Holometabola
Modes of Development in Pterygota (flying insects)
First proposed by Jan Swammerdam – dissections
helped reveal that pupa were a transitional stage
between larva and adult
Ametabolans (plesiomorphically wingless hexapods): no
morphological transformations (continue to moult even
at adult stage)
Hemimetabolans: hatch as nymphs with morphology
similar to that of the adult and grow progressively
until adult stage, gain full flying wings and functional
genitalia, non-edopterygote winged insects, often
described as incomplete metamorphosis
Early instars superficially similar to adults in
morphology, but degree to which this is true is very
variable (wings develop with each moult), no ecdysis in
adults
- Hemimetabolous nymphs resemble the adult but lack fully developed wings and
genitalia
- External wing bunds
Holometabolans: hatch as larva morphologically different from the adult, then
progressively grow through successvie moults until last larval instar, after which
they moult into the pupal stage – often quiescent and similar to the adult and then
to adult stage, with flying wings fully formed and functional genitalia
- Complete metamorphosis – larva differs in morphology and behaviour from the
adult, their trimorphic development is marked by a transitional pupal stage and
interna wing development
- Holometaboly is an autopomorphy of Endopterygota – single evolutionary origin
(Grimaldi and Engel, 2005)
- Holometabolous larvae specialised for feeding
- Internal wing and genital discs
- Final moult generates pupa
- Discs evert to generate adult structures
- 4th type is a variant: represented by highly modified group of dipterans
,Holometabola phylogeny
Diversification coincides with Angiosperm
radiation and development of flowering plants
(Weigman et al., 2008)
Among 3 types of metamorphosis – more
modified is holometabolan which matches
monophyletic clade Endopterygota
In most cases, primordia of adult organs are
placed within the larva of imaginal discs (2n),
and complete development and eversion of them occur in the metamorphic transition
- Larval tissues are polyploid and destroyed during metamorphosis
Best known orders are Coleoptera, Hymenoptera, Lepidoptera and Diptera
Hypermetamorphosis also occurs in some groups – morphological changes in
successvie instars one or more times before reaching the pupal stage (dramatic
changes are associated with specialised predatory habits or with parasitic life
styles) (Neuroptera – Mantispidae, several Coleopters – Meloidae, Rhipiphoridae, all
Sterpsiptera) (Wheeler et al., 2001)
Holometaboly occurred in the Palaeozoic (Nel et al., 2007)
Range of competing theories regarding evolution of holometaboly
Coleoptera
Body usually heavily sclerotized, forewings
sclerotized and modified as rigid covers
(elytra) over hindwings
Hindwings membranous – often reduced or
absent
350 000 – 275 000 species – largest order
of insects
Occur in all environments: rhinoceros beetles can carry up to 100 times their own
weight for short distances, or 30 times their weight indefinitely
Diptera
Most pterygota have 4 wings, ancestors of Diptera
had 4 wings, but hind wings of Diptera modified in
balance organs (halters) act as a gyroscope, providing
high level of fine control during flight
100 000 species – Tsetse fly for example
Mouthparts of flies are designed to suck/pierce
rather than bite, all liquid feeders
Three main suborders: Nematocera, Cyclorrhapha,
Brachycera
, - Cyclorrhapha once reclassified as an infraorder within Brachycera (renamed
Muscomorpha)
- Cyclorrhaphan monophyly supported by the invagination of the larval head capsule,
modifications of the larval mouthparts and pupation within the skin of the last larval
instar
- Fly with a bristle on the upper side of third antennal segment is likely to belong to
the Cyclorrhapha – Brachycera larvae have mouthparts but those of the
Cyclorrhapha (eg. house fly) have virtually disappeared and have evolved into a tiny
pair of hooks maggots
Halteres greatly facilitate controlled flight (McGavin, 2000)
Lepidoptera
120 000 described species – minute to large, sucking
mouthparts, mandibles ususally vestigial, maxillae coupled
Basal Lepidoptera have chewing mandibles
Proboscis an adaptation of maxillae facilitates feeding on
herbaceous plants
Ditrysia contain 98% of species
Scales provide functions including insulation, thermoregulation, aiding gliding flight
and colour patterning
Heliconius butterflies are a long-standing example of mimicry and geographic
diversification (Joron et al., 2006)
- Mullerian co-mimics, H. Melpomene and H. erato look identical in any one site, but
their pattern changes between geographic locations (look up hybrid speciation)
- In H. heurippa colour pattern is involved in mate choice and may arise through
hybridisation
- Evidence from laboratory crosses, biogeographic distribution of colour patterns and
genomics (Mavarez et al., 2006)
Metamorphosis
In Coleoptera and Lepidoptera appendages and
genitalia generated from groups of imaginal cells
set aside during embryogenesis that proliferate
and evert during metamorphosis
In higher Diptera (Drosophila) all head, thorax,
genitalia epidermis is derived from discs (abdomen
from ‘histoblasts”)
Juvenile Hormone and Ecdysone
Wigglesworth initially identified JH through parabiosis experiments in Rhodinus
prolix
