1. Egg  Juvenile instars  (Pupa) 
Aquatic Insect Life Cycles – variable responses to multiple selective forces
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 asynchronously
(all at once) (spread over time)
Aquatic
Terrestrial
Autumn
Winter
Spring
Summer
Egg  Juvenile instars  (Pupa) 
GROWTH & DEVELOPMENT
Adult
EMERGENCE
Time
Temperature

2. EVOLUTION – maximizes individual fitness
Aquatic
Terrestrial
Autumn
Winter
Spring
Summer
Egg  Juvenile instars  (Pupa) 
GROWTH & DEVELOPMENT
Adult
EMERGENCE
Eggs can hatch:
Immediately  single juvenile cohort
Delayed, then all at once
Continuously  multiple cohorts
Adults can emerge:
synchronously asynchronously
(all at once) (spread over time)
Females can lay eggs all at once (short-lived)or multiple times (longer-lived)
EVOLUTION
– 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

3. What are sources of mortality risk?
Aquatic
Terrestrial
Autumn
Winter
Spring
Summer
Egg  Juvenile instars  (Pupa) 
GROWTH & DEVELOPMENT
Adult
EMERGENCE
Eggs can hatch:
Immediately  single juvenile cohort
Delayed, then all at once
Continuously  multiple cohorts
Adults can emerge:
synchronously asynchronously
(all at once) (spread over time)
Females can lay eggs all at once (short-lived)or multiple times (longer-lived)
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!

4. 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.

5. 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

6. Thermal optima are species-specific
Differences in species growth rates (Fig. 5.18)
Geographic range reflects thermal optima and tolerances (Fig. 7.5)

7. Also true for fish

8. Most aquatic species have positive growth over the winter
- unlike terrestrial insects (resting eggs or adult diapause)
Some species extremely cold-adapted (Fig (Ward 1992)) - even under ice!

9. Insect Eggs When laid, they can Phenotypic Plasticity
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

10. 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?)

11. Growth rate and development:
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)
Note: There can also be strong selection for reducing development time to produce more than one generation per year (i.e., even if adults are smaller), especially in environments where likelihood of juvenile mortality is high. More later; now focus on growth.
(previous examples of variable voltinism)

12. 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?

13. 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)
Temperature
Adult
body size
(growth)
Energy to maintenance (resp. loss)
Energy to adult tissue

14. 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?

15. Center of the species range!
Why?
Temperature
Adult
body size
(growth)
Energy to maintenance (resp. loss)
Energy to adult tissue
Small body size because most energy to adult tissue
Small body size because high maintenance costs (cellular respiration cost)
What environmental factors might be complicate this expectation?

16. Adult emergence: If temperature too cold, then what?
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 Sweeney et al. 1992]
What is Advantage? (Hint: cue)
b) fixed number of degree-days > 0°C [draw on board]

17. 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
WHY LATER EMERGERS SMALLER? NOT OPTIMAL TEMPERATURE, MORE RESPIRATORY LOSSES
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.

18. 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

19. 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

20. Stream temperatures vary with latitude:
and position in a stream network

21. 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

22. 3 Types of life cycles (1) Slow Seasonal (2) Fast Seasonal
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

23. 3 Types of life cycles [Fig. 5.2, Merritt & Cummins 1996]
(1) Slow Seasonal
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?
Stable, perennial streams
time
instars

24. 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?
Stable, perennial streams to avoid competition with similar species
Streams with short growing period
Intermittent streams
Perennial streams with
competitors present
(e.g., Anagapetus bernea)
time
instars

25. 3 Types of life cycles (3) Non-seasonal - Where adaptive?
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)
- Where adaptive?
 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
instars

26. 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?

27. 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