Caddisflies: Architects Under Water

Glenn B. Wiggins
Fig. 1. Trophic categories
of North American
genera in three orders of
aquatic insects, based on
functional feeding groups.
Ephemeroptera (mayflies)
are mainly collectors
and scrapers; Plecoptera
(stoneflies) are mostly
shredders and predators.
In Trichoptera, all functional
feeding groups are
represented by a substantial
proportion of the genera.
Note that the sum of feeding
groups in each order may
exceed the total number
of North American genera
because some genera are
assigned to more than one
group. (Modified from G. B.
Wiggins and R. J. Mackay
1978; data for feeding groups
from relevant chapters in
Merritt and Cummins 1996.)
78
the histograms of Fig. 1. The data analyzed are derived from North American genera in three orders
of aquatic insects, but they are extrapolated globally on the premise that North America provides
a representative sample of the freshwater systems
of the world.
North American genera in three orders of
aquatic insects are categorized by functional feeding groups in Fig. 1: Shredders feed on leaves and
other large pieces of plant material often colonized
by aquatic fungi and bacteria. Collectors gather
small organic particles, by definition <1 mm, which
include feces from shredders and other aquatic
invertebrates, all colonized by fungi and bacteria,
which are the principal food resource. Scrapers
graze films of diatoms and periphyton from rocks
and other substrates. Predators eat other invertebrates.
The histograms in Fig. 1 reveal complementary
feeding behavior between Ephemeroptera and
Plecoptera. Mayfly genera are predominantly collectors and scrapers; stonefly genera are mainly
shredders and predators. An incumbency principle
derived from the competitive exclusion concept of
basic ecology states that organisms well adapted
to one environment or niche impede invaders from
gaining entry to that niche. I infer that natural
selection moved the ecological and taxonomic
diversification of mayflies and stoneflies along conventional pathways of least resistance: each order
became mainly specialized for feeding in ways the
other did not. In the historical context, other insect
orders could have been involved in bringing about
this ecological tension, but mayflies and stoneflies
have persisted successfully since the Carboniferous
and Permian periods and would have been competitors in stream habitats throughout the past 200 to
300 million years.
In contrast, evidence available for Trichoptera
indicates an aquatic existence that began some
100 million years later in the Triassic period of
the early Mesozoic. The histogram for Trichoptera
in Fig. 1 shows no marked suppression in genera
among functional feeding groups comparable
with Ephemeroptera and Plecoptera. Because food
American Entomologist • Summer 2007
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C
addisflies are known primarily for the cases constructed and carried about
by their larvae. However, that is but one part of their architectural repertoire. Portable cases are suited for active foraging, but stationary tubes and
retreats, often provided with filter nets of silk mesh, are constructed by sedentary
caddis larvae for gathering food delivered by stream currents. And pupation offers
an entirely new sector for construction behavior; in different groups of caddisflies,
distinctive cocoons and shelters are constructed for metamorphosis.
In this article, I explore the evolutionary significance of different constructions
by caddis larvae and consider other features of their life cycles that reveal unusual
evolutionary strategies.
An appropriate beginning would be to ask why larval construction behavior is
so fundamental throughout the order Trichoptera because different construction
behavior related to food gathering and to pupation occurs through each of the three
major groups of caddisflies. The answer to this question, I believe, is to be found in
Retreat-Making Caddisflies (suborder
Annulipalpia)
The ecological strategy of larvae of the retreatmaking families is to harness the energy of moving
water in transporting food materials to their fixed
locations. In this way, energy expended in active
foraging for food is conserved, and the associated
risks of predation are reduced. There are, in all,
eight families of caddisflies in which larvae collect
food from fixed retreats, and seven of these families
occur in North America.
Outstanding examples of the subdivision of
feeding niches are the silken filter nets made by retreat-making caddis larvae. Funnel-shaped nets up
to 12 cm or so long fixed in place in slow currents
by larvae in the family Polycentropodidae filter out
small organisms carried in suspension. These nets
filter food from the ambient water column, space
unavailable to mayflies and stoneflies, which are
confined to bottom substrates. Tubular nets of silk
spun by larvae in the family Philopotamidae have
the smallest meshes of all retreat-makers, allowing these caddis larvae to filter very fine organic
particles.
