Two photos, same larva. On the right picture the larva is taking in air with the tip of its abdomen. The white belly has grown fat by eating midge larvae. Wesenberg Lund ¹ thought the thin belly skin might make skin breathing possible. The round spots on the segments are stigmata of the air funnels (tracheae). These stigmata are close with the 1st and 2nd instar larva. Here, on the 3d instar (e.g. moulted twice) larva they are open. The light dots mean little airbubbles that have a silvery appearance. The stigmata are small and equipped with water repellant hairs through which the air cannot escape. The reason they are open might be that the larva can breath through them when its crawled out of the water to pupate.
Please mark the many bell animalcules on the hornwort (the white dots).

¹ Biologie der Süsswasserinsekten, Wesenberg Lund (1943)

Left a detail picture with the stigmata. When watched attentive one may see little air channels under the armoured skin that run from the stigmata and branch within the body of the larva. The segment most left is the one before the last tail segment. That last tail segment also has two stigmata, but those are the breathing holes on the tip of the abdomen, with which the larve gets air at the surface.