Evolutionary history of insects

Topic: EVOLUTIONARY HISTORY OF INSECTS
Name: Bhumika Kapoor
Course name: Classification Of Insects
Course title: ENTO-503

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.

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.

A 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.

♠ 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.

 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.

 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.

 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 ORTHROPTEROID GROUP THE HEMIPTEROID GROUP
The 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.

Complete 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

 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

Evolutionary history of insects

Evolutionary history of insects

Evolutionary history of insects

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