Insecta
Arthropod subphylum Hexapoda comprises class Insecta and three other small,
closely related, wingless insect-like groups: Collembola, Protura + Diplura
Hexapoda are united on basis of:
a) Distinct body plan of head, thorax, abdomen – unique synapomorphy of Hexapoda
b) three pairs of thoracic legs, one pair of antennae
c) three sets of “jaws” (mandibles, maxillae, labium)
d) aerial gas exchange system composed of tracheae and spiracles
e) Malpighian tubules formed as proctodeal (ectodermal) evaginations and amont
the Pterygota, (wings)
Hexapods are fundamentally terrestrial arthropods, groups inhabiting aquatic
environments have secondarily invaded those habitats through behavioural
adaptations and modifications of aerial gas exchange systems
Most spectacular evolutionary radiation among Hexapoda = insects – inhabit every
conceivable terrestira, freshwater habitat and less commonly, sea and marine
littorial region – oil swamps, seeps, sulphur springs, glacial streams, brine ponds
Published described species established range from 890 000 to over a million,
abundant and diverse
Evolutionary exploitation of developmental genes working on segmented and
compartmentalised bodies, coevolution with plants (particularly angiosperms),
miniaturisation, invention of flight
Insects are key predators of other invertebrates, diets of many terrestrial
animals, major role as reducer-level organisms in food webs – due to sheer numbers,
also constitute much of matrix of terrestrial food webs, biomass and energy
consumption exceed vertebrates
Phylogeny
Arthropod tree following
the Mandibulata hypothesis
(Edgecombe et al, 2012)
Subphylum Hexapoda
comprises the class
Insecta/Ectognatha and
other wingless groups (class
= Entognatha)
Entognatha – base of
mouthparts hidden within
head capsule, mostly soil
inhabiting detrivores
Fossil Record
, First undisputed fossil Hexapod early Devonian (410 mya) but molecular clock and
trace fossils suggests insects arose from common ancestor at the Silurian-
Ordovician boundary approximately 434-421 (myr BP) (Gaunt and Miles, 2002)
First definitively winged insects appear in the Early Carboniferous (after 345 mya)
(Garrouste et al., 2012) describe putative winged insect (based on mouthparts) from
late Devonian (360 mya)
Earliest undisputed fossil records of hexapods are early Devonian (390 mya)
wingless creatures resembling modern springtails (Collembola) and jumping
bristletails (Archaeognatha), first winged insect fossils make appearance later in
the Devonian Silurian trace fossils are very hexapod-like, molecular clock data
suggest early Silurian origin for the insects
Characteristics + Morphology
- Body composed of 19 true segments and divided into 3 tagmata – head, thorax and
abdomen
- Head (5 segments) bear: compound eyes + ocelli, one pair of antennae, clypeolabrum,
mandibles, first maxillae, fused second maxillae form a labium or lower lip
- Thorax (3 segments) bears: 3 pairs of uniramous walking legs
- Abdomen (11 segments) without fully developed legs
- Gas exchanged by spiracles and trachea
- Ectodermally derived malpigian tubules
Diversification + Evolution
Proposed innovations that have acted as drivers for hexapod richness: origin of
insect body plan, flight, the capacity to fold the wings, origin of complete
metamorphosis, ecological opportunities (evolution of flowering plants and
parasitism) (Nicholson et al., 2014)
Large degree of disparity existing among the major sub-clades – eg. orders
Zoraptera and Coleoptera differ in richness by
four orders of magnitude (32 and 350,000
described extant species, respectively)
Phylogenetic distribution of extant richness
suggests Metamorphosis is a key innovation driving
diversification in insects
(Nicholson et al., 2014)
Both fossil family origination and extinction rates
increased in groups that have wings but not the
other key innovations, while insects with complete
, metamorphosis have lower extinction rates than their sister group without this
innovation
Tests for the effect of the insect bauplan (comparing non-insect Entognatha with
ectognathan Apterygota) or wing folding (comparing Palaeoptera with Polyneoptera)
