Making a Case for the Caddisfly
By Daniel Manget
The hermit crab outgrows its shell and finds a new one; but it doesn’t design and build it himself, it just finds a new one when once it’s outgrown one, big deal. Termites make impressive mounds that can survive for hundreds of years, but it just looks like a big pile of mud with little strategy involved and it takes a thousands of insects to pull it off. The well-known Beaver can stop the flow of a giant river and transform a forest into a pond almost overnight, however, this feat is one of strength and endurance of a group and doesn’t compare to the architectural beauty and innovation of the greatest secret of the animal world, the Caddisfly. One of nature’s most remarkable engineers is a tiny larval grub that lives in the creek near your house and it doesn’t get the credit it deserves. The larva of this insect, which lives in a wide variety of aquatic environments throughout the world, makes the most intricate, beautiful, practical, and most evolved living quarters on the planet; which is why every nature enthusiasts needs to hear the case for the Caddisfly.
In 1400’s England, salesmen would travel throughout the countryside selling pieces of cotton, silk, and yarn that they had pinned to their clothing for advertisement. These pieces of cloth were referred to as Caddices and the salesmen where known as Caddice Men. So Caddisfly makes a fitting name for an insect that travels the river attaching various objects to itself using silk in order to make a living.
Around 234 million years ago, Caddisflies broke away from Butterflies and Moths becoming their own group but have retained some of the characteristics of their ancient cousins; mainly the ability to make silk from modified salivary glands. Now they have become the largest group of aquatic insects worldwide in number and variety. There are over 12,000 different species of Caddisfly (discovered so far) occupying every continent but Antarctica. North America has over 1,200 different species alone. These numbers reflect the success this genius water-dwelling creature has had in being able to use its silk to adapt and survive in new environments.
Larva from different Caddisfly families use their silk in different and creative ways to help with survival which naturally separates them into 3 different groups. The Free-swimming Caddis’ are the oldest and most primitive of the Caddisflies, based on the fossil record, making them the first group to split from the Butterflies and Moths. Their evolution began in small cold mountain streams, and it’s where they remain still, swimming around under rocks and in the substrate looking for plankton to eat. Free-swimmers use silk to anchor themselves to certain points in order to not be swept down by the stream current. They also use silk to build a dome-shaped “Cocoon” to protect the pupae during metamorphosis.
The Net-spinning and Retreat-making Caddis’ were the next group to evolve and use silk in a few unique ways to catch food and to protect themselves. Some make a “Seine Net” in the current to catch food that is floating down stream. Some are “Finger net” builders and make long cylindrical nets underneath rocks with the open end facing upstream. The Caddisfly sits at the back of the net and waits for bacteria and algae to get caught in the net and grow before grazing over them and scraping them off. This gardening technique has proven to be able to produce more food than the surrounding environment in which they live. These sedentary bugs onlyhave to move when something is trying to raid their food plot. The “Trumpet net” makers build more of a funnel-shaped net and like spiders they wait for larger insects to get caught before sensing their vibration and ambushing them.
Recent evidence shows that Net-spinning Caddisflies are highly successful (and responsible) because they have the luxury of being able to choose the “healthy veggies” that get caught in their net over the “junk food.”
The Case-maker Caddisflies, the group most recently to evolve, have taken the use of silk to a new level of ingenuity. These master larval architects have moved past solving the problems of survival in an aquatic habitat and are now just showing off, probably to the chagrin of their primitive free-floating cousins but to the delight of nature-lovers and artists. These larvae us their silk to assemble immaculate cases made from natural materials that reflect the surroundings that the species has evolved to live in. Each species has a genetically predetermined blueprint for its design ingrained into its DNA based upon environments that the species has historically lived. While some species are capable of improvisation in their design based on available materials, most are so specific that they can be identified by their case pattern alone.
When considering the challenges a small wormy-caterpillar-looking thing must face in a big, roaring, predator-filled river, the brilliance of their skill is quickly appreciated. The first challenge, the fast moving current, works to advantage of the predator because all it takes is one slip of the larvae and it is washed downstream to the waiting hungry trout. So, the first function of a case is act as a weight or ballast to anchor to rocks and the stream bottom, some even using silk to glue themselves down. All cases are designed to be hydrodynamic, facing upstream to minimize drag, and there seems to be an evolutionary competition to see who can build the most streamlined case. In some species there is a direct correlation between the speed of the current and the size of the stones used to make their case.
Cases are also great tools to avoid predators, not only because of the perfect camouflage, but because what predator wants to get a mouthful of rocks just to eat a tiny grub. The Caddis just drags its case around a rock with its front legs while safely grazing on algae and diatoms, and when it’s time to pupate it can glue itself to a rock and build a cocoon right inside the case avoiding the risky open water while transforming into a flying insect. The moth-like Adult lives for relatively brief period with the main purpose of laying eggs in the river to start the cycle again.
Leading the proverbial competition for survival innovation has to go to the tube-case makers. While beavers work hard to make a huge area habitable for themselves, this genius larvae only needs the case that it has built to be able to inhabit almost any type of aquatic habitat, regardless of quality. This is the main reason for this insect’s worldwide dominance in aquatic habitats and is due to its ability to produce its own oxygen in places where there is very little. With a few undulations of the larvae’s abdomen, a current is produced to flow through the tube. Moving water always carries more oxygen then still water so the gills of the insect are constantly being replenished whenever needed. So, warm puddles with little oxygen may be a barrier for most water creatures but not for this smart bug. While most Caddisflies are sensitive to pollution and are therefore consistent indicators of good water quality, a lot of these little buggers have learned to drive environmentalists nuts by adapting to streams with very poor quality. Some have even the audacity to adapt to live ONLY in streams that have been recently disturbed by construction, again to the chagrin of everything else in the stream.
So now the secret is out and they’re hiding in your creek and mine. What looks like a discreet little water bug swimming around in the mud is really a master craftsmen and artist who has learned to create beauty in its survival.
Angrisano, E. 2004. Trichoptera (caddisflies). In B. Grzimek, D. G. Kleiman, V. Geist, and M. C. McDade, Grzimek’s Animal Life Encyclopedia. Detroit: Thomson-Gale. ISBN 0307394913.