Caddis Flies (also Caddisflies), like Moths, are mainly nocturnal and attracted to light. There are some 200 species in the UK alone. Like Stoneflies and Mayflies, their larvae are aquatic.
(Oxford Dictionary spells it "Caddis-flies," but I've not seen that elsewhere, and says the larvae are "Caddis-worms" or just "Caddis.")
The adults have extremely long antennae, often as long as the body, and varied patterns on their wings but usually in drab colours (many are plain).
They are not true flies, having two pairs of wings. Related to Lepidoptera, their wings have hair rather than the scales of Moths/Butterflies.
Caddisfly larvae "construct" cases, tubes or nets of silk, gravel and other materials under water to hide, catch prey or pupate.
Some larvae are predators, some vegetarian. Larvae and adults are an important food source for fish, birds, amphibians and other insects.
The presence of many larvae is also an indication of the cleanliness of a water source.