indicate no significant differences in net diversification rates
Body size variation in insects (Chown and Gaston, 2009)
- Oxygen availability probably influenced early diversification of arthropods and
diversity of insects began to increase only following the end of a low oxygen period
(Romer’s gap) during early Carboniferous
- Diversification disrupted by notable, large extinction events but continued till
present with some periods showing important patterns, such as significant increases
in herbivory during periods of high CO2 partial pressure (Currano et al., 2008)
- Oxygen and gigantism: Gigantism taxonomically widespread in the late Palaeozoic
(including Protodomata: wingspans 710mm, Paleodictyoptera: wingspans 560 mm,
Ephermeroptera: wingspans 450mm)
- One highly supported mechanism for occurrence of gigantism (also occurred for
other invertebrate and lower vertebrate groups) = hyperoxia + hyperbaria in the
Paleozoic atmosphere, leading to relaxation of constraints on tracheal diffusion and
power demands of flight musculature of winged species (Dudley, 2002)
- Oxygen availability would also have been enhanced in aquatic larval stages of many
groups but gigantism also common in terrestrial species
- Oxygen pulse hypothesis also consistent with subsequent loss of these forms with
increasing hypoxia in the late Permian (Huey and Ward, 2005) and evolution of large
size in at least one group (Mayfly, Hexagenitidae) during second oxygen peak in the
Cretaceous
- Hypothesis well explored empirically and theoretically from the perspective of
changes in tracheolar density and gas exchange mechanics that might offset
alterations in ambient oxygen availability
- Significant relationship between maximum size and atmospheric O 2 concentration
until Early Cretaceous (~130 Mya) – Fitting different statistical models to data on
fossil wing span and O2 concentration (Clapham and Kerr, 2012)
- Role of biotic interactions in limiting size
Extreme case of giant forms during the Paleozoic –
aerial predator Protodonata – wingspan of 71cm (Shear
and Kukalova-Peck, 1990)
Size, Fitness and Oxygen concentration
Arthropod subphylum Hexapoda comprises class Insecta and three other small,
closely related, wingless insect-like groups: Collembola, Protura + Diplura
Hexapoda are united on basis of:
a) Distinct body plan of head, thorax, abdomen – unique synapomorphy of Hexapoda
b) three pairs of thoracic legs, one pair of antennae
c) three sets of “jaws” (mandibles, maxillae, labium)
d) aerial gas exchange system composed of tracheae and spiracles
e) Malpighian tubules formed as proctodeal (ectodermal) evaginations and amont
the Pterygota, (wings)
Hexapods are fundamentally terrestrial arthropods, groups inhabiting aquatic
environments have secondarily invaded those habitats through behavioural
adaptations and modifications of aerial gas exchange systems
Most spectacular evolutionary radiation among Hexapoda = insects – inhabit every
conceivable terrestira, freshwater habitat and less commonly, sea and marine
littorial region – oil swamps, seeps, sulphur springs, glacial streams, brine ponds
Published described species established range from 890 000 to over a million,
abundant and diverse
Evolutionary exploitation of developmental genes working on segmented and
compartmentalised bodies, coevolution with plants (particularly angiosperms),
miniaturisation, invention of flight
Insects are key predators of other invertebrates, diets of many terrestrial
animals, major role as reducer-level organisms in food webs – due to sheer numbers,
also constitute much of matrix of terrestrial food webs, biomass and energy
consumption exceed vertebrates
Phylogeny
Arthropod tree following
the Mandibulata hypothesis
(Edgecombe et al, 2012)
Subphylum Hexapoda
comprises the class
Insecta/Ectognatha and
other wingless groups (class
= Entognatha)
Entognatha – base of
mouthparts hidden within
head capsule, mostly soil
inhabiting detrivores
Fossil Record
, First undisputed fossil Hexapod early Devonian (410 mya) but molecular clock and
trace fossils suggests insects arose from common ancestor at the Silurian-
Ordovician boundary approximately 434-421 (myr BP) (Gaunt and Miles, 2002)
