ENTM Lec 6 Insect Life Histories

what is life history

• life history describes the schedule & duration of key events in an organism’s lifetime

  • I.e Life cycle + factors that influence life cycle

how does life history vary

They vary with:

• Voltinism

• Polymorphism & Polyphenism

• Dormancy

• Mechanisms for survival under extreme conditions

what is voltinism

The number of generations completed in one calendar year

what is univoltine life cycles

during specific months (or seasons) they have a planned routine

what is Multivoltine life cycles

where an insect completes three or more full generations within a single year

what is semivoltine life cycles

where insects takes more than one year to complete a single full generation (cicadas)

Voltinism may or may not be a fixed species trait meaning…

Some insect species, the generation per year is hardwired into their DNA while for other species, it is flexible & changes based on weather or location

what are examples of short lived adult insects & long lives adult insects

Short

• mayflies

long

• queen ants

• queen termites

• cicadas

what is polymorphism/polyphenism

Many insects exhibit multiple ‘morphs’ or phenotypes

– The production and maintenance of

phenotypic differences between individuals is

termed polymorphism when it is controlled

genetically

– When there is no underlying genetic basis,

rather it is determined by environmental

conditions it is termed polyphenism

example:

• sexual diphormism (gender)

• winged & wingless morphs - antagonistic pleiotropy

• Environmental cues

  • colour polyphenism

• 
  

polyphenism + polymorphism in bees

• drone vs worker= genetic polymorpishm

  • turn into more drones since only have one set of chromosomes

• queen vs worker

  • another worker if light feeding

  • queen if heavy feeding +royal jelly

How do insects survive in harsh weather conditions

Ex: extreme cold/heat/aridity/ temporal uncertainty

By migration

• moving away from an original home range to avoid unfavourable conditions

• facilitated by distinctive pre- & post-movement behaviours

how do insects respond to cold

Insects overcome cold by combinations of:

• Temperature regulation

  • Behavioural and/or morphological responses

• Temperature compensation

  • Cryoprotection

• This may or may not involve periods of dormancy (particularly during winter or dry periods)

Examples:

• shivering

• basking (direct sunlight to absorb environment heat)

• insulation (fur)

• changing colour (dragon fly)

• aggregation (grouping together)

what is dormancy

Interruption in development of insect by a period of dormancy

– During Winter = Hibernation

– During Summer = Aestivation

• Dormancy involves one of two processes:

1. Quiescence

2. Diapause

what is quiescence

A halted or slowed development in direct response to unfavorable conditions

– Indeterminate duration

– Development resumes immediately when favorable conditions resume

What is diapause

Arrested development combined with adaptive physiological changes

– Development does not necessarily resume when suitable conditions resume

– Major environmental factors responsible for inducing or terminating diapause include:

1. photoperiod (day length)

2. temperature

3. food quality

types of diapause

• Obligatory diapause

  • Occurs at a fixed time regardless of environmental conditions

    • Seen in univoltine insects, often occurs during winter in temperate and arctic climates

• Facultative diapause

  • Optional dormancy

    • Seen in many bi- and multivoltine insects in the generation that experiences unfavorable conditions (i.e. winter)

what is winter diapause

• Injury due to severe cold occurs two ways:

• Non-freezing injury

  • Cold shock, results in membrane failure, enzyme and/or ion imbalances

• Freezing injury

  • Mechanical damage from recrystallization

  • Osmotic shock due to ice reducing levels of liquid water

• This injury must be avoided to survive the winter

  • Insects exhibit multiple strategies to achieve this

what is cryoprotection - freeze tolerance

• Control where freezing occurs using Ice nucleating agents

  • Ice nucleating agents (Cryoprotectants) are produced seasonally to encourage freezing outside of cells

  • Additional substances allow supercooling and maintain liquid at freezing temperatures for cellular function

what is cryoprotection - freeze avoidance(/intolerance)

• Prevent freezing to avoid ice damage by using cryoprotectants and proteins

  • Cryoprotectants include Glycerol, ethylene glycol, trehalose

  • Antifreeze proteins bind to the surface of ice crystals as they form

how do insects respond to heat

• High temperatures can cause severe damage to insects by denaturing proteins, altering membrane and enzyme structures and by loss of water

• Strategies to deal with high temperatures include behavioural responses, morphological/physiological adaptation and aestivation

Examples:

• Obelisking (dragonfly tilting tail to minimize surface area

• Stiliting (fully extending legs for more airflow under abdomen)

• Nocturnal activity (burrowing in the day)

• heat shock proteins (first aid kit to keep cells alive)

• Aestivation (summer dormancy)

how do insects repond to aridity

• Aridity (Heat + low humidity)

  • Temperature and humidity are often intimately linked

  • These conditions make maintaining water balance and mechanisms for cooling critical

Example: Fog catching tenebrionid beetles

• Drink dew in sand

• drink dew on plants

• build ridge in sand to catch dew

• make hyrdrophobic regions so water runs down back into mouth