were the first living organisms to mater flight and for more than 100 million years until the first flying reptiles appeared in the Jurassic period.

HOW , WHEN AND WHERE DID WINGS EVOLVED ?????

This question puzzled entomologists since the days of Charles Darwin.

Fossil records are not very but fully winged insects were suddenly seen in the CARBONIFEROUS PERIOD (about 380 million yrs ago) with no clear evidence of ancestral precursors.

Some biologists have speculated that wings may developed:

• As modification of tracheal gills in aquatic insects (GILL THEORY);

• From flaps of integuments covering the thoracic spiracles( SPIRACULAR FLAP THEORY);

• From pre-existing appendages near the legs ( THE ENDITE-EXITE THEROY);

• Or as lateral extensions of dorsal plates on the thorax (THE PARANOTAL LOBE THEORY).

But all these theories could not able to explain how or why such appendages acquired a hinge- joint mechanism, and neural complexity, etc necessary for sustained flight.

In 1994, James Marden and Melissa Kramer proposed an attractive theory and they suggested that flight may have originated among aquatic insects living on the water surfaces .

Such insects could have used their gills covers as OARS (protowings) to row across water. Selective Pressure for a more efficient rowing mechanism could lead to larger oars and stronger muscles.

In fact, Surface – Skimming Behaviour has been observed in both MAYFLIES (EPHEMEROPTERA) & STONE FLIES (PLECOPTERA).

There is direct evidence of a correlation between wing size and skimming velocity in Taeniopteryx burski, a small winter stone fly with feeble mastery of flight.

Marden and Kramer’s Theory is noteworthy because it accounts for progressive development of wings as well as the muscles needed to power them.

Fig. representing Taeniopteryx

Based on comparative morphology of wing structure , venation and flight mechanism, it appears that the ability to fly evolved only once in the Class Insecta.

Some Pterygote Insects Like Fleas And Lice

Secondarily Wingless

Changes leads to Hemimetabolous development ( incomplete metamorphosis)

Insects with Hemimetabolous development grouped together as EXOPTERYGOTA [Greek word ‘exo’ – outer and ‘pterygo’-

wing ]

Functional Wings Occur Only In Adult Insects.

THE PALEOPTEROUS ORDERS All orders that shares the “PRIMITIVE” conditions are grouped

as PALEOPTERA, a sub division within the sub class Pterygote. Based on fossil records , entomologists recognized up to seven

orders of paleopterous insects. Most of these insects are abundant in The Carboniferous

Period, but they disappeared suddenly in The Permian (about 250 mya).

The largest insect that ever lived, Meganeura monyi Brongniart , was one of the member of one of these extinct orders, the PROTODONATA.

It was damsel-fly like insect with body length of 30 cm (1 foot) and awing span of nearly 75 cm (2.5 feet).

Of all paleopterous orders, only two have survived that are: Ephemeroptera Odonata

THE NEOPTEROUS ORDERS The progeny of the wing-folders are placed in the sub division

NEOPTERA, meaning “NEW WING”. Such an adaptation first appeared 350-400 million years ago in

The Late Devonian Or Early Carboniferous Period. These insects represent a remarkably successful lineage; they

became the ancestors of all “higher” orders of insects. During the burst of adaptative radiation, the CLASS INSECTA

into at least three distinctive branches. First branch – Orthopteroids; having chewing mouthparts and

Hemimetabolous development. Second branch – Hemipteroids; having hemimetabolous

development and specialized mouthparts. Third branch – Endopterygota ; having complete

metamorphosis.

THE ORTHROPTEROID GROUP THE HEMIPTEROID GROUPThe Hemipteroid Complex arose from a common ancestors during the CARBONIFEROUS PERIOD, around 290 mya. Adaptive radiation within this lineage eventually led to the evolution of two sisters groups (clad.5.)one giving rise to Psocoptera & Phthiraptera; the others including- Thysanoptera & Hemiptera.

There is extensive controversy over phylogenetic relationships within The Orthropteroid Complex. The First Orthropteroid Insects were scavengers and/or herbivores. They are regarded as the earliest group of Neoptera. It probably dates from the early CARBONIFEROUS PERIOD, around 360 mya.

THE ENDOPTERYGOTE GROUPComplete metamorphosis is the most distinctive characteristic of all Endopterygote insects.The ancestors of today’s holometabolous insects may have first appeared near the Carboniferous Period around 290 mya.—probably not long after Hemipteroids and Orthopteroids diverged from their ancestral forms.Endopterygote insect order represents about 1/3rd of living insects orders, they contain over 4/5th of known insects species.It is suggested that two sister groups i.e. Mecoptera and Neuroptera diverged from a primitive endopterygote ancestor in the Permian period and that all living members descended from one of these two lineages.(clad.6)

Fig.: Phylogeny of insect orders

Major events in

insect evolution

Origin Of Wingless Hexapods

Evolution Of Wings

Evolution Of Wing Folding

Evolution Of Metamorphosis

End Permian Extinction

Angiosperm (Flowering Plant)

Radiation

REFERENCES E.L.Jordan, and Dr.P.S. Verma, Invertebrate

Zoology, S. Chand & Company Ltd., New Delhi Cedric Gillot, Entomology: Second Edition,

Plenum Press, Newyork And London D.B.Tembhare, Modern Entomology, Himalaya

Publishing House https://projects.ncsu.edu/cals/co......... https://en.m.wikipedia.org/wiki/hexapoda https://en.m.wikipedia.org/wiki/evolution... Diversity.pdf