The BioFresh Cabinet of Freshwater Curiosities

Micropterna in an artificial case (image: Gerhard Laukötter)

Artistic riverine insects create colourful cases from unusual materials

Guest curators: Prof. Daniel Hering (University of Duisburg-Essen) and Gerhard Laukötter

Many animal species are protected from predators, desiccation or disturbance by a thick shell or skin. Only few, however – leaches, midge larvae and butterfly larvae – are capable of building cases to artificially protect them from the environment. Unsurpassed as artistic architects of such artificial cases are tiny caddis larvae, which live amongst the rocks, vegetation and rubbish on river beds.  These unique little creatures have developed the curious ability to use these raw materials to create colourful and unusual protective outer tubes.

Lithax in an artificial case (image: Gerhard Laukötter)

Caddis larvae are the larval stages of caddis flies (Trichoptera), of which about 12,000 species are known worldwide. Larvae of almost all species are aquatic. Larvae of about half of the species construct transportable cases, protecting soft-skinned parts of the body and in which the larvae can retreat in case of danger. All species protect their defenceless pupal stage with artificial cases, which are firmly attached to the river bed.

Depending of the larval habitat size and form of caddis cases vary, resulting in a large number of unusually constructed cases: round and square tubes, cases in the form of a turtle shell or a snail shell, sand tubes punctuated by thick stones and multi-story cases with sophisticated ventilation systems! Fascinatingly, caddis larvae use very diverse materials for case construction, including: self generated silk; sand of a defined grain size or of different grain sizes; small pieces of wood cut at an exact size by the larvae’s mouthparts; and small parts of leaves, roots or of reed stalks.

Sand

Sand: Sericostoma (image: Gerhard Laukötter)

Helicopsyche (image: Gerhard Laukötter)

Sericostoma larvae bind small sand grains together in a seemingly jointless, curved tube. Similar material is used by representatives of the genera Molanna (in flat tubes) and Helicopsyche (in winded cases, amazingly similar to a snail shell).

Silk

Silk: Micrasema (image: Gerhard Laukötter)

Larvae of the genera Micrasema and Setodes are specialized weavers, with cases made of pure silk. The diameter of the tube increases when the larvae growths.

Wood and vegetation

Crunoecia (image: Gerhard Laukötter)

Some caddis larvae species (e.g. Crunoecia which inhabits springs), cut wood fragments to a standardized size with which perfectly squared cases are built. Other species don’t care for geometry at all and assemble chaotic cases using all available wood and leaf material without any real construction plan. The important outcome – protecting the larvae – is nevertheless achieved.

Unusual and artificial material

When these preferred materials are not available, most species resourcefully change to building cases out of other more unusual material, with a range of strange and curious results.

Micropterna (case of artificial material, image: Maren Hering)

In springs with low current flow, coarse sand and gravel is often absent; and the riverbed is covered by fine sand. Species usually preferring coarse particles have to change to completely different items: using seeds, small mussel and snail shells, regardless of whether they are empty or still inhabited! On rare occasions, a fascinating form of kleptomania can be observed. Here, the cases of small larvae are used by larger larvae for building their own cases. As with snails, this is done regardless whether or not the cases are still inhabited.

Artificial material is also used by caddis larvae, and sometimes even preferred. Small fragments of red bricks or cement, fibrous tissue, even small pieces of paper or plastic have been observed as parts of colourful caddis cases.

Limnephilidae (case of Bythinella shells, image: Gerhard Laukötter)

As caddis larvae are generalists in selecting building material, several scientists have exposed larvae in laboratories. In some cases there was a scientific rationale for this. For example, larvae can be marked to observe their migration, as colourful cases are more easily found in a stream. Caddis larvae reared on a bed of small glass pieces may build a transparent case – which proves useful for scientists hoping to observe the behaviour of the larvae inside.

Limnephilus (case made of snails, image: Gerhard Laukötter)

Some biologists have offered fragments of corals, nacre, opal, malachite, lapis lazuli, garnet, rock crystal or turquoise to caddis larvae. These materials result in precious and colourful cases.

Some specimens alternately reared on two different materials some species build ringed cases.

Beautiful, natural art made by insects in seemingly everyday landscapes.  There’s often a lot more to the curious world of freshwater ecosystems than first meets the eye…

More information:

• Higher resolution versions of these caddis larvae photos and others at the BioFresh flickr
• More information on caddis larvae from the Field Studies Council
• Wikipedia page

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