Electronic Reserve Reading from Evans re: Development,
Hormones, Molting. http://www.lib.washington.edu/types/course/
OUTLINE
• Metamorphosis and Holometabolism
• Stages of Development
• Unusual Variations in Holometaboly
• Molting and Its Control
• Polymorphism
• Longevity, Aging, & Senescence
Embryology
• The initial action following fertilization is multiplication of
the zygote nucleus and proliferation of protoplasm at the egg
periphery without cell division, the forming of a syncytium.
• This is peculiar to insects and has to do with dense and
voluminous yolk within the egg.
• Cell membranes form shortly thereafter, making the
blastoderm.
• Insect embryos reveal some aspects of early evolution,
including formation of the mouthparts from limb segments.
• In holometabolous species, adult features form from
imaginal discs within the larval body.
Typical insect embryo at different stages.
Insect Development & Life Histories
Main Points:
• Metamorphosis is a transition in form.
• With wings, most important factor in insect evolutionary progression
& diversity.
• Growth in arthropods requires molting.
• The intervals between molts are “stadia”; the form at each interval
is the “instar”.
• There are 3 basic variations of development (metamorphosis) in
insects: ametaboly, hemimetaboly, and holometaboly.
• Holometaboly involves distinct larval, pupal (transitional), and
adult stages.
• Advantages to holometabolous life history include:
reduced larval-adult competition, better timing of activities
with resources, greater efficiency in both larval and adult
phases. Disadvantages include vulnerability of the pupal stage and
complications in larval-adult transition.
Human development, one type of metamorphosis.
Unusual intermediate types (single orders)
Major types of insect metamorphosis
Holometabolous
~90%
whiteflies
thrips
Hemimetabolous
~9.9%
mayflies
Ametabolous
~.1%
Relative species
diversity by
development type
none
1
2
“pads”
partial
3
4
5
complete
6
Hemimetabolous development in a bug (HEMIPTERA, Heteroptera).
Each stage shows progression toward the adult form, best tracked in
the external development of the wings.
Names of hemimetabolous immatures
General: nymph
Terrestrial: nymph
Aquatic: nymph or naiad
First instar cricket - its main function is to emerge from
the subtrranean-laid egg and squirm to the surface, after
which it quickly molts into a 2nd (feeding) instar.
from Gullen & Cranston, 2000
Growth and development (molting & metamorphosis) in a
chironomid midge, a holometabolous insect.
Growth & holometaboly
in Danaus plexippus,
the monarch butterfy
1st instar and egg chorion
last larval instar
Chrysalis, outer cuticle is
skin of last larval instar
imago, or adult
DIPTERA
COLEOPTERA
HYMENOPTERA
Some insect larval types. In terms of numbers and biomass, most
insect life at any one time consists of larvae.
From Borror, Triplehorn, & Johnson, 1989
from Evans 1984
Distribution of
imaginal discs in a
Drosophila larva
(DIPTERA). Many of
the adult features are
preformed and
packaged in the larval
stage. This simplifies
pupal transition.
An example:
development of
wing buds in a
caterpillar larva
(LEPIDOPTERA).
from Evans 1984
Imaginal discs in Drosophila
courtesy of Dr. James Truman
Growth and morphogenesis of Drosophila leg imaginal discs
embryo
L1
courtesy of Dr. James Truman
L2
L3
size
wander pupariation
The formation of late-forming discs requires feeding
during the last larval stage
Starvation suppresses the release of insulin-like growth factors and
causes elevated levels of JH.
courtesy of Dr. James Truman
butterfly
moth
parasitic
wasp
beetle
muscoid fly
beetle
A, B: obtect,
C, D, E: exaerate,
F: coarctate,
limbs appressed
limbs loose, movable in
some spp.
enclosed in last
larval skin
Major insect pupal types. The insect pupa
represents a stage of tissue reorganization.
Pupal eclosion of a
muscoid fly showing
inflated ptilinum.
Normal adult face with
ptilinum withdrawn.
from Evans, 1984
in Poropoea, a chalcidoid
parasite of beettle larvae,
HYMENOPTERA
Hypermetamorphosis
in a Rhipiphorid beetle, COLEOPTERA
Hormones and Molting
Main Points
• Molting is necessary in all arthropods in order for growth to
occur; the “instar” is the particular stage, the “stadium” is
the interval between molts.
• Molting is a complicated, delicate, and precarious act.
• Molting can be divided into 7 steps, as per Evans, 1984.
• The new cuticle is formed before the old is shed; part of the
old cuticle is recycled; the new instar stretches into the new
exoskeleton.
• Major endocrine centers are the brain, corpora allata,
corpora cardiaca, & prothoracic gland.
• Major hormone groups that affect molting include juvenile
hormone (JH), ecdysial hormones (“ecdysone”), &
prothroacicotropic hormone (PTTH).