First definitively winged insects appear in the Early Carboniferous (after 345 mya)
(Garrouste et al., 2012) describe putative winged insect (based on mouthparts) from
late Devonian (360 mya)
Earliest undisputed fossil records of hexapods are early Devonian (390 mya)
wingless creatures resembling modern springtails (Collembola) and jumping
bristletails (Archaeognatha), first winged insect fossils make appearance later in
the Devonian Silurian trace fossils are very hexapod-like, molecular clock data
suggest early Silurian origin for the insects
Characteristics + Morphology
- Body composed of 19 true segments and divided into 3 tagmata – head, thorax and
abdomen
- Head (5 segments) bear: compound eyes + ocelli, one pair of antennae, clypeolabrum,
mandibles, first maxillae, fused second maxillae form a labium or lower lip
- Thorax (3 segments) bears: 3 pairs of uniramous walking legs
- Abdomen (11 segments) without fully developed legs
- Gas exchanged by spiracles and trachea
- Ectodermally derived malpigian tubules
Diversification + Evolution
Proposed innovations that have acted as drivers for hexapod richness: origin of
insect body plan, flight, the capacity to fold the wings, origin of complete
metamorphosis, ecological opportunities (evolution of flowering plants and
parasitism) (Nicholson et al., 2014)
Large degree of disparity existing among the major sub-clades – eg. orders
Zoraptera and Coleoptera differ in richness by
four orders of magnitude (32 and 350,000
described extant species, respectively)
Phylogenetic distribution of extant richness
suggests Metamorphosis is a key innovation driving
diversification in insects
(Nicholson et al., 2014)
Both fossil family origination and extinction rates
increased in groups that have wings but not the
other key innovations, while insects with complete
, metamorphosis have lower extinction rates than their sister group without this
innovation
Tests for the effect of the insect bauplan (comparing non-insect Entognatha with
ectognathan Apterygota) or wing folding (comparing Palaeoptera with Polyneoptera)
indicate no significant differences in net diversification rates
Body size variation in insects (Chown and Gaston, 2009)
- Oxygen availability probably influenced early diversification of arthropods and
diversity of insects began to increase only following the end of a low oxygen period
(Romer’s gap) during early Carboniferous
- Diversification disrupted by notable, large extinction events but continued till
present with some periods showing important patterns, such as significant increases
in herbivory during periods of high CO2 partial pressure (Currano et al., 2008)
- Oxygen and gigantism: Gigantism taxonomically widespread in the late Palaeozoic
(including Protodomata: wingspans 710mm, Paleodictyoptera: wingspans 560 mm,
Ephermeroptera: wingspans 450mm)
- One highly supported mechanism for occurrence of gigantism (also occurred for
other invertebrate and lower vertebrate groups) = hyperoxia + hyperbaria in the
Paleozoic atmosphere, leading to relaxation of constraints on tracheal diffusion and
power demands of flight musculature of winged species (Dudley, 2002)
- Oxygen availability would also have been enhanced in aquatic larval stages of many
groups but gigantism also common in terrestrial species
- Oxygen pulse hypothesis also consistent with subsequent loss of these forms with
increasing hypoxia in the late Permian (Huey and Ward, 2005) and evolution of large
size in at least one group (Mayfly, Hexagenitidae) during second oxygen peak in the
Cretaceous
- Hypothesis well explored empirically and theoretically from the perspective of
changes in tracheolar density and gas exchange mechanics that might offset
alterations in ambient oxygen availability
- Significant relationship between maximum size and atmospheric O 2 concentration
until Early Cretaceous (~130 Mya) – Fitting different statistical models to data on
fossil wing span and O2 concentration (Clapham and Kerr, 2012)
- Role of biotic interactions in limiting size
Extreme case of giant forms during the Paleozoic –
aerial predator Protodonata – wingspan of 71cm (Shear
and Kukalova-Peck, 1990)
Size, Fitness and Oxygen concentration
