750,000 years ago, the formerly mighty Aedes semelparosus group had long since entered a long period of decline, splintering into dozens of isolated daughter species scattered across Apterra. One by one, these died out with a whimper, outcompeted by other groups like the pansanguinophages. Finally, there was only one left, the sole member of the genus Ailoxaedes. In the freshwater lake separating the two great continents of Ailuropia and Loxodia, this species was all that remained of what was once the most common flying animal on the entire planet. In most years, only a few hundred males and females emerged from the water. During one particular spring, their numbers were lower than ever before. The entire population of about five dozen larvae was now relegated to a small, shallow depression on the lake's western shore. They wriggled around, eating zooplankton and rotting plants, growing a bit more with each passing day. A few of them outpaced the others; these carry two copies of a recessive trait that increases their size dramatically. In earlier generations, it would've been unlikely that this mutation would ever display itself, as it was exceedingly rare in the population. Until that year, in fact, no individual had ever carried two copies. Due to the shrinking population of Ailoxaedes, though, two closely-related mosquitoes bred the previous year, resulting in some of their inbred offspring expressing the defective gene.
The increase in body size was not the only impact of the disorder. As their conspecifics pupated and flew off to begin the adult stage of their life cycle, the handful of mutants failed to complete their metamorphosis. In all respects except their reproductive organs, they simply continued to resemble large larval mosquitoes. As they reached breeding age, they mated with each other and released their eggs into the same water they dwelt in. With their increased body mass, the half-dozen of them ended up producing nearly three thousand offspring by the time winter came. On the other hand, their flying relatives only lasted a few more years. Some of them also harbored the mutant allele responsible for neotenic development, and as their population continued shrinking, more individuals were born with two copies. This bolstered the gene pool of the larviform giants, who soon thereafter found themselves the sole remaining members of their family. The aberrant dipterans found success with their new body plan, spreading across the lake with each subsequent generation. The lake became nearly choked with them, as they bullied and overpowered the smaller young of other mosquito families. In under a century, they'd returned from the brink, reaching numbers that hadn't been seen in nearly 200 millennia.
Over time, they spread to nearby rivers, other lakes, and even some brackish regions on the Loxodian coast. Dozens of species diverged from these early paedomorphic forms over the generations. Some now specialize on algae, while others feed on copepods and other small aquatic creatures. One species even preys on small minnows, grabbing them with its hook-like mandibles. On the whole, they are still a minor component of aquatic ecosystems, and their geographic spread has been slow. Meanwhile, other mosquitoes that share their waterways have adapted to cope with the competition. The neotenic semelparous mosquitoes remain restricted to the northern continents, where they've carved out a niche for themselves in a world that shows no mercy to those who can't change with the times.