Efficiency in filter feeding is enhanced in the
family Hydropsychidae through nets with a wide
range of mesh sizes. Thus larvae in the hydropsychid subfamily Macronematinae spin nets with
American Entomologist • Volume 53, Number 2
small meshes in slow currents of rivers; larvae in the
subfamily Arctopsychinae (Fig. 2) spin the largest
meshes and feed primarily on insect larvae carried
in the currents of fast-flowing streams. Larvae in
other subfamilies, such as the Hydropsychinae,
spin meshes of intermediate size, occupy a wide
range of current velocities, and filter food particles
ranging widely in size.
Another ingenious device for filter-feeding is
built by caddis larvae in the family Dipseudopsidae.
Larvae in the genus Phylocentropus, widely distributed through eastern and central North America,
build branching tubes of silk and sand grains
beneath the surface of sedimentary deposits in
Fig. 2. Examples of the
subdivision of feeding
niches through larval
construction include the
stationary silken nets spun
by retreat-making caddis
larvae. This larva in the
family Hydropsychidae
spins widely spaced silken
meshes suited to capturing
small insects carried in
strong currents where
it lives. Larvae of other
species in this family live in
slow currents where they
spin filter nets with smaller
meshes. (Photograph: J. C.
Hodges)
streams and lakes (Fig. 3). Larvae induce a current
of water to move through the tubes by extending
the intake and outflow openings above the surface
of the sediment; they divert current carrying food
materials in suspension into the buried network
and undulate their bodies from within to enhance
the flow of water. The larva spins a mass of silken
threads in its buried retreat to filter food particles
from the circulating water.
These examples show that silk and construction
behavior underlie a multiplier effect in enlarging
ecological opportunities for retreat-making caddisflies.
Case-Making Caddisflies (suborder
Integripalpia)
An entirely different strategy is followed by the
case-making families. These larvae forage actively
for food and build portable cases, which they carry
with them as they move over the stream bottom
(Figs. 4, 10). Principal foods of case-making larvae
include decaying plant materials such as leaves that
fall from stream-side trees. The leaves are carried
by the current until they settle in slow-flowing
stretches of streams or are caught up by obstacles in
the current such as logs and rocks. In either event,
the leaves accumulate in patches, and portable
cases allow these larvae, bearing their own predator
protection, to move about to find aggregations of
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resources in aquatic habitats are the same for all
aquatic insects, the means of obtaining food would
make the difference between success and extinction. Consequently, I infer that when caddis larvae
began to exploit freshwater resources, space and
conventional means of obtaining food were largely
monopolized by mayflies and stoneflies. For the
later-arising caddisflies to gain a share of the energy
resources in established freshwater communities,
as the histogram shows they certainly did, natural
selection seized on an unconventional asset—the
silk that caddis larvae produced for constructing
cocoons. Behavior became the focal point for
natural selection in using silk, and construction by
larvae was established as the ladder for ecological
penetration by Trichoptera.
I infer from Fig. 1 that through conventional
feeding behaviors, mayflies and stoneflies populated
freshwater communities to levels approaching saturation. However, caddisflies, which did not appear
until some 100 million years later, were still able to
exploit substantial ecological space in these communities; and I believe this happened because of the
unconventional feeding behavior of caddis larvae
through their varied constructions of cases, retreats,
and filter nets. Fine-tuning of their construction
behavior through natural selection has allowed
caddisflies to insinuate themselves into feeding
niches not fully exploited by other aquatic insects
that lack silk and the behavior for building devices
to enhance their feeding and pupation efficiency,
and hence their survival and reproduction.
Examples of unconventional construction and
feeding behavior are numerous in each of the three
suborders of Trichoptera.
leaves. Fungi and bacteria growing on the decomposing leaves are important sources of nutrients for
these caddis larvae. Other case-making larvae move
over the exposed upper surfaces of rocks, scraping
films of diatoms and other algae.
