There are several species of insect that spend most of their life in water as a larvae before climbing in to the open air, pupating and then emerging as a flying adult with the sole purpose of mating and then dying when their part of that process is complete. Dragonflies and damselflies are probably the best known examples along with mayflies of course but caddis flies do this too. Caddis flies are insects of fresh water rivers and are best known for the habit the larvae have of coating themselves with sand grains to protect them from predation.
There are about ten species of caddis fly and this one is the brown sedge caddis (Anabolia nervosa). The adult has wings about half an inch long and are quite a small insect. Like mayflies large numbers tend to hatch at the same time and so finding them is not difficult as one tends to encounter a swarm of them. Most numerous in August and September, widely distributed in or rivers and not uncommon.