“Submarine cottages”

Post by Jen Berlinghof

Late spring and early summer are busy seasons for children visiting the Lake County Forest Preserves for pond study programs. The shorelines of ponds pulse with the excitement of students, nets in hand, ready to discover the macroinvertebrates teeming under the water’s surface. The most delightful find this season by students has to be what Henry David Thoreau once called the “submarine cottages” of caddisfly larvae.

Students patiently fill their buckets with wiggly tadpoles and hulking crayfish. What appears to be an insignificant, teeny stick in a student’s pond bucket suddenly transforms when placed under a microscope. The “stick” suddenly springs to life, as students notice the head of a caddisfly larva unsuspectingly pop out of one end.

Caddisflies are insects in the order Trichoptera, one of the largest orders of aquatic insects. The larvae are widely distributed in various freshwater bodies, and a few species inhabit marine environments. Some consider caddisfly the aquatic cousins of moths. Only terrestrial for a brief period as adults, caddisflies spend most of their lives as larvae and pupae in a wide variety of waterways.

Their success as an order is directly tied to their unique ability to make silk strands, which is used to create portable homes and, in some cases, nets for filtering food. These unique silk retreats have some real advantages when it comes to camouflage, physical protection, food acquisition, and respiratory efficiency.

Caddisfly larvae resemble caterpillars tucked into “sleeping bags” adorned with stones, sand, sticks, and bits of plants. The larvae have silk glands in their lower lips and use this silk to create temporary homes and affix their cases to large rocks or woody debris. By attaching to substrate, they avoid being carried away by the current. The natural materials attached to the outside of the cases not only help weigh them down to prevent drifting, but also serve as amazing camouflage.

Some species create flat, thin extensions on their cases to increase the surface area, which allows them to sprawl on top of soft sediments. Others encase themselves in super long tubes burrowed into the mud, secure inside, never even contacting the muck surrounding them.

While most caddisfly larvae eat algae and plants, some species forgo homebuilding all together and use their silk to catch food instead. These net-spinning species construct mesh silk nets that strain edible particles of plants, crustaceans, and insects from the water.

Caddisflies will spend anywhere from two months to two years in the larval stage before pupating in the water. The case where they spent the larval stage will be modified into a cocoon with the addition of a porous, silk sieve plate to the rear of the case. This allows water in, and with it a fresh supply of dissolved oxygen, while keeping predators out. The cocoon is capped on top with stones or plants and the whole thing is glued in place with silk to a rock or log.

After a few weeks, the caddisfly will cut the pupal case open with its sharp jaw. Many species use the pupal skin like a raft, drifting to the edge of the water to dry their wings before flying to nearby vegetation. Adult caddisflies are mostly nocturnal and live about one month, feeding on nectar with sponge-like mouthparts.

These insects play an important role in the function of freshwater ecosystems. Most species are sensitive to pollution, so the diversity and abundance found in our wetlands can provide useful information about the environmental condition of our waterways.

With the exception of net-spinning caddisflies, each species creates its own, unique abode. Currently, there are 15 documented species of caddisfly in Lake County, Illinois, and likely even more out there—waiting to peek their heads out under the microscope.

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