Caddisfly Larvae

Scientific Name: Trichoptera

Number of Species/distribution
Caddisflies are one of the largest groups of aquatic insects with about 7,100 described species worldwide.
  • There are approximately 1,340 species in North America.
  • Caddisfly larvae have successfully adapted to a range of different microhabitats.
Diet/Feeding
The larval stage of caddisflies have very diverse diets and feeding strategies, occupying a range of trophic levels and functional feeding groups from predators and filter feeders.
  • Larvae are mainly herbivorous scavengers, feeding mainly on fragments of plant material, living vegetation, and other living and dead organisms.

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Caddisfly Larvae

  • They can be characterised as collectors, shredders, scrapers, and predators; feeding strategies may vary seasonally as the food supply changes throughout the year, or as the larvae develop and become larger.
  • Different families of caddisflies use silk to produce different types of 'capture nets' to collect food from the environment. Water flows into the net, which captures food particles from the water.
  • The larvae can then graze on the captured food in the net. They may also be selective feeders, feeding on the more nutritious foods.
  • Adult caddisflies can only feed on liquids, since they do not have well-developed mouthparts. They feed only on plant fluids, such as nectar, or may not feed at all.
Habitat
Caddisfly larvae are most diverse in cool, flowing water, but have invaded a wide range of habitats. They are known to construct cases out of silk and various other materials, for shelter.
  • Most caddisfly larvae can be found in benthic habitats in temperate lakes, streams, and ponds. They can tolerate low oxygen concentrations.
  • Habitats can include streams, both cool and warm, lakes, marshes, and ponds.
  • Each species of caddisfly has larvae that are adapted to specific water temperatures and speeds, mineral and pollutant concentrations, and sunlight exposure.
  • Due to the specific habitat preferences of different species, many species can co-exist in a single stream or river.
  • Larvae can construct cases out of silk woven with sand grains, fragments of wood, and other materials from their surroundings. The silk is produced by the larvae through a special gland called the 'labium'.
  • Adult caddisflies are terrestrial. They tend to be most active at night, hiding in cool, moist habitats (such as riparian vegetation) during daytime.


Lifecycle
Like all flies, they undergo complete metamorphosis, meaning that they pass through four complete life stages. These are the egg, larval, pupal, and adult stages.

Eggs
  • Shortly after mating, adult females lay their eggs in or near the water. They walk or dive into the water, and cement their eggs to the base of aquatic vegetation or to stones.
  • Caddisfly eggs will not hatch until moisture is present.
  • The eggs are laid within a gelatinous matrix; the presence of moisture triggers hatching, and the larvae break out of the gelatinous matrix to begin spinning nets of silk or building cases.
Larvae
  • Caddisfly larvae usually go through 5 stages of development (called instars).
  • Upon hatching, the first instar of the larvae sometimes remains in the gelatinous mass for a period of time, after which they break out of the matrix and begin constructing a case or silk net.
  • When environmental conditions are favourable, the larvae continue to development through instars. When conditions are not favourable, the development process is delayed until conditions improve.
  • During winter, larvae living in shallow water may be covered embedded in ice, sometimes for a period of up to 6 months, tolerating temperatures as low as -10dC. Most caddisfly larvae overwinter as larvae, having very little growth during this time.
  • Some larvae, however, do not freeze when the water surrounding them does; others overwinter eggs inside the gelatinous matrix, which provides protection.
  • Thus, caddisflies may hibernate during the winter as either larvae or eggs.
  • The fifth instar of larval development lasts the longest; during this stage, the larvae does the most feeding to prepare to pupate.
  • During the final instar, the larvae produce a pupal case, either by sealing off their existing case or constructing a new one.
Pupae
  • After sealing the case, the final instar of the caddisfly larvae becomes stiff, shorter, and broader, losing flexibility in the head and abdomen. This stage is known as the 'prepupal resting stage'.
  • This process occurs under water, and is influenced by many factors, including water temperature, light exposure, altitude, and the availability of nutrients.
  • The pupal stage usually lasts about 2 to 3 weeks, but some species may overwinter as pupae.
  • When the pupae has developed into an adult and is ready to emerge from the case, it uses specialized appendages, called mandibles, to chew its way out of the case and swim to the surface of the water.
Adults
  • Adult caddisflies have a lifespan ranging a few weeks to several months, depending on the species and on environmental factors.
  • They are generally nocturnal.
  • Upon emergence from the papal stage, adults are ready to mate. They may mate while in flight, on nearby vegetation, or on the ground.
  • Mating rituals may involve the use of chemicals to attract females (called pheromones), while males of other species gather in large swarms and display mating dances to attract females. Some may also make species-specific sounds.
  • Sperm is transferred directly from the male to eh reproductive organs of the female. During mating, the two partners may stay together for just a few minutes, or for several hours. Both male and female may mate multiple times with several other partners.
  • Most species of caddiflies have a life cycle that spans about one year. In colder climates, some larger species may need more than a year to develop.

Role in Food Chain
Caddisfly larvae play a vital role in many aquatic ecosystems; they are a significant food source for many fish and water bird species.

Interesting Facts
  • While caddisfly larvae tend to closely resemble caterpillars, caterpillars have many appendages along their abdominal segment (called prolegts). Caddisfly larvae, however, have only a single pair located near the tip of the abdomen.
  • The cases that caddisfly larvae construct provide protection from predators, but also provide camouflage, helping them blend into their surroundings. Caddisfly larvae have very soft bodies, and the case also acts as a barrier from the abrasive substrate.
  • Caddisflies are closely related to butterflies and moths.
  • The shape of the cases, along with the types of materials used to create them, vary between different caddisfly species.
  • When the female goes underwater to lay her eggs, she can stay under for up to 30 minutes while she glues her eggs to submerged rocks and vegetation. She does this by using air that is trapped on her tiny hairs for oxygen.
  • An artist named Hubert Dubrat uses caddisflies to create unique sculptural forms. He removes caddisfly larvae from their existing cases, and then places them into an environment containing such materials as gold flakes, precious gems, and pearls, and leaves them to make cases out of these materials. Since he started experimenting in the 1980s, other companies have used this method to create jewellery that can be sold.

Sources
Animal Life Resource (2010). Caddisflies: Trichoptera - Caddisflies And People. Available here.
Brusca, R. C, and Brusca, G. J., 2003. Invertebrates, 2nd ed. Sinauer Associates, Inc., Publishers, Massachusetts. Page 600.
Canada's Aquatic Environments (2002). Trichoptera. Available here.
DMI International Corporation (2003). LaMotte Aquatic Macroinvertebrate Insect Identification Flashcards.
McMaster University (2010). Trichopetera. Available here.
Soil & Water Conservation Society of Metro Halifax (2004). Order Trichoptera. Available here.
The Commonwealth Scientific and Industrial Research Organization (2010). Trichoptera. Available here
Virgina Polytechnic Institute and State University, Department of Entomology (1996). Trichoptera (Caddisflies). Available here.

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