Experiments conducted under controlled conditions in Sweden by Anita Johansson (1991) and
Christian Otto (2000) show that caddis larvae
deprived of their cases are more vulnerable to
predatory fish than are larvae within cases, and
fish tend to pass over larvae that are inactive and
withdrawn into their cases. Moreover, cases made
of different plant and mineral materials fastened
together with silk in various ways allow larvae to
be less conspicuous in their habitats. Case-making materials of different buoyancy assist larvae
in maintaining positions in stream currents or in
drifting downstream as some caddis larvae do.
The extraordinary variety of architectural forms
and materials in portable caddis cases tells us that
the ecological subtleties of case-building are still far
from being fully understood. About 30 families of
case-making caddisflies are recognized in the world,
and 15 of them occur in North America.
80
Cocoon-Making Caddisflies (suborder
Spicipalpia)
Four families, all occurring in North America,
are assigned to this group: Rhyacophilidae, Hydrobiosidae, Glossosomatidae, and Hydroptilidae.
Despite repeated investigation, relationships of
these four families remain clouded by conflicting evidence and, until some level of congruence
Fig. 4. Many case-making larvae construct portable
cases of small rock fragments, as do these members
of the family Limnephilidae. (Photograph: J. C.
Hodges)
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Fig. 3. Larva and
tube in the retreatmaking family
Dipseudopsidae
(genus
Phylocentropus).
Larvae construct silklined tubes of sand in
deposits of sediment
on the bottom of
streams and lakes.
By undulating its
abdomen, the larva
regulates the flow
of water through the
tube, where silken
threads strain out
suspended food
particles. Sealed
branches may
represent filter nets
of earlier instars;
inset of silk threads
from interior surface
of filter net where
larva feeds. (Arrows
show direction of
water flow; illustration:
P. StephensBourgeault).
emerges, assignment of all families to a third
suborder, Spicipalpia, seems the least disruptive
classification.
These four families share several significant
features, most notably construction of cocoons
entirely closed to ambient water currents (Fig.
5); consequently they are known collectively as
the closed-cocoon-makers or just cocoon-makers.
Of course, all caddis larvae construct cocoons for
metamorphosis, but larvae of retreat-makers and
case-makers make cocoons with meshed openings
at each end, permitting freshly oxygenated water
to flow through the cocoon for respiration of the
developing pupa. In contrast, respiration for pupae
of cocoon-making caddisflies depends solely upon
oxygen diffusing through the silken wall of the
closed cocoon.
The real enigma of these closed cocoons,
however, is that the walls function as osmotically
semipermeable membranes. Scanning electron micrographs reveal that the parchment-like ovoid cocoons are made of two dense layers of silk threads;
their rigid form and darkened color are probably a
result of tanning the silk with phenolic compounds
released by the larvae, as in the sister order Lepidoptera. Osmolarity experiments by workers in
Germany, for example, Wilfried Wichard and coworkers (1993), have confirmed the semipermeable
properties of these caddis cocoons. This contrast
in cocoon construction between the Spicipalpia
and the other two suborders is a focal point in
the unresolved issue of higher level evolutionary
relationships in Trichoptera and is identified as the
cocoon paradox (see Wiggins 2004).
The closed semipermeable cocoons in these four
families are spun within fixed dome-shaped enclosures of rock fragments, although the pupal domes
are of stout silk in some hydroptilid genera. After
completing its cocoon, the larva molts to the pupal
stage. Organic fluids emitted by the molting larva
cannot pass out through the semipermeable wall
of the closed cocoon, and those concentrated ions
set up an osmotic pump that draws surrounding
water, with a lower ionic concentration, through
the silken wall. Positive pressure within the cocoon distends the walls. Respiration is sustained
by diffusion of dissolved oxygen at the boundary
between the semipermeable wall of the cocoon and
the ambient water flowing through spaces between
rock fragments of the enclosure. Darkened turgid
cocoons of Rhyacophilidae and Glossosomatidae
occur commonly in their domed enclosures on
rocks in cool streams where they can be readily
found. When a closed silken cocoon is punctured,
release of the turgor pressure can be observed as
a gush through the opening. The function of this
turgor pressure remains an enigma, but construction of an osmotically semipermeable membrane
Fig. 6. Caddis larva in the family Glossosomatidae;
protected by its domed pupal cocoon enclosure
of rock fragments constructed precociously at the
beginning of the first larval stage, the larva grazes
diatoms from upper exposed surfaces of rocks in
streams. (Illustration: A. Odum)
American Entomologist • Volume 53, Number 2
by any creature using its own raw materials has
to be an exceedingly rare event throughout the
animal kingdom.
