What is the only insect that can survive year-round in Antarctica?

The Antarctic midge is a tiny flightless insect that lives in Antarctica year-round. When it is cold outside the midge burrows a centimeter below the ground and provides a protective jelly for its eggs so they don't freeze. It is the largest purely terrestrial animal endemic to the continent. However, it is worth noting that the habited area of these midges is on one of the coastal peninsulas. So you won't find them at the South Pole.

The flightlessness of B. antarctica may be an adaptation to prevent wind from blowing it into inhospitable areas. It can survive freezing, but though local air temperatures may reach as low as -40 °C, this insect cannot survive temperatures below -15 °C. This is comparatively milder than other cold-adapted insects. The reason for this relatively low freezing tolerance is due to thermal buffering: just burrowing at a depth of 1 cm, temperature is stable between 0 and -2 °C for 10 months out of 12, and it seldom goes lower than -7 °C all year round. Ice and snow cover also helps keep the temperature stable. Freezing tolerance is enhanced by cold hardening.

https://en.wikipedia.org/wiki/Belgica_antarctica