Case-building as a primary defense behavior in larval caddisflies
(Trichoptera)
Matthew Malick
Abstract
Although only a small number of terrestrial insect species have aquatic
developmental stages, these larvae compose a large portion of the macroinvertebrate
biomass found in most aquatic ecosystems. In some systems, caddisfly larva
(Trichoptera) constitute a substantial portion of this biomass. Aquatic insect
forms demonstrate an immense array of behavioral and physiological adaptations in
order to successfully occupy a given habitat. Many larval Trichopteran species have
evolved the ability to build a mobile case which serves as a primary, mechanical
defense against both vertebrate and invertebrate predators. These tubular cases are
constructed from silk and debris and display a high amount of species-specific
construction. Both generalized and specialized crypsis occur in case construction
depending on habitat type. Many species utilize fine substrate particles (sand and
organic detritus) to mimic their average habitat type. Other species, however, may
employ hollowed plant material to mimic specific detritus. It has been shown that
vertebrate predators prefer non-cased larvae over case-building species, however,
there is also predator-selection among cased larvae. For this reason, there is a
high amount of intra-species competition among Trichopteran larvae for non-predator
selected cases. Cases have also been shown to act as foraging and respiratory
devices, and to aid in the resistance of entry into stream drift. Case-building
species, therefore, may have an advantage in occupying feeding patches and habitats
which non-case-building species would not inhabit for risk of predation. Thus,
case-building caddisfly species have developed a defense suitable for aquatic
environments that allows them to utilize optimal microclimates which other non-case-
building species cannot because of predation pressures.
Introduction
Although only a small number of terrestrial insect species have aquatic
developmental stages, these larvae compose as much as 95% of the macroinvertebrate
biomass found in some aquatic systems (Ward, 1992). Aquatic insect forms have,
therefore, developed an immense array of behavioral and physiological adaptations in
order to successfully occupy a diverse range of habitats (Merritt and Cummins,
1984). Caddisfly larvae (Trichoptera) sometimes comprise a large portion of this
macroinvertebrate biomass. Species of Trichoptera occur on every continent except
Antarctica and consist of about 10,000 species worldwide. 1200 species occur in
North America alone (Ward, 1992).
Although life histories among Trichopterans are diverse (Merritt and Cummins,
1984), most are holometabolous and have aquatic larvae and pupae, and terrestrial
adults. Univoltinism is most common, however, some species complete more than one
generation per year while others require two years for development (Peckarsky 1990).
Caddisflies, like most other aquatic insects, probably evolved in cold, fast
flowing environments (Peckarsky, 1990; Mackay and Wiggins, 1979), but quickly
colonized both lentic and lotic systems due to subsequent morphological adaptations.
Trichoptera are a sister group of Lepidoptera (Mackay and Wiggins, 1979) and also
have the ability to produce silk. Silk production probably supported rudimentary
case and net-spinning construction in early Trichopterans which allowed exploitation
of habitats with otherwise unfavorable conditions. This silk production has
contributed to diversification of feeding habits, defensive capabilities, and
microhabitat selection.
Five groups within the three superfamilies of Trichoptera have been identified
based on case-building behavior alone (Peckarsky 1990). This behavior has enhanced
defensive capabilities which has allowed subsequent improvements in habitat
selection and ecological diversity. Case-building behavior is usually species-
specific although construction may vary depending upon available habitat. Cases
function as ballast camouflage, and mechanical defenses (Peckarsky 1990).
The ability of larval Trichopterans, therefore, to construct cases from silk
and surrounding materials has led to their ecological diversification and
utilization of habitats unavailable to other aquatic macroinvertebrates.
Discussion
Case Building Behavior
Material and Ontogeny
Probably the most important aspect of ecological diversity among Trichopterans
is the ability to produce silk. Silk production has enabled caddisflies to exploit
a wide range of aquatic habitats. Silk utilization is different in most families
and has more or less defined the ecological role of caddisflies. According to
Mackay and Wiggins (1979), three modes of existence have resulted from silk
utilization. Some families such as the caseless, predatory Rhyacophiloidea spin
only a thin thread while moving along the substrate. Other more sedentary larvae
such as the Hydropsychoidea spin nets or fixed shelters which serve as food capture
devices. The third, and probably most significant utilization of silk production is
the construction of mobile cases by such families as the Limnephiloidea.
