Aquatic Insect Life Cycles – variable responses
to multiple selective forces
Females can lay eggs all at once (shortlived)or multiple times (longer-lived)
Eggs can hatch:
-Immediately  single juvenile cohort
-Delayed, then all at once
-Continuously  multiple cohorts
Adults can emerge:
synchronously
(all at once)
time)
Aquatic
Terrestrial
Egg  Juvenile instars  (Pupa) 
GROWTH & DEVELOPMENT
Winter
Spring
Temperature
Autumn
asynchronously
(spread over
Time
Adult
EMERGENCE
Summer
Females can lay eggs all at once (short-lived)or
multiple times (longer-lived)
Eggs can hatch:
-Immediately  single juvenile cohort
-Delayed, then all at once
-Continuously  multiple cohorts
Adults can emerge:
synchronously
(all at once)
Aquatic
Egg  Juvenile instars  (Pupa) 
GROWTH & DEVELOPMENT
EVOLUTION
Autumn
Winter
asynchronously
(spread over time)
Terrestrial
Adult
EMERGENCE
Spring
Summer
– maximizes individual fitness
 maximize growth (g) and minimize mortality risk (µ)
Why maximize g?
How to maximize g?
Avoid competition?
Larger female size  more eggs
Females can lay eggs all at once (short-lived)or
multiple times (longer-lived)
Eggs can hatch:
-Immediately  single juvenile cohort
-Delayed, then all at once
-Continuously  multiple cohorts
Adults can emerge:
synchronously
(all at once)
Aquatic
Egg  Juvenile instars  (Pupa) 
GROWTH & DEVELOPMENT
Autumn
Winter
asynchronously
(spread over time)
Terrestrial
Adult
EMERGENCE
Spring
Summer
How minimize µ?
What are sources of mortality risk?
Biotic
Abiotic
Juvenile (aquatic)
Predation (invertebrates, fish,
predator numbers)
Disturbance (frequency and
intensity)
Adult (terrestrial)
Predation (insects, spiders, birds)
Air temperature (limited
aerial season)
These factors can vary a lot in different streams and regions!
Upshot:
Over evolutionary time species have “balanced” the
interacting selective factors that influence growth and
mortality of both juveniles and adults in an attempt to
maximize fitness. This results in a very wide wide range
of life cycle strategies, including:
- number of generations per year
- egg development time and synchrony of hatching
- timing and synchrony of adult emergence
These strategies can sometimes vary within a species
from stream to stream (or region), reflecting the local
environmental selection forces that affect the balance of
growth and mortality in the juvenile and adult life stages.
Temperature
Temperature is important because it:
(1) directly controls metabolic rates of ectothermic
organisms (invertebrates, fish)
• Affects growth and development
• Sets species ranges (latitude and altitude)
• Species adapted to different temperatures
(2) controls dissolved oxygen concentrations
(3) is relatively predictable over annual cycle and
can provide an evolutionary cue for adaptation
Temperature and thermal regimes
• Thermal optima are species-specific
– Differences in species growth rates (Fig. 5.18)
– Geographic range reflects thermal optima and tolerances (Fig. 7.5)
• Also true for fish
Most aquatic species have positive
growth over the winter
- unlike terrestrial insects (resting eggs or adult diapause)
– Some species extremely cold-adapted (Fig. 6.12 (Ward 1992)) even under ice!
Insect Eggs
• When laid, they can
– a) all hatch immediately
– b) hatch over time (Fig. 3.17)
– c) delay hatching
• (diapause = state of arrested
metabolism)
• Some eggs require cold winter
temperature to break diapause
• Phenotypic Plasticity
– e.g., some stoneflies
• perennial streams – eggs hatch immediately
• intermittent streams – diapause for 9-11 months
– How much flexibility? Genetic vs. Phenotypic?
For most species, don’t know
• What happens when temperature changes?
– Change in thermal regime  change in cues
– Example: Hypolimnetic (deep) release Dam on
Saskatchewan River, Canada
• Many aquatic insects in
far North have winter egg
diapause.
• Rising spring temps after
ice-out are required to
break diapause [Fig. 3.14]
4°C
• Dam warms winter water
temp. to 4°C for 10s of
kilometers
Biotic response:
30 genera and 12 families eliminated
 1 family remains! (any guesses?)
Growth rate and development:
Growth:
The elaboration of new cells and tissue by an organism
Development:
The gradual increase in complexity of an organism’s cellular
and tissue organization
For aquatic insects, culminates in production of adult
tissues (wings and gonads)
Adult body size depends on growth rate,
which is temperature dependent
If you extend the temperature axis out
(warmer), you’ll see a peak (and then
decline) in growth rate for each species
Figure 10: Same insect species in 2 different
streams in the same catchment. For this
species, which stream has a more
“optimal” temperature?
