Electronic Reserve Reading from Evans re: Development, Hormones, Molting. http://www.lib.washington.edu/types/course/

• Metamorphosis and Holometabolism

• Stages of Development

• Unusual Variations in Holometaboly

• Molting and Its Control

• Polymorphism

• Longevity, Aging, & Senescence

OUTLINE

EmbryologyEmbryology

• 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 HistoriesMain 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.

Major types of insect metamorphosis

Unusual intermediate types (single orders)

Ametabolous~.1%

Hemimetabolous~9.9%

Holometabolous~90%

whitefliesthrips

Relative species diversity by

development type

mayflies

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

none partial “pads” complete

1 2 3 4 5 6

First instar cricket - its main function is to emerge fromthe subtrranean-laid egg and squirm to the surface, after which it quickly molts into a 2nd (feeding) instar.

Growth and development (molting & metamorphosis) in a chironomid midge, a holometabolous insect.

from Gullen & Cranston, 2000

Growth & holometabolyin Danaus plexippus,the monarch butterfy

1st instar and egg chorion

last larval instar

Chrysalis, outer cuticle isskin of last larval instar

imago, or adult

From Borror, Triplehorn, & Johnson, 1989

Some insect larval types. In terms of numbers and biomass, most insect life at any one time consists of larvae.

DIPTERA

HYMENOPTERA

COLEOPTERA

An example: development ofwing buds in a caterpillar larva (LEPIDOPTERA).

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.

from Evans 1984

Imaginal discs in Drosophila

courtesy of Dr. James Truman

Growth and morphogenesis of Drosophila leg imaginal discs

L1 L2 L3 wander pupariation embryo

sizecourtesy of Dr. James Truman

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

Major insect pupal types. The insect pupa represents a stage of tissue reorganization.

A, B: obtect,limbs appressed

butterfly moth parasitic wasp beetle beetle muscoid fly

C, D, E: exaerate,limbs loose, movable in some spp.

F: coarctate,enclosed in last larval skin

Pupal eclosion of a muscoid fly showing inflated ptilinum.

Normal adult face with ptilinum withdrawn.

in a Rhipiphorid beetle, COLEOPTERA

in Poropoea, a chalcidoidparasite of beettle larvae,HYMENOPTERA

Hypermetamorphosis

from Evans, 1984

Hormones and MoltingMain 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).

from Evans 1984

Neurosecretory aspects of the insect brain. Neurosecretory cell clusters may contain as few as a single cell each.

Basic hormones, pathways, & control of molting in holometabolous insects.

= PTTH

=ecdysone

= JH

from Gullen & Cranston 2000

REVIEW:basic insect cuticle structure

from Evans 1984

The seven basic phases of insect molting.

from Gullen & Cranston 2000

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 its orchestration is one of the major elements of evolutionary change between them.

Molting phases and hormonal influences in a caterpillar.

post-eclosion insects in teneral phase (= “callows”)

Cricket, ORTHOPTERA

True bug, HEMIPTERACockroach, BLATTODEA

Molting (ecdysis):

• Usually takes place in early morningbecause of peak humidity.• Precarious because of helplessnessof molting insect.• Faulty molting is a major cause ofmortality.

PolymorphismDef.: 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 aphida), b) ovoviviparous, apterous forms Summer, plentiful, rich foodc) 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).

housekeeping, nursing signaling

foraging

Some discrete age-related worker tasks:

Age-related polyethism in the honey bee, Apis mellifera. Responsive (to colony & environment), structured (by age), but flexible (contingent on colony needs).

from Winston 1987

Parasite-causedpolyphenism ina solitary bee, Andrena sp.(HYMENOPTERA)

Parasite:

Insect Longevity

Determining Factors Genetic Environment Mortality Factors (season, life stage) Physical Factors (temperature, humidity) Timing (especially season)

• 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.

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 InsectsCicadas: 17 years (mostly as nymphs)Some wood-boring beetles: many yearsQueen honey bees: ~12 yearsQueen termites: > 20 years

Long-lived beetle.emergence holeimported sculpture

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.

from Gullen & Cranston 2000

The most reliable age grading of larvae depends on rigid body parts, e.g. head width &/or mandible dimensions.

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