Antarctica and the sub-Antarctic islands are considered natural laboratories to study and understand the influence of environmental variable and patterns of variation therein on the biota, including the influences of climate change.  The Antarctic terrestrial fauna consists only of small invertebrates, with just two native species of holometabolous insects, Parochlus steinenii and Belgica antarctica, and two established non-native species, Eretmoptera murphyi and Trichocera maculipennis. Studies of the life history, phenology and stress tolerances of insects are critical to better understand adaptations to natural environmental stress and the ecological consequences of recent and ongoing climate change. In this context, we characterized the habitat preferences, life cycle and phenology of P. steinenii, the winged Antarctic midge, in a lake on King George Island, South Shetland Islands, maritime Antarctic.  Based on the data obtained, we hypothesize that P. steinenii has a multi-year life cycle that may span as much as four years. The species is very restricted in terms of its preferred microhabitat distribution, being highly abundant only at shallow depths close to the edge of the lakes in which it is found. A combination of further field and laboratory studies are now required to assess how the length of P. steinenii life cycle is influenced by the scale of temperature variation typically experienced in its natural habitat, and as predicted under different climate change scenarios, as well investigating the ability and timing of larval movement to take advantage of the conditions of specific microhabitats at different times of year.

Habitat preferences of Parochlus steinenii: We studied the habitat preferences and the life history of P. steinenii in Lake Kitiesh, Fildes Peninsula, King George Island (62°S, Maritime Antarctica) over the austral summer seasons from 2014-2019, during short (several week) field expeditions organized by the Chilean Antarctic Institute (INACH); longer duration, including year-round, field sampling is not possible at this location due to logistic limitations, as is commonly the case in Antarctic field research. This lake hosts an abundant population of P. steinenii and is easily accessible from Professor Julio Escudero Station (INACH, Chile), allowing for continuous site monitoring during the expeditions and the deployment of year-round environmental monitoring equipment.  Antarctic populations of P. steinenii are restricted to permanent deeper lakes, which are ice-covered 9-10 months each year. To characterize the terrestrial habitat in which adults are found, we established five line transects around the perimeter of Lake Kitiesh. Two transects were located parallel to the main inflow and outflow streams, while the other three were oriented approximately in N, NE and S directions, in each case perpendicular to the lake and starting at the lake edge. Along each transect, we quantified adult densities using 50 × 50 cm quadrats (subdivided into 10 × 10 cm grids) at 0, 2, 4 and 6 m from the lake shore. Adults of P. steinenii were not observed beyond 6 m from the lake edge. Numbers of adult flies in each 10 × 10 cm grid were recorded, and a digital photograph of each entire quadrat was taken to allow assessment of habitat preferences (Canon PowerShot D20). We recorded rock, snow, mud, moss and water proportions within each quadrat. We assessed differences in P. steinenii adult density around the lake shore using Permutational Multivariate Analysis of Variance (PERMANOVA), based on a Bray-Curtis similarity matrix. We also investigated the relationship between P. steinenii density (individuals/m2) and habitat type (rock, snow, mud, moss and water) using Distance Based Linear Models (DistLM). Both analyses were run in PRIMER-E v7 with Permanova+add-on (Anderson 2005).

Thermal environment and growing degree-days: To assess the thermal characteristics of the aquatic environment in which P. steinenii develops from egg to adult, we anchored one temperature data logger (HOBO® U22 Water Temp Pro V2) at a depth of 50 cm in Lake Kitiesh, corresponding to the depth at which high densities of larvae are commonly found during the austral summer. Minimum, maximum and average daily water temperature (°C) were recorded from January 2014 to January 2019. Using these data, we calculated growing degree-days (GDD), using the pollen package in R (Nowosad 2019, R Core Team. 2021), which calculates GDDs available for development as the accumulated product of time and temperature above 0°C (noting that lake water temperatures are not likely to exceed the upper developmental temperature of P. steinenii, although this has not been formally assessed) and that the larvae do not survive freezing) using daily maximum and minimum temperature data (Nufio and Buckley 2019).

To describe the life cycle of P. steinenii, we collected reproductive adults (n = 200) from rocks along the lake shore. Small rearing chambers (10 containers of 250 mL) were set up under controlled temperature conditions (4°C) in the laboratory of Prof. Escudero Station as described by Harada et al. (2014). Each rearing chamber contained lake sediment, rocks, and water from the midge’s habitat. Groups of ~ 20 reproductive adults were placed in each container, where mating occurred followed by oviposition on the surface of the rocks. The egg masses were then transported in the rearing chambers to the Wankara Laboratory (Universidad de Magallanes) in Puerto Williams, Navarino Island, Chile. At the Wankara Laboratory, the chambers were maintained in an incubator with a 16 h light and 8 h dark cycle (LD = 16:8 h) at ~ 4°C (range achieved 4-4.5°C), monitored by a temperature logger (Hobo ® U22 Water Temp Pro V2) recording from the first day of observation until the hatching of the adults. Weekly observations of larval development were made until they reached the adult stage. Larval instar was assigned based on size, initially by eye and then followed by detailed measurements of head width and length (mm) using a microscope with built-in camera (Leica DM750), calibrated for each image. Head capsule length (mm) was measured from the anterior margin of the frons to the posterior margin of the head’s sclerites, while the width (mm) was measured as the distance across the eyes on the head capsule. This enabled larval development to be followed through four distinct developmental instars (L1, L2, L3, L4). Growing Degree Days (GDDs, thermal units) needed to complete a cycle from egg to adult were then calculated. We obtained the GDDs required for each developmental stage and compared these with the GDD accumulation curve obtained from the water temperature recorded at 50 cm depth in Lake Kitiesh between 2014 and 2019.