Growth rate and development:
– High temp  high metabolism so high respiration loss
(metabolic heat)
– Low Temp  slow metabolism and low growth potential,
available energy allocated to adult tissue
• There is an optimal temperature that maximizes adult body
size
– Established experimentally for several insect species by Vannote and
Sweeney (1980)
– Consider an insect species that must complete development in a fixed time
(e.g., univoltine)
Small body size because
most energy to adult tissue
Small body size because high
maintenance costs (cellular
respiration cost)
Adult
body size
(growth)
Energy to maintenance (resp. loss)
Energy to adult tissue
Temperature
Thermal Equilibrium Hypothesis
(Vannote & Sweeney 1978)
– The optimal thermal regime for species
maximizes
• adult body size (and fecundity)
• metabolic efficiency
• population abundance
Where in a species’
geographic range would
individual body size and
population size be
greatest?
Center of the species range!
Why?
Small body size because
most energy to adult tissue
Adult
body size
Small body size because high
maintenance costs (cellular
respiration cost)
(growth)
Energy to maintenance (resp. loss)
Energy to adult tissue
Temperature
• What environmental factors might be complicate this
expectation?
Adult emergence:
• Juveniles develop adequately to become sexually mature
• If temperature too cold, then what?
– 1) delay maturation, e.g., go from bivoltine to univoltine -not all species can do this
– 2) population cannot persist locally
• Species differ in timing
– a) fixed temperature of
emergence
• [Fig. 7.13 Sweeney et al.
1992]
• What is Advantage?
(Hint: cue)
– b) fixed number of
degree-days > 0°C
[draw on board]
• Photoperiod can also be a trigger
– Why?
• good predictor of air temperature for adult
– May be interaction between water temp and photoperiod ?
• 2 Types of emergence:
– Synchronous, i.e., all adults
appear within a few hours
• Mayflies - Adults don’t feed
• Evolutionary reason for
synchronous emergence?
– swamp predators
– find mate
– Asynchronous, i.e., adults
emerge over time
• Often variation in size of adult
– Later emergers usually smaller (Fig. 10) possibly because growth
rates in stream are not same for all individuals (depends on food
acquired) and smaller individuals may wait longer to emerge so they
can close the gap in body size yet still find an aerial mate.
• What determines thermal regime for a stream?
– Stream thermal regime depends on environmental and landscape
“setting” (Fig. 3.1 (Giller & Malmqvist 1998))
Environmental
controls on stream
water temperature
– Also, thermal regime has components or features that influence
growth and development (Fig. 6.1 (Ward 1992)) [note similarity to flow
regime components]
Environmental
Controls
Biologically-important
components of the thermal
regime
Stream temperatures vary with latitude:
and position in a stream network
For several streams in the same catchment:
-Why different thermographs?
-Which has more degree-days >0 per year?
Hint
Note: Multiply
Y-axis by 10
• 3 Types of life cycles
• Seasonal cycles
– immatures within species of roughly similar age (“cohorts”)
– (1) Slow Seasonal
– (2) Fast Seasonal
• Nonseasonal
– individuals of several ages are present at all times.
– (3) Non-seasonal
• 3 Types of life cycles
• (1) Slow Seasonal
[Fig. 5.2, Merritt & Cummins 1996]
– Eggs hatch quickly, juveniles grow slowly, maturity in ~1, 2, 3 years
• Species may have extended hatching period
• Examples -- many univoltine stoneflies, some mayflies and caddisflies
– Where adaptive?
time
• Stable, perennial streams
instars
• 3 Types of life cycles
(2) Fast Seasonal
– Long egg or juvenile diapause, followed by rapid growth to maturity
• Some species emerge in Spring, Summer, Fall
– Where adaptive?
– Intermittent streams
– Perennial streams with
competitors present
(e.g., Anagapetus bernea)
time
• Stable, perennial streams to avoid competition with similar species
• Streams with short growing period
instars
• 3 Types of life cycles
• (3) Non-seasonal
– Many size classes present
• overlapping cohorts of multivoltine species (chironomids)
• poorly synchronized (extended mating/egg-laying)
• species life cycle spans > 1 yr (stoneflies, dobson flies)
- In frequently disturbed streams
with high juvenile mortality (“bet
hedging” life history strategy)
- In cold, stable streams that
require more than one year to
complete life cycle.
time
- Where adaptive?
instars
• In cold weather (e.g., February) you can find
small aerial aquatic insects along Poudre R.
(midges, Chironomidae)
• How do they survive?
• Which life cycle type would you put them in?
• What kind of voltinism would they exhibit?
• What emergence cue would they use?
• Functional Feeding Groups
(Cummins 1973)
– Invertebrates that obtain food resource in
similar way
• Main Functional Groups
• Shredder
– Eat coarse particulate organic matter
(CPOM, > 1 mm)
• Collector-filterer
– Eat suspended fine particulate organic
matter (FPOM, < 1mm)
• Collector-gatherer
– Eat sedimentary (benthic) FPOM
• Grazer
– Eat algae, microbial biofilm
• Predator
Number of North American Genera by
Functional Group for 3 Major Orders
Shredder Collector
Grazer Predator
Ephem
5
50
25
10
Plecop
35
10
15
45
Trichop
65
25
50
30