Development and diapause Flashcards
how do insects grow with an exoskeleton
- Have to break out of old exoskeleton and make new, larger one (called molting)
- Means their growth is stepwise, not continuous
define instar
each step in the insect’s development
instar - what are immatures called
nymphs or larvae
instar - what is the final instar
adult
Insect development follows what general pattern
- egg
- first immature stage
- feeding and growing
- metamorphosis
insect development - egg
embryo develops in the egg, which hatches into the next development stage
insect development: first immature stage - In Hemimetabolous insects
called a nymph
insect development: first immature stage - In Holometabolous insects
called a larva
insect development - feeding and growing
The first instar nymph/larva feeds and grows, molting into successively larger insects, until the next development stage
insect development - metamorphisis
- the final molt into the adult (sometimes called imago) form
- focused on mating and reproduction (also dispersal)
What happens during molt?
- Apoysis
- Begin new cuticle development
- Continue new cuticle development
- Ecdysis
- Expansion
- Complete development of new cuticle
what happens during molt? - apoysis
epidermis separates from cuticle
what happens during molt? - Begin new cuticle development
- Begin new cuticle development under the existing old cuticle
- excrete molting gel
what happens during molt? - Continue new cuticle development
activate enzyme in molting fluid to digest (and recycle some of) old cuticle
what happens during molt? - Ecdysis
cast off old cuticle
what happens during molt? - Expansion
expand body to fill new cuticle (swallow air or water)
Hemimetabolous development
- Stingbug (Hemiptera)
- Grasshopper (Orthoptera)
- Dragonfly (Odonata)
Hemimetabolous development - stingbug (Hemiptera)
nymphal instars are more or less similar, but adults have wings (and genitals)
Hemimetabolous development - gasshopper (orthoptera)
- nymphal instars are more or less similar, but adults have wings (and genitals)
- partial development of wings in 4th and 5th instars
Hemimetabolous development - dragonfly (odonata)
- While nymphs still similar from instar, adult form can be very different, especially in groups with aquatic nymphs and winged adults
- old last nymphal instar exoskeleton: nymph climbs out of water before metamorphosis
explain insect state after molting (ecdysis)
- Insect is vulnerable (soft and immobile) for several hours after molt until new cuticle hardens
- Especially so after imaginal (adult) molt, as wings are very delicate
insect state after molting (ecdysis) - what is the soft form called
‘callow’ or ‘teneral’ adult
insect state after molting (ecdysis) - what does the new adult often do?
New adult often swallows air to ‘inflate’ new cuticle; often hangs down to assist with wing emergence
Holometabolous development (aka ‘complete metamorphosis’) - life stages
- Egg
- Larva
- Pupa (this is distinct from Hemimetabolous insects)
- Adult (or imago)
Holometabolous development (aka ‘complete metamorphosis’) - larva
Larvae very distinct from adults
Holometabolous development (aka ‘complete metamorphosis’) - pupa
- ‘resting stage’
- no growth, but massive physiological reorganization for larva-adult transition
- Often involves silk or other cocoon
- Whiter during earlier stages, when its about to emerge, more color is present
Different types of larvae
- Larval forms can be diverse
- Some may have > 6 legs (=prolegs) or no legs at all
Hormonal control of molt
- Juvenile hormone (JH)
- PTTH (Prothoracicotrophic Hormone)
- Ecdysone (or ‘ecdydteroid’)
hormone control of molt - Juvenile hormone (JH)
- high titers maintain larval molts
- secreted by corpora allata (neurosensory organ) when stimulated by neuropeptides from the brain
hormone control of molt - PTTH (Prothoracicotrophic hormone)
secreted by the brain, stimulates prothoracic gland to produce ecdysone
hormone control of molt - ecdysone (‘ecdyteroid’)
stimulates production of new cuticle, molt
explain the hormone levels that control molt
- If ecdysone is high -> molt
- If JH is high -> molt is larva-larva
- If JH is low -> molt is larva-pupa
- If JH is absent -> molt is pupa-adult
Some variations on development
- Diapause
- Genetic polymorphism
- Environmental polyphenism
variations on development - diapause
- Arrested development to get through unfavorable period
- Usually overwintering, or waiting out dry period
- Often egg or pupa (no growth), but all stages diapause in at least some species
variations on development: diapause - what breaks diapause?
usually photoperiod, temperature, moisture, or a combination of them
variations on development - Genetic polymorphism and mimicry
- forms depends on genotype
- genetic polymorphism (Mendelian alleles) maintains discrete forms of mimic
variations on development - environmental polyphenism
- forms depends on environmental influences (developmental plasticity)
- aphids are an example
variations on development: environmental polyphenism - aphid example
Alternate between winged/wingless sexual/asexual forms
variations on development: environmental polyphenism - why can aphids alternate?
- Asexual reproduction and wingless morphology lets aphids exploit resource bonanza on host and channel resources to reproduction (not mating, flight)
- but the ability to product winged forms, mate in autumn preserves benefit of dispersal and sexual reproduction
