Macun high-alpine streaM, swiss alps (Diptera: chironoMiDae)

Male adult of Pseudorthocladius immezensis sp. n is diagnosed and described based on material collected in the high alpine cirque of the Macun area (Immez Lake basin, alt. 2616 m a.s.l.). A combination of morphological characters found in the male adult has allowed us to consider this new species as a local biogeographical representative of the eastern part of the Swiss Alps: coronal suture with lateral extension, low antennal ratio (AR 0.30); distal half of wing hairy; anal point broadly triangular with a characteristic enlarged base; inner apical margin of gonocoxite truncate; superior volsella large lobe-like; inferior volsella double, dorsal lobe nose-like and distinctly bent apically; virga present, with 4 spines; crista dorsalis absent. A differential diagnosis is given, in which some distinguishing morphological characters are summarised. Comments on the ecology and geographical distribution of the new species are also provided.


Introduction
The relatively harsh environmental conditions, such as long winters, thick snow and ice cover and low temperatures prevail in alpine freshwaters. These limiting conditions contribute to the settlement of a highly specialized fauna (especially insects) (Ward 1994). Moreover, the insular nature of alpine regions constrains the distribution of species colonizing and inhabiting alpine streams and lakes (Hieber et al. 2005). In alpine headwaters, Chironomidae dominate most stream assemblages in terms of abundance (Lods-Crozet et al. 2001, Ilg and Castella 2006, Robinson et al. 2016, Alther et al. 2019). In the remote area of the highalpine cirque of Macun (Swiss National Park), rare and climate sensitive species colonize spring-fed streams, ponds and lake shores (Lods-Crozet et al. 2012). New chironomid species have been re-cently described from here (Moubayed-Breil and Lods-Crozet 2018).
In this paper, P. immezensis is described as new to science and its morphology compared to the morphologically similar P. cristagus Stur & Saether, 2004;P. curtistylus;P. filiformis (Kieffer, 1921) and P. pilosipennis Brundin, 1956. A differential diagnosis that highlights morphological characteristics and relevant distinguishing characters is given.

Material and methods
Male adults of P. immezensis sp. n were collected using a Malaise trap, preserved in 80-85% ethanol and cleared of musculature in 90% lactic acid (head, thorax, abdomen and anal segment) for about 60 to 80 minutes. The specimens were checked under a binocular microscope after 20 minutes to determine how the clearing was progressing. When clearing was complete, the specimens were washed in two baths of 70% ethanol to ensure that all traces of lactic acid were removed.
The studied material was mounted in polyvinyl lactophenol. Before the final slide mountings in dorsal view, the hypopygium including tergite IX and anal point, the gonocoxite and the gonostylus, were viewed ventrally and laterally, in order to examine and draw all the necessary details of the species, from both sides. For a better examination of the specific features of the hypopygium, the anal point and tergite IX were removed and the hypopygium was illustrated in a lateral view separately. Remaining part of the abdomen and the halters are preserved in 85% ethanol for an eventual DNA analysis. Morphological terminology, abbreviations and measurements follow those of Saether (1980) and Langton and Pinder (2007). : crystalline water, conductivity 5.9 µS/cm; temperature (min-max, 3.9-19.5 °C; mean, 11.6 °C), pH 6.7.
Holotype (mounted on 1 slide; GBIFCH 00597051) is deposited in the collections of the 'Musée cantonal de Zoologie, Palais de Rumine, 6 place de la Riponne, CH-1014 Lausanne (MZL), Switzerland. Paratypes, Switzerland. 1 male adult, mounted on 1 slide, is deposited in the collection of the senior author.
Etymology. The name 'immezensis' refers to the Immez Lake basin where the type-material was collected.

Diagnostic characters
P. immezensis sp. n. can easily be distinguished from other related species by the following combination of characters. Head. Base and median part of coronal suture with atypical lateral and median extensions; temporal setae 10-11; antenna 1280 µm long, last flagellomere about 300 µm long, apical seta absent, AR 0.30. Sensilla coeloconica absent on palpomere 3. Clypeus sub-trapezoidal, with 8 setae. Thorax. Lobes of antepronotum in contact; humeral pit half ellipse-like. Wing. Distal half of membrane densely covered with macrotrichia (hairy cells are: r 4+5 , m 1+2 , m 3+4 ; cells cu and an bare); squama with 5 setae. Legs. Sensilla chaetica present on tarsomeres ta 1 -ta 5 of PI-PIII. Abdomen. Tergite IX broad basally, narrowed distally. Anal point markedly enlarged at base, with 17 setae mostly located close to the lateral margin. Virga with 4 spines. