Ray-Finned Mosquitoes

Tailtube worms are a group of neotenic mosquitoes that have embraced a fast-paced, free-swimming lifestyle, drastically improving their endurance by making their gills into a part of their propulsion system. They can also gulp air through their siphon, which forms the top half of the tail fin, though they don't possess fully-developed spiracles like their airgill relatives, so larger tailtubes can't fully oxygenate their body this way. Instead, they are always swimming, using their own momentum to move water over their breathing apparatus. However, this ancestral morphology has some serious drawbacks, most notably that the surface area of the tail gills is quite low, especially in genera that have specialized for efficient, fast swimming. This has also restricted tailtubes from attaining body sizes comparable with other Apterran nekton like fish and Pelagarthrids. One subfamily, called the Ray-Finned Tailtubes (Actinopterygioculicinae), has stumbled upon a novel solution to this problem: creating new breathing surfaces by modifying their bristles into hydrodynamic fins.

Even the earliest tailtube worms loosely arranged their bristles above and below their thorax to serve as a crude approximation of dorsal and anal fins, adding a bit of drag but in exchange helping keep the mosquitoes stable in the water. This subfamily has refined this feature into orderly rows of bristles, with one set on the back, one on the belly, and one to eight pairs running along the sides of the thorax and sometimes the abdomen. The individual hair-like structures are connected by a thin, transparent cuticular membrane that serves as both a fin and a supplementary gill. Ray-finned tailtube worms are thus able to live in ways and places none of their other family members can, though they still aren't as diverse as fish or aquatic isopods. 

One type of environment where Actinopterygioculicine flies truly excel are warm, slow-moving, low-oxygen backwaters such as are common across Apterra's tropics and subtropics. No other tailtubes can survive in these areas, though their cousins the gillywogs can often be seen clinging to the water's surface, finding plenty of oxygen in the air above. For one genus of ray-finned tailtubes, these slow-moving Semperinfantids are the perfect food source, one for which they face little competition. Reaching about five centimeters in length, the Bristlebrush (Paracaecovenator spinatus) lives in the tannin-rich, oxygen-poor oxbow lakes and sluggish streams of the Aglirian lowlands. Visibility is so poor in these waters that the bristlebrush's eyes have become greatly reduced in size and complexity, to the point that they can only barely form images and are mostly used to distinguish light and dark. It hunts mainly by touch, using the fine sensory bristles on its face to detect prey around it. These filaments may be nearly the length of the mosquito's body, and they are tipped with chemoreceptors that can quickly identify whatever they're touching. If that object turns out to be a smaller animal, the bristlebrush lunges forward, grabbing it with a pair of short, upward-curving mandibles before it can escape. The mosquito then retreats to somewhere dark and secluded to process its meal, which may sustain it for several weeks. By slowing its metabolism during digestion, the bristlebrush eliminates the need to move at all times; a gentle flapping of its fins, along with occasional ventures out of its hiding place, are more than enough to provide all the oxygen it requires.

Other ray-finned tailtubes have a much more active lifestyle, made possible by the cool, oxygen-rich waters of the South Perithalassic. Along the southern edge of Perinesia lives the largest insect in all of history; the Mosquituna (Thunnoculex gigas), at around two kilograms, would be on the large size for a pelagic baitfish. Like the shoaling fish and Pelagarthrids it shares the sea with, the mosquituna is a vital food source for larger marine animals, but as its name suggests, it's a speedy and powerful hunter in its own right - at least within its size class. With a crescent-shaped tail, dorsal and pectoral fins that can fold closely against its exoskeleton, and a sleek, bullet-like body packed with muscle, this animal is built for both speed and stamina, and though it isn't the most extreme example of either trait, it combines the two in a way that makes it a terrifying threat to anything smaller than itself. It employs a long-distance pursuit strategy, relying on smell and sight to keep track of prey that may be able to outswim it in short bursts, but which can rarely maintain its lead for long. When it finally catches up to its weary target, the mosquituna must execute a killing bite and then eat quickly, for there are hundreds of other species in this part of the world that would eagerly take the opportunity for a free meal.