Returning to case-making behavior as a malleable asset for gathering food, larvae in the
spicipalpian families Glossosomatidae and Hydroptilidae feed on algae. Glossosomatids graze the
biofilms that flourish on exposed surfaces of rocks,
locations where they would be highly vulnerable
to predatory fish. Construction by glossosomatid
larvae is confined to cocoon enclosures of rock
fragments to moor and protect the pupa during
metamorphosis. Natural selection has, however,
contrived to advance this construction behavior
from the ancestral condition at completion of the
final larval stage to the beginning of the first stage
shortly after hatching from the egg. Consequently,
through precocious construction of cocoon shelters, glossosomatid larvae are able to graze diatoms
from exposed surfaces
completely under the
cover of their domed pupal enclosures (Fig. 6).
Most larvae in the
spicipalpian family Hydroptilidae build domed
pupal enclosures, too;
although, as in no other
Trichoptera, two domes
are fastened together at
their bases to form a
portable case resembling
the shell of a clam (Fig.
7). Again, a shift in timing in the Hydroptilidae
has advanced construction behavior from the
conclusion of the final
fifth larval stage to the
beginning of that stage.
Earlier instars of all hydroptilids are extremely
small and their stages
are brief; therefore, these larvae build no cases
at all. The two-domed structures form the purse
cases typical of most hydroptilid genera (Fig. 7),
and the narrow profile is well suited to moving
through clusters of algal filaments on which these
larvae feed.
The other two families of the Spicipalpia,
Rhyacophilidae and Hydrobiosidae, are unique
among all Trichoptera in their free-living, predatory lifestyle, constructing nothing until the close
of the final larval stage, when they build a domed
enclosure of rock fragments for the cocoon. Among
all Trichoptera, this is the simple behavioral condition that most closely resembles primitive groups
in the sister order Lepidoptera, such as the family
Micropterigidae and could represent ancestral
construction behavior in Trichoptera. Rhyacophilid larvae are common in cool streams of North
America; hydrobiosid larvae in North America are
confined to the Southwest.
In summary, construction behavior throughout
Fig. 7. Caddis
larvae in the family
Hydroptilidae;
larval cases in this
family consist of
two precocious
domed cocoon
enclosures partially
joined together to
form the typical
hydroptilid purse case.
(Illustration: A. Odum)
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Fig. 5. Turgid
closed cocoons
of the family
Rhyacophilidae
removed from
the domeshaped pupal
enclosure of
rock fragments;
prepupal larva
on the right,
pupa on the left.
Organic fluids
emitted by the
molting larva are
unable to pass
outward through
the silken
semipermeable
wall of the
closed cocoon, and those concentrated ions set up an
osmotic pump that draws ambient water, with a lower
ionic concentration, through the silken wall. Positive
pressure within the cocoon distends the walls; the
function of this turgor pressure remains an enigma.
(Photograph: J. C. Hodges)
the suborder Spicipalpia is distinctive among all
Trichoptera in that it is derived solely from pupation behavior. For all three suborders of Trichoptera, it is construction behavior that has opened
access to food resources in ways that would not
have been possible without silk and diversified
architecture.
Fig. 8. Movement of
water generated by
abdominal undulation
(ventilation) through
the case of a typical
case-making caddis
larva. (Illustration: A.
Odum)
Temporary Pools
Ecological penetration of standing waters by
caddisflies has taken one more step—to temporary
or vernal pools. In North America, only three families are known in which natural selection has made
this very substantial leap. The leap is substantial
because vernal pools hold surface water for only
a few months and are dry basins for the rest of
the year—a huge problem for aquatic animals. At
the latitude of southern Ontario, where I studied
vernal pools, these basins retain surface water from
snow melt in March and April usually until July,
about four months (Fig. 9). Few species of aquatic
insects of any kind can survive the alternating wet
and dry phases of temporary pools, but every one
that can live under these conditions has accrued
a set of remarkable survival strategies through
natural selection.
Looking into a temporary pool on a sunny
day in April or May, one sees caddisfly larvae
carrying their cases of plant pieces (Fig. 10). Most
of the animal species in these pools are aquatic
insects—dragonflies, damselflies, mayflies, beetles,
bugs, and midges and other true flies (Diptera) of
diverse sorts, as well as caddisflies. Each species is
adapted in elegant ways to meet the challenge of a
short wet phase and a prolonged dry phase. With
few exceptions, every animal species in temporary
Fig. 9. A typical
temporary pool basin
in southern Ontario
fills with water
when snow melts in
spring. These pools
of water dotting the
countryside for a few
months every year
have something to
teach us about the
way the world works.
(Photograph: E. C.
Wiggins)
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Case-Building and Respiration
Increased access to the energy resources of
freshwater habitats is one aspect of construction
behavior in Trichoptera, but there are others. A
substantial body of evidence shows that portable
tube cases are an asset in respiration. Inside their
cases, larvae and pupae undulate the abdomen,
causing water to move through the case from front
to rear (Fig. 8). An opening is always built into the
posterior end of the case to provide an exit for this
ventilatory flow of water. In this way the tubular
case channels a flow of oxygenated water over the
body and abdominal gills of the insect, facilitating
the gaseous exchange of respiration, and enabling
the insects to regulate their own supply of oxygen.
Under experimental conditions, the rate of abdominal undulation for ventilation increases as levels of
dissolved oxygen decline.
All families of Trichoptera are represented in
cool running waters where breathing is easy, and
phylogenetic analysis indicates that caddisflies
first became aquatic animals in cool streams.
However, among the 15 families of case-makers in
North America, species in 6 of them have become
fully adapted to the standing waters of lakes and
marshes. Experimental evidence suggests that
construction of tubular cases contributed to an
increase in respiratory efficiency to levels allowing
larvae and pupae in these families of Trichoptera
to exploit independently the lentic waters of lakes
and marshes (see Wiggins 1996). This is a remarkable outcome; through the production of silk,
and through flexible larval behavior for construction, a fundamental physiological advantage was
gained, greatly expanding ecological penetration
by Trichoptera.
American Entomologist • Volume 53, Number 2
until the shortening days of late summer initiate
maturation of eggs. This suspension of development, or diapause, occurs in many insects and at
different points in the life cycle. Because of the delay
in ovarian development caused by diapause, these
caddisfly eggs are spared exposure to desiccating
summer conditions in the dry basin of the pool,
but are ready to be laid after cooler and usually
wetter weather follows in September. Soil of the
pool basin absorbs moisture from autumn rains,
and more of the moisture is retained because the
sun’s heat is less intense. In this way, oviposition
has become independent of free water, allowing
these species to deposit eggs on the pool basin after
surface water has disappeared. Eggs are placed on the underside of moist logs
on the waterless pool basin, encased in a thick
gelatinous mucopolysaccharide matrix that can
resist desiccation for months. Eggs begin to develop
immediately, and the tiny larvae hatch from the egg
within two to three weeks (Fig. 12), but only after
being flooded with water do the larvae leave the
egg matrix. Where pool basins are not flooded until
snow melts in spring, the tiny larvae can remain
within the gelatinous matrix through autumn and
winter, a remarkable period of about six months.
When the matrix is flooded with water, the larvae
immediately crawl out of it, construct a crude case,
and begin to forage for food (Wiggins 1973).
Fig. 10. Caddis larva in
the case-making family
Limnephilidae carrying
its portable case as it
forages for food. This
species, Limnephilus
indivisus, is common
in temporary pools of
northeastern North
America. (Photograph:
W. A. Crich)
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pools has been derived from ancestors that lived
in permanent waters. If life cycles in these species
have been shaped through natural selection over
long periods of time, there must be advantages to
living in such unlikely freshwater habitats.
At least two advantages can be recognized. One
involves predation. Fish are major predators of
aquatic insects and other small freshwater creatures,
but fish, at least species native to temperate regions
of the globe, cannot survive the dry phase and hence
are eliminated from temporary pools. Predatory insects such as aquatic beetles and bugs and dragonfly
nymphs do live in temporary pools, but there are far
fewer species, and therefore a less efficient predator
fauna, than in permanent waters. Moreover, populations of the insect predators in temporary pools
have to develop every year from colonizing adults
or from drought-resistant eggs. All this means that
the impact of predation is substantially reduced for
aquatic animals that have adaptations to bridge the
dry phase of temporary pools.
A second advantage is concerned with nutrition and growth. Temporary pools, especially in
wooded sites, are fueled primarily by the energy
and nutrients stored in fallen leaves and other
decomposing plant detritus. Furthermore, the dry
phase of temporary pools usually supports a community of specialized herbaceous plants that can
tolerate springtime flooding but grow rapidly in the
moist, organically enriched soil after surface water
disappears; these plants contribute to the detrital
base every year.
Detritus decomposes faster when exposed to
air during dry periods and has a higher protein
content upon flooding in spring than when submerged continuously in permanent water. The terrestrial fungi causing the decomposition of plant
detritus in the absence of surface water flourish
where oxygen is not limiting. In permanent waters
where detrital materials are covered by water,
oxygen is not plentiful, and decomposition is
brought about by aquatic fungi and bacteria that
do not grow as abundantly as terrestrial fungi.
Colonizing fungi contribute much of the protein
of decomposing plant materials. In feeding experiments, case-making caddis larvae of temporary
pools showed a preference for decaying leaves
with higher protein levels. Consequently, rapid
development, which is so important to animals in
transient water, would be enhanced by the proteinrich detritus as well as by the higher temperatures
of small bodies of water.
Only a tiny fraction of the caddisfly species
living in permanent waters of lakes and marshes
can complete their life cycle in temporary pools.
Adaptations of caddisflies to transient waters are
best understood in certain species of the case-making family Limnephilidae, where several critical
features coincide in the life cycle.
In most temporary-pool Limnephilidae, adults
emerge from the pupal stage as the pool recedes in
June or July (Fig. 11). European researchers Novák
and Sehnal (1965) showed that ovarian development in the females is suppressed for several weeks
Fig. 11. Adult of a case-making caddisfly in the family Limnephilidae that has
just cast off its pupal skin. This species, Limnephilus indivisus, is common in
temporary pools of northeastern North America. (Photograph: W. A. Crich)
83
Fig. 12. The
gelatinous egg
matrix of limnephilid
caddisflies of
temporary pools
is deposited in
September on moist
wood debris lying on
the waterless basin of
the pool. Larvae hatch
from the eggs within
3 weeks but remain
in the matrix until it
is flooded with water,
which may not be until
the following spring—
an interval of about 6
months. (Photograph:
W. A. Crich)
Fig. 13. Case-making caddis larvae in the family
Phryganeidae construct portable cases from pieces of
plant debris cut to size. The cases provide protection
from predators as the larvae forage for food, and are
also an asset in respiration. (Photograph: J. C. Hodges)
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American Entomologist • Summer 2007
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This complex life cycle has been fully understood for a relatively short time. Previously, the
occurrence of adults of the same species in the
spring and autumn of each year was attributed to
two generations, even though larvae to produce
a second generation were never found. Now we
know that larvae of these same species live in
transient waters, and that females reappear in
autumn for oviposition after an inactive summer
period of several weeks in diapause, and we know
also that the first instars are protected in a durable
gelatinous matrix until it is flooded with water.
These two critical adaptations combine to equip a
few caddisflies for life in temporary vernal pools
(Wiggins 1973).
The workings of natural selection are made
clear by comparing the life history of case-making
caddisflies of temporary pools with the ancestral
or primitive conditions retained in most case-making caddisflies. The case-makers are restricted to
permanent waters where adults of related species
in the Limnephilidae are short-lived and lack a
diapause, and females enclose their eggs in jelly
that liquefies shortly after the larvae hatch from
the eggs. Other case-making caddis larvae inhab-
iting temporary pools are members of the family
Phryganeidae (Fig. 13).