Most case-building species construct cases of material from their immediate
surroundings. Otto (1980) divides case construction into organic and mineral groups.
The case size, shape, and material choice are usually species-specific although some
modification may occur due to limited resource availability. This availability may
determine the quantity and quality of building material in some species and may
impose a preferential sequence if material is limited (Hansell, 1972; Otto, 1980).
In addition, many species demonstrate an ontogenic association to case-
building and material. Upon hatching, early instar larvae of case-building species
immediately initiate case construction. It has been demonstrated (2Otto, 1987;
Rowlands and Hansell, 1987) that caseless larvae are preferentially preyed upon more
than cased individuals and avoid both cased and uncased Trichopteran larva.
Selection of initial construction material varies from species to species although
many demonstrate a preference for certain resources. Although initial building
material may be produced from a certain resource, many Trichopterans such as
Lepidostoma hirtum may change building strategies during larval development
(Hansell, 1972). L. hirtum constructs a tubular, sand grain case immediately upon
hatching, however, building material abruptly changes from mineral to vegetative
resources during the 3rd instar. This behavior has also been observed in other
species (Anderson, 1980; Elliot, 1970; Otto, 1980; Rowlands and Hansell, 1987).
This change in resources, however, may differ among species i.e. mineral to
vegetative or vegetative to mineral.
One obvious reason for this transition is resource availability. Many species
alter their construction material when a more valuable or abundant resource becomes
practical. A larger number of summer species make cases from mineral resources as
compared with autumn species which show a predominance of organic cases fashioned
from fallen leaves (Otto, 1980).
Consequently, different species relying on similar building material may
occupy the same habitat by developing temporal niches to avoid strict competition of
resources (Mackay and Wiggins, 1979). Species with distinct developmental rates may
segregate resources by utilizing them at different times when others have either
completed or just begun their development and do not require similar items. This
allows for optimal utilization of mineral and vegetative resources by multiple
species.
Competition and Energetics
Inter- and intra- species competition for cases and case material, however,
does occur (Otto, 1980; 1Otto, 1987; 2Otto, 1987). Specific case shapes, sizes, and
compositions are seemingly in demand. Otto (1974) estimates that the energetics of
silk production in Trichopterans amounts for about 12% of the total energy content
of the larvae. Consequently, this energy expenditure may be considerable in less
productive systems.
Larvae seem to prefer building material which involves the least amount of
energy investment without compromising necessary aspects of predator avoidance and
movement over the substrate. Case construction and selection of Agrypnia pagetana
illustrates these energetic trade-offs (2Otto, 1987). A. pagetana constructs cases
from small vegetative material or alternately uses a natural hollow stem. Energetic
costs of silk production in vegetative cases is high compared to the use of hollow
stems which require only a silk lining. If hollow stems are not readily available,
however, an early instar larvae will construct a vegetative case. Because of the
higher energy investment allocated towards construction of vegetative cases, they
are not as readily abandoned or captured by other larva as compared with hollow stem
cases (2Otto, 1987). Therefore, owners of vegetative cases will strongly defend
their past energy investment against opponents while hollow stem owners readily
surrender their cases. Owners of hollow stem cases more than 2 days old will
voluntarily exchange the old case for a new, more rigid stem if one is encountered
(2Otto, 1987).
In Potamophylax cingulatus the transition of case material from leaf discs to
mineral resources may be due to energetic tradeoffs of early development (Otto,
1980). The use of abundant leaf discs by early instar larva during certain times of
the year is less costly than the silk requirement for constructing mineral cases.
This allows P. cingulatus to assimilate more energy for early growth. In addition,
the use of more resistant mineral cases may be advantageous to larger, later instar
larvae which have a greater probability of predation by vertebrate predators.
These energetic tradeoffs in early developmental stages may, therefore,
conserve energy required for later predator avoidance, pupation, and reproduction.
Consequently, the energetic cost of case materials may ultimately affect future
fecundity.