Neurosecretory aspects
of the insect brain.
Neurosecretory cell
clusters may contain as
few as a single cell each.
from Evans 1984
= PTTH
= JH
=
ecdysone
Basic hormones,
pathways, & control of
molting in
holometabolous insects.
REVIEW:
basic insect
cuticle structure
from Gullen & Cranston 2000
The seven basic
phases of insect
molting.
from Evans 1984
Molting phases and hormonal influences in a caterpillar.
In early instars, ecdysteroid initiates apolysis and formation of new larval cuticle to allow
for growth. Following the last larval instar the homone mix & timing changes and allows
for the formation of pupal cuticle, which forms beneath the (retained) last larval cuticle.
The imago forms within the pupal cuticle and finally emerges from the last larval skin.
Details of the process differ significantly between different groups (orders) of insects and
from Gullen & Cranston 2000
its orchestration is one of the major elements of evolutionary change between them.
Molting (ecdysis):
• Usually takes place in early morning
because of peak humidity.
• Precarious because of helplessness
of molting insect.
• Faulty molting is a major cause of
mortality.
Cricket, ORTHOPTERA
Cockroach, BLATTODEA
post-eclosion insects in
teneral phase (= “callows”)
True bug, HEMIPTERA
Polymorphism
Def.: Marked differences in appearance or behavior
within the same species.
Terms & Determinants:
Polymorphism per se, genetic, e.g. butterfly
mimicry clines, rings. Also the general term (refers to all 3 types).
Polyphenism, environmental:
a. climate, nutrition, e.g. aphids (HEMIPTERA)
b. pollution, e.g. lady beetles (COLEOPTERA)
c. colony-influenced (social/eusocial insects),
e.g. ants, bees (HYMENOPTERA),
termites (ISOPTERA)
d. parasite-influenced, e.g. stylopization
(HYMENOPTERA)
Polyethism, behavioral, hormones, developmental stage, colony conditions
& feedback especially social insects, e.g. caste polyethism in honey bees.
[Wigglesworth: developmental stages “another form of polymorphism”,
ref. especially hypermetamorphosis]
Polyphenism in aphids.
Determined by season, food quality,
crowding, & predator pressure.
Mediated by hormones. In many
spp., involves assexual & sexual
reproductive phase, apterous and
winged phases.
Sunflower aphid
a), b) ovoviviparous,
apterous forms Summer,
plentiful, rich food
c) sexual alate (lays eggs)
Fall, decreasing food
quality, crowding
Polymorphism in social
insects: ants
It involves several axes of
differentiation:
1) sexual [(male vs.
queen (female)] h vs g,
2) reproductive (vs. non
reproductive) h+g vs. a-f,
3) worker castes (grades
of morphology &
behavior) a vs. c vs. f vs. d.
Temporal polyethism in the honey bee,
Apis mellifera (HYMENOPTERA).
Some discrete age-related worker tasks:
housekeeping, nursing
foraging
signaling
from Winston 1987
Age-related polyethism in
the honey bee, Apis
mellifera. Responsive (to
colony & environment),
structured (by age), but
flexible (contingent on colony
needs).
Parasite-caused
polyphenism in
a solitary bee, Andrena sp.
(HYMENOPTERA)
Parasite:
Insect Longevity
• Life cycle duration, (egg to egg) may be dependent on season.
• Adult form may be short-lived seldom survives beyond
reproduction.
• Immature phase almost always longer duration.
• One stage may diapause, extending life duration with no
activity.
Determining Factors
Genetic
Environment
Mortality Factors (season, life stage)
Physical Factors (temperature, humidity)
Timing (especially season)
Age Determination
Usually relative age more meaningful,
i.e. “what instar” vs. “how many days”.
Correlation with size is tenuous
Difficult in larvae (few rigid body parts
to measure)
Factoids: Longest-lived Insects
Cicadas: 17 years (mostly as nymphs)
Some wood-boring beetles: many years
Queen honey bees: ~12 years
Queen termites: > 20 years
imported sculpture
emergence hole
Long-lived beetle.
Why “age-grade” insects?
Some practical examples:
1. Pest population outbreak prediction
Agriculture/phytophage, e.g. Caterpillar or weevil infestations in
alfalfa require timing of management program (spray or harvest).
Medical/disease vector, e.g. mosquito control relies on assessment of
stage of growth, which determines state of population relative to
potential for disease spread.
2. Forensic Entomology
Indicator species, e.g. blowflies. Stage of development of larvae on
corpse indicates approximate time of death.
The most reliable age grading of larvae depends on rigid body
parts, e.g. head width &/or mandible dimensions.
from Gullen & Cranston 2000
Predator-inflicted wing “strikes”, an element of adult wear.
Dispensable wing edges is a common survival strategy.
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long-lived wood-boring beetle