In Australia, where phryganeids do not occur
and the few limnephilids are restricted to permanent cool streams, the caddisflies of temporary
pools are species of the case-making family Leptoceridae. Females in these species retain their eggs
until the larvae hatch. By depositing first instars
directly into pools, the caddisflies reduce their
development period in the receding water.
Quite unexpected in my fieldwork was the
discovery that certain species of retreat-making
caddisflies in the family Polycentropodidae live in
temporary pools, too. Evidence suggests that the
females lay their eggs in the water, where they fall
to the bottom and remain in the soil after the pool
disappears. Development appears to be suspended
by diapause, probably until the eggs are subjected
to the low temperatures of winter, followed by
longer days and warmer temperatures of spring
after the pool is filled again, when the eggs hatch.
Because these larvae are predators, hatching in the
spring coincides with the appearance of many prey
organisms in the pool.
The evolutionary picture for caddisflies is that
these insects entered temporary pools along independent adaptive family pathways. The species
of case-making Phryganeidae and retreat-making
Polycentropodidae are chiefly predacious, and temporary pools offer invertebrate prey in abundance.
However, species of the Limnephilidae are the most
abundant caddisflies in temporary pools, and these
larvae are supported through fungi and bacteria
growing on decomposing detritus; from their
method of feeding these larvae are assigned to the
functional group of shredders. Because of their size
and relative abundance, limnephilid caddis larvae
probably represent a substantial part of the shredder biomass in temporary pools. A byproduct of
shredding is fine organic particles, which are a basic
food for filter-feeding and other collector larvae. In
temporary pools these include mosquitoes, midges
and some other Diptera, mayflies, and crustaceans
such as fairy shrimp. Consequently, because detritus provides the energy base for temporary pools,
shredder larvae of the Limnephilidae would be a
keystone group with broad ecological influence in
these communities.
Natural selection has brought about such
marked change in the life cycles of a few caddisflies, allowing them to exploit the resources
of temporary pools, that it became clear that
there must be general patterns of development
through the entire community of aquatic insects
in transient pools. Accordingly, my colleagues
postdoctoral associate Rosemary Mackay and
doctoral candidate Ian Smith and I undertook an
intensive study (Wiggins et al. 1980) of evolutionary and ecological strategies of insects and other
invertebrates in temporary pools. We found that a
restricted and predictable set of species comprising
dragonflies, damselflies, mayflies, beetles, bugs,
mosquitoes, midges, phantom midges, soldier
flies, marsh flies, water mites, crustaceans, snails,
American Entomologist • Volume 53, Number 2
every year have things to teach us about the way
the world works.
This article was adapted from Glenn Wiggins’s
book Caddisflies, The Underwater Architects, published by the University of Toronto Press in 2004;
background information and other references are
available there.
References Cited
Graham, J. B. 1997. Air-breathing fishes; evolution, diversity, and adaptation. Academic Press, New York.
Graham, J. B., R. H. Rosenblatt, and C. Gans. 1978.
Vertebrate air breathing arose in fresh waters and not
in the oceans. Evolution 32: 452–463.
Johansson, A. 1991. Caddis larvae cases (Trichoptera,
Limnephilidae) as anti-predatory devices against brown
trout and sculpin. Hydrobiologia 211: 185–194.
Merritt, R. W., and K. W. Cummins [Eds.]. 1996. Introduction to the aquatic insects of North America, 3rd ed.
Kendall/Hunt Publishing Company, Dubuque, IA.
Niklas, K, J. 1994. One giant step for life. Nat. Hist.
6/94: 22–25.
Novák, K., and F. Sehnal. 1965. Imaginaldiapause bei den
in periodischen Gewässern lebenden Trichopteren, p.
434. In Proceedings of the XII International Congress
of Entomology, London, 1964.
Otto, C. 2000. Cost and benefit from shield cases in caddis
larvae. Hydrobiologia 436: 35–40.
Wichard, W., H. H. Schmidt, and R. Wagner. 1993. The
semipermeability of the pupal cocoon of Rhyacophila
(Trichoptera: Spicipalpia), pp. 25–27. In C. Otto [Ed.].
Proceedings of the Seventh International Symposium
on Trichoptera, Umeå, Sweden, 1992. Backhuys
Publishers, Leiden, the Netherlands.
Wiggins, G. B. 1973. A contribution to the biology of
caddisflies (Trichoptera) in temporary pools. R. Ont.
Mus. Life Sci. Contrib. 88.
Wiggins, G. B. 1996. Larvae of the North American
caddisfly genera (Trichoptera), 2nd ed. University of
Toronto Press, Toronto.
Wiggins, G. B. 2004. Caddisflies, the underwater architects. University of Toronto Press, Toronto.
Wiggins, G. B., and R. J. Mackay. 1978. Some relationships between systematics and trophic ecology in
Nearctic aquatic insects, with special reference to
Trichoptera. Ecology 59: 1211–1220.
Wiggins, G. B., R. J. Mackay, and I. M. Smith. 1980. Evolutionary and ecological strategies of animals in annual
temporary pools. Arch. Hydrobiol. Suppl. 58: 97–206.
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clams, and flatworms lives in temporary pools.
Their survival strategies range through various
types of resistant eggs or cysts to diapause at different points in the life cycle leading to shifts in
the time of oviposition as in the caddisflies (see
above). Moreover, instead of coping with drought,
strategies of some of the insects involve avoiding
drought altogether by overwintering in permanent
ponds and dispersing over the countryside in
spring to deposit eggs in newly flooded temporary
pools. Adaptations of parasitic water mites to accommodate the varied life cycles of their aquatic
insect hosts, already altered to establish places in
temporary pools, verge on amazing.
Seasonally fluctuating waters such as those
characteristic of temporary pools have a long
history associated with evolutionary change. The
first plants to colonize the barren landscapes of
Earth more than 400 million years ago arose in
freshwater environments of the Silurian period.
Paleobotanist Karl Niklas of Cornell University
attributed (1994) the evolution of moisture-conserving adaptations such as the waxy cuticle,
stomata, and cutinized spores that are crucial for
land plants to selection under periodic drought
in habitats with fluctuating water levels. The
first vertebrates to breathe atmospheric air were
fish in fresh waters of Silurian tropical lowlands.
Evidence gathered by University of California
physiologist Jeffrey Graham (1997) and with
co-workers (Graham et al. 1978) shows that low
levels of dissolved oxygen brought on by high
temperatures during recurrent seasonal drought
would have provided conditions that favored
selection for the evolution of air breathing in certain fish through proto-lungs. These vascularized
outgrowths of the esophagus enabled the fish
to respire oxygen from air gulped at the water
surface. Lungfish of today are survivors of these
ancient fish with dual systems of respiration
using lungs and gills. In modern gill-breathing
bony fish, the lungs have become hydrostatic
gas bladders, through regulated buoyancy that
substantially reduce the energy expended by fish
in rising off the bottom and maintaining their
preferred level in the water column. Moreover,
the lungs of the ancient fishes also evolved to
open terrestrial living for Devonian ancestors of
amphibians, followed through time by the entire
panoply of air-breathing vertebrates. Hence the
long view is that seasonally fluctuating fresh
waters initiated the evolution of terrestrial plants
and of air breathing in fish, which opened the
way for terrestrial vertebrates.
This independent congruence of seminal evolutionary events in seasonally fluctuating fresh
waters fully justifies description of these habitats
as crucibles of evolution. And largely unnoticed,
a community of caddisflies and other freshwater
invertebrates has gained sanctuary from predacious fish through natural selection and evolution
stimulated by the same recurring wet and dry
phases of temporary waters. The transient pools
of water dotting the countryside for a few months
Glenn B. Wiggins is Curator Emeritus, Department
of Natural History, Royal Ontario Museum, 100
Queen’s Park, Toronto ON M5S 2C6, and Professor Emeritus, Department of Ecology and Evolutionary Biology, University of Toronto. His early
research on the biosystematics of Trichoptera was
largely directed to the taxonomy and biology of
the immature stages throughout North America,
summarized in his 1996 and 2004 books. Studies
were broadened in investigation of the intriguing
evolutionary questions posed by aquatic insects
adapted to life in temporary vernal pools (e.g.,
1973, 1980).
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