Most large insects are inactive over winter. An exception are larvae of the caddis family Limnephilidae. They eat dead leaves that fall into freshwaters and that resource is at its peak over winter; many also use dead leaves to make their larval cases.
One common species is the Mottled Sedge Glyphotaelius pellucidus. It cuts perfect circles from the leaves to make a flat and beautifully camouflaged case when seen from above against a background of dead leaves; fish are often absent from the places where it lives but crows are known to seek out these juicy morsels.
Dead leaves may not sound like good eating but they are once wet and colonised by fungi and bacteria that decay them. The larva’s gut enzymes are focussed on breaking down bacteria and fungi and not the dead leaf tissue; a good example of ingestion and digestion not being obviously linked. The caddis larvae are in a feeding group entitled primary shredders, i.e. they break down the big leaf into small pieces that they pass out as excrement.The pieces in the excrement are quickly colonised by bacteria which tend to stick to surfaces by atomic forces and secrete their enzymes externally to digest the leaf further. The films of bacteria are a food source for another group of animals that eat small particles. However, a lot of particles must be eaten as a bacterial film is not very nutritious and some scirtid beetle larvae eat so much so fast they need to poo several times a minute.
Wet dead leaves vary in their food value for caddis and other shredders. Soft leaves such as sycamore and alder are nutritious and easy to eat whilst pine needles are, not surprisingly, hard to eat and not good value. Oak is also good, but willow may be poor due perhaps to chemicals left in the leaf by the plant. Beech is very poor and the contrast between use of oak and beech leaves can be seen in a small summer-dry pond in Neilson’s Plantation, Arrowe Park, Wirral (SJ26978513) near where I live. This pond is overhung by Turkey Oak and beech. Out of preference, Glyphotaelius larvae seem to eat the oak leaves but make their cases from the beech. This is sensible because the oak leaf resource may become used up and cases made of oak may be cannibalised. Predation of the larva by another larva is rare but they are very conspicuous and, surprisingly, larvae out of a case are much less efficient at extracting oxygen from the water and oxygen may be at low levels in ponds full of rotting leaves. The species passes the summer as an adult.
If you see quite large holes cut from leaves in stagnant ponds it is almost certainly this species. In flowing water another limnephilid Potamophylax cingulatus make similar cases. Note that leaf-cutter bees make small holes that can persist in leaves, but they rarely go for tough tree leaves.
Ian Wallace, UK Caddis Recording Scheme