Order Trichoptera
“Hairy wings”
Trichopterans, known
colloquially as caddisflies, are natural engineers. Though the adult form
achieves little other than mating and congregating mindlessly around
incandescent light bulb, the Trichoptera larvae actively compete with other
inhabitants of their freshwater habitats. Some are simply vicious predators. But the
vast rely on self-made “cases” to give them a competitive advantage against
their neighbors. Their designs vary from species-to-species, ultimately
displaying a range of creative instinct.
Of the known 7,000 species of
caddisflies worldwide, over 1,200 inhabit North America. As earlier stated, the
adult forms are common frequenters of accessible light sources. The caddisfly’s
adult life is very short-lived and many may not even eat. As larvae, however,
they have enormous impacts on their environment. Caddisflies are the third
largest aquatic holometabolous insects, following Coleoptera and Diptera. They
are also a part of the well-known acronym ETP, which stands for Orders
Ephemeroptera (mayflies), Trichoptera (caddisflies), and Plecoptera
(stoneflies). The presence of these three orders is an established indicator of
good stream health. Caddisflies, for instance, rely on the presence of rocks,
pebbles, sand, and aquatic plants with which to build their protective cases.
Having a stream stocked with these small invertebrates attracts predators of
higher trophic levels,
and the entire ecosystem thrives.
The
Order name Trichoptera (“hairy wing”)
refers to one of the main features to distinguish caddisflies from moths. While
moths and other Lepidopterans’ wings are covered with minute scales, those of
caddisflies are covered in small hairs. In basic morphology, however, the two
are not very distinct. The main difference is found in each orders’ respective
larval development, which are very different indeed.
Caddisfly larvae are
eruciform, like Lepidopteran caterpillars, but instead of arming themselves
with bristles or bright colors like their terrestrial look-alikes, they create personal,
portable shelters for themselves. Trichopterans do this by secreting rings of
silk to string small bits of the surrounding substrate into a protective tube,
or case. Some of these cases are mobile, and are carried everywhere with the
larva, but others are plastered to the sides of submerged rocks or other
surfaces. These structures can be used as shelter, personal armor, or even as
netting to catch prey. The very morphology of the larvae themselves is suited
to this lifestyle. For instance, all six of each larvae’s legs are grouped
toward the front of their bodies, with a long abdomen trailing behind. Some
species, because they require moving water, can even manipulate the setae along
the sides of their abdomens to create a current through their cases. Other
species (notably free-roaming ones) have prominent gills in the same place.
Different caddisfly species use
many techniques in their daily survival. As stated earlier, many free-roaming
species, such as members of Family Rhyacophilidae, are free-living predators,
spreading their numbers more thinly across freshwater habitats. Battling the
current with a combination of hook-like abdominal prolegs and silken lifelines,
these Trichopterans scrabble over rock surfaces to find prey. This is usually
the extent to which members of this family use their silk glands, but like
their case-building cousins, Rhyacophilids still construct rudimentary shelters
for pupation.
Members
of Hydropsychidae are also predatory, but they do use their silk more
constructively. After constructing a net facing the current, Hydropsychids lie
in wait beneath nearby rocks, monitoring their net to eat any small prey and
organic matter caught by it (Clifford 316).
The most spectacular
constructions are used for personal protection. Caddisfly species instinctively
will use any resources available to construct their cases, though each species
has a favored medium. In experiments, naked caddisflies placed in a controlled
tank can be observed to make a case from whatever substrate is provided, from
purple rocks, to glass, to gold! (Marshall 240). Some caddisfly families,
including Glossosomatiadae, use small pebbles to construct their living
quarters.
Standing out from the majority of
cylinder-forming Trichopterans, the Helicopsychidae family fuses grains of sand
with their silk excretions to form delicate snail-like shells (Marshall 240).
From these cases they can move about with relative mobility, feeding on
detritus and algae. Some Helicopsychid species last the whole winter sealed up
within their shells, attached to plant stems (Clifford 316).
Some of the smallest caddisflies
belong to the Hydroptilidae family, and adult forms seldom exceed 5 mm in
length. They start out as free-ranging larvae, feeding on diatoms and other
tiny foods. Then, upon their fifth instar, these diminutive
species’ abdomens swell up, and they seal themselves inside of silken
purse-shaped cases until molt.
The dependable diversity
represented by Order Trichoptera is astounding, and many different families can
be found within the same square foot of streambed. Though temperature, food
availability, and current velocity control the specifics of their actions, even
in the same environment, the species-specific survival methods mentioned above
hold true. How such diversity came to evolve within a limited number of
freshwater environments is unknown. But it is impressive to see the efforts and
skill that these larvae display as they thrive in their habitats.
Works
Cited
Clifford, Hugh F. Aquatic
Invertebrates of Alberta. Edmonton, Alberta: U of Alberta, 1991. Print.
George W. Gallepp. The American
Midland Naturalist
Marshall, Stephen A. Insects:
Their Natural History and Diversity. Buffalo, NY: Firefly (U.S.), 2006.
Print.
Whitfield, James B., John T. Doyen, Alexander H. Purcell,
and Howell V. Daly. Daly and
Doyen's
Introduction to Insect Biology and Diversity. New
York: Oxford UP, 2013. Print.
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