Predator Avoidance
Crypsis
Although these examples demonstrate the relative costs and benefits of case
construction throughout larval development, the most apparent, although sometimes
disputed (Williams, 1987), purpose of case construction in Trichopteran larva is
defense and prey avoidance.
Because building material is obtained from the immediate surroundings, larva,
in most cases, are naturally camouflaged against the surrounding habitat. Cryptic
defense, therefore, is inherent in case construction. Larvae that construct mineral
cases are more likely to be preyed upon if they stray on to vegetative substrate
(Otto, 1980). It is likely then, that larvae maintain a home-range upon substrate
which resembles its particular case construction in order to avoid predation.
Resistance to Accidental Drift
In addition, many lotic aquatic insect species, including caddisflies, inhabit
microhabitats which expose the larvae to extremes of current velocity. Although
stream drift is a typical mode of dispersal for many aquatic larva, accidental entry
may occur. A strong current may dislodge larvae from the substrate to the drift
where they are more likely to be consumed by predators. Case design may impede or
completely prevent accidental entry into the current drift of lotic systems
(Waringer, 1989). Waringer (1989) has shown that stone cases are most effective on
gravel substrates, however, are less effective on vegetative or sandy bottoms. The
same reasoning applies to vegetative cases although to a lesser extent. While
significantly heavier stone cases may offer added weight in high flow conditions and
limit accidental displacement, the energetics of producing and carrying these
titanic dwellings is not reasonable for an average species. A resistance
coefficient of 0.8 has been calculated for cylindrical, smooth stone cases while an
average, streamlined body has an approximate value of 0.05 (Waringer, 1989).
Although case material increases the amount of drag forces incurred, it is
probably more beneficial for early instar larvae and smaller species which lack the
strength to adhere to the substrate during high current velocities. It has been
shown that larger, caseless larva are not dislodged until current velocities reach 2
ms-1. Although this is almost twice the current resistance of case-building species
(Waringer, 1989) most smaller species lack the physical strength to resist these
high flow conditions. Therefore, case development as a means of preventing
accidental displacement from the substrate is advantageous in smaller species.
Predator Affects and Microhabitat Distribution
Microhabitat distribution and predator avoidance is probably the most significant
aspect of case-building behavior in Trichopteran larvae. Case construction allows
for crypsis and mechanical protection. Tinbergen (1967), however, points out that
camouflage is only effective if accompanied by specific types of behavior. Thus,
larval Trichoptera utilize microhabitat distribution, temporal niche selection and
defense behavior, in addition to case construction, as a means of avoiding predation
and optimizing food and habitat resources.
Case construction material seems to have an overall effect on predator attack,
capture, and ingestion (Johansson, 1991). Although mineral cases seem to have a
higher crushing resistance to vertebrate predation than some vegetative cases, this
may not offer an overall advantage (Otto, 1980). Predators are assumed to determine
prey choice by the minimal amount of handling and search time that would maximize
the energy per unit foraging time (Pyke et al, 1977). Many mineral cased larva are
readily preyed upon by vertebrate predators, however, are ejected shortly thereafter
because of the difficulties of breaching the resistant case (Johansson, 1991).
Certain sizes and shapes of vegetative cases, however, are equally difficult to
handle due to added protuberances or long case length (Johansson, 1991).
One would assume that selective pressure would favor those individuals that
construct heavy, structurally sound cases which offer significant crushing
resistance. Mineral cases, although providing an excellent mechanical defense, are
energetically costly to construct and maintain. Many vegetative cases, however,
provide a greater amount of cryptic defense while providing similar mechanical
capabilities along with less energy expenditures. Longer cased species exhibit an
overall advantage to predator avoidance when compared to those constructing short
cases (Johansson, 1991) due in part to the difficulty of ingestion by vertebrate
predators.
Case rigidity is of little benefit if parts of the larva are exposed to
predators. This, however, may be of little importance to invertebrate predators
such as larval Dytiscus spp. (Coleoptera). The relative size and aggressive
behavior of these and other invertebrate predators allows for rapid extraction of
cased Trichopteran larva. The relative handing time of cased Trichopteran larvae by
Dytiscus spp. is considerably more than that of vertebrate predators (Johansson,
1992). In most cases, Dytiscus spp. will simply wait for the apprehended larva to
expose a portion of itself beyond the protective confines of the case. Case design
does, however, show some resistant adaptations to this predation (Johansson, 1992).
Many species have also developed behavioral adaptations to augment the
defensive character of case construction. It is assumed that Trichopteran larvae do
not purposely make themselves conspicuous to predators unless accidentally displaced
from their normal habitat. Since most larvae can only recognize predators by direct
contact, the chances of avoiding predation in a different habitat are small
(Johansson, 1991). Larval movement has proved to be the best predictor of risk for
macroinvertebrates to predation by vertebrate predators (Ware, 1973). Many benthic
feeders rely almost exclusively on sight to locate food. Johansson (1991) has shown
that immobile larvae stand a better chance of predator avoidance under these
conditions. Some species feign death longer than others if the threat of predation
persists (Johansson, 1991). Once again, this amount of death feigning may be a
function of the relative case strength. Potamophylax cingulatus exhibits only a
small amount of death feigning behavior due to its rigid case which offers adequate
protection (Johansson, 1991).
All of these defensive behavioral adaptations have allowed Trichopteran larvae
to optimize microhabitat distribution of the aquatic environments which they
inhabit. Habitat selection by aquatic insects is crucial due the amount of
variability normally encountered in aquatic environments (Statzner, 1981). Many
aquatic insect species, including some Trichopterans (Elliot, 1970), exhibit diel
fluctuations in habitat selection which affords better refuge from predators. Some
species of Plecopterans, Ephemeropterans, and other insects that lack similar forms
of primary defensive capabilities exhibit negative phototactic responses (Ward,
1992) and favor undersides of stones and gravel during diurnal periods to escape
predation. During these increased times of predation, however, many case-building
Trichopteran species are abundant on substrate surfaces where food availability is
high (Koetsier, 1989; Personal Observation). Although studies have shown that
vertebrate predation alone does not significantly decrease overall density of
aquatic insects, cased Trichopterans do seem to have an advantage in some situations
(Allan, 1982; Koetsier, 1989).
Exposed rock surfaces provide better foraging for grazer species utilizing
preiphyton communities as a food source. These exposed surfaces increase available
light energy for primary production (Steinman and McIntire, 1986) and provide rich
feeding patches for Tichopteran grazers. Although periphyton growth itself affords
some amount of refuge for invertebrate grazers, the advantage of case construction
under these exposed conditions cannot be discounted. In addition, lotic net-
spinning caddisflies are usually not evenly distributed along a watercourse (Otto,
1985) and instead aggregate in areas of high resource availability. These rich
patches, however, are usually more risky because of their increased exposure to
predation. Catch-net constructing species usually inhabit downstream reaches of
lotic environments where fish are regularly encountered. Because nets are usually
constructed in exposed areas where drift is easily accessible, case-building species
may have an advantage over non-
case builders. These strategies allow Trichopteran larvae to utilize rich feeding
patches which other macroinvertebrates find too risky. Exposed substrate surfaces
offer productive feeding opportunities for grazers and net-spinning species. Case
construction, therefore. allows for colonization and utilization of rich
microhabitats that are otherwise inaccessible to most macroinvertebrates.
Conclusions
Ecological diversification is important to the survival of any organism and
behavioral adaptations are the basis for many successful taxa which have succeeded
in colonizing numerous habitats. Case-building behavior of caddisfly larva is an
obvious advantage in most circumstances. Resource and habitat acquisition is
facilitated by the mechanical and cryptic defensive applications of larval cases.
The construction of portable cases has enabled some caddisfly larvae to avoid
otherwise considerable predation pressures which may prevent colonization and
utilization of certain resources. Intense competition for sufficient resources in
aquatic environments has enabled caddisflies to evolve a means of directly occupying
more suitable habitats. This acquisition of rich resources has extended the habitat
of Trichopterans to a variety aquatic environments.
Case construction may contain a complex succession of behaviors which allows
species-specific adaptations that further habitat utilization, predator avoidance,
and ultimate reproductive success.
Case-building in caddisfly larva, therefore, is a considerable advantage for
those species which utilize this behavior.
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