Insect Life History: Mating, Metamorphosis & Growth

High fecundity

Insects lay lots of eggs

High fertility

Many of these eggs hatch

oviparous

Insects are mostly, laying eggs

Where these eggs are laid depends on

the life history of the insect

Ametabolous

No metamorphosis

• Primitive

• Develop with little change to body form

• Adults look largely like immatures

Hemimetabolous

• Incomplete metamorphosis

• Three distinct stages: Egg, Nymph, Adult

• Gradual change in body form, no pupal stage

• ex. milkweed bug life cycle

  • egg

  • instar 1

  • instar 2

  • adult

Hemimetabolous:

Nymphs

• Nymphs resemble adults, have thin

exoskeleton but lack wings

• Undergoes multiple stages of development:

nymphal instars

• Never enters a pupal stage and final molt

results in an adult

Holometabolous

Complete metamorphosis

• Four life stages: egg, larva, pupa, and adult

• Immature stages of very different from adult

• Morphology, physiology, and behavior of each

stage are adapted for different activities

Holometabolous:

Larvae

• Distinct from adult, generally worm-like

• Exploit different food resources than adult

• Many adults lay eggs directly onto food

source, begin eating soon after hatching

• Undergoes multiple stages of

development: larval instars

• Larvae accumulate resources to grow and survive

• Insects consume largest amount of food during this stage

• Excess nutrition stored for adulthood, crucial for metabolically

expensive behaviors: flight

• Larvae eat enough in insects who cannot eat during pupation

Holometabolous:

Pupae

• Pupae is a non-feeding stage

• Sessile, but some species, mosquitoes, are mobile

• During pupation adult structures of insect are

formed while larval structures are broken down

• Pupation may last weeks, months, or years,

depending on temperature and species of insect

• To prepare for pupation, larvae seek protected

sites or construct a protective cocoon

• Silk or own feces

• Some insects undergo diapause as pupa where

both internal and external features change

• Diapause: period of reduced metabolism in

response to environmental cues

Stages of Diapause

1. Initiation: Stimulus signals preparation for diapause

2. Maintenance: metabolism slowed; development halted

3. Termination: intensity of diapause decreased,

either abruptly or in response to stimuli

4. Post-diapause: period of slowed metabolic

activity to allow determination of conditions

• Many insects undergo diapause to survive winter

• Shorter days are key stimuli signaling preparation for diapause

• Some insects accumulate special proteins to lower freezing

point of their body like antifreeze

• Insects eclose from pupae by splitting pupal case

• Most butterflies emerge in morning while

mosquitoes emerge in the evening

• Once pharate adult (new adult) has eclosed from

the pupa, empty pupal exoskeleton is exuvia

Holometabolous

metamorphosis led to

radiation

Majority of insects are holometabolous.

Benefits of Holometabolous Development

• Reduced competition between

immature and adult of the same species

• Potential to occupy different

ecological niches

• Pupal diapause allows insects to better

survive environmental conditions

Insect Growth

Exoskeleton cannot expand with growth

• Must be shed and new one formed: molting

• Stage between successive molts: instar

• Molt allows:

• Change in form of exoskeleton and structures

• Wound healing

• Renewal of exoskeleton

Once adulthood reached,

growth ceases

• Growth only occurs during

the immature stages

Reproduction

• While the immature stages focus on growth

and feeding, adults focus on reproduction

• Reproductive behaviors are extremely diverse,

Often incorporate multiple forms of

communication and stimuli

• Behavior is often sex specific, dimorphic within

a species

• Reproduction can be viewed from resource

perspective, costs and benefits associated with choice

• Takes time: sexual signals and gametes are

metabolically expensive, renders many immobile

• However, effective courtship may provide males access

to “choosy females”

Sexual Selection

• In natural selection, individuals compete for resources

• Sexual selection: competition arises when individuals

compete for mate access

• Mate choice requires one sex to exhibit a trait, and

other to have a heritable preference for said trait

• Leads to the development of elaborate secondary

sexual characteristics that may not be directly

involved in reproduction

Sperm Competition &

Cryptic Female Choice

• Multiple mating females: post copulatory sexual selection

occurs through sperm competition and cryptic female choice

• Sperm competition results as gametes of two or more males

compete to fertilize an egg

• Males may adopt a suit of traits to avoid sperm competition

• Most strategies reduce likelihood

that a female will re-mate

• Energetically costly, but may

benefit male by reducing amount

of sperm needed

• Last male to inseminate female is

often capable of fertilizing

majority of eggs

Sperm Competition

• Males may guard females post copulation, after fertilization

has occurred

• Some males create a “mating plug” to prevent other sperm

from accessing eggs

• Mating plug been shown to deter other males, and cause

females to be less receptive to mating

Traumatic Insemination

• Mating practice: male pierces the

abdomen of the female to avoid mating plug

• Bed bugs reproduce exclusively by this method

• Sperm carried through hemolymph (insect

blood) to ovaries

• Ability to transfer sperm is due to insect open

circulatory system

• All organs interact with hemolymph

Female Adaptation

• Females evolved sperm-receptacles at site of

penetration: spermalege

• Ectospermalege: visible external target

• Mesospermalege: sac attached to inner

abdomen, under ectospermalege

• Sperm injected through the male's

aedeagus into the mesospermalege and

sperm migrate to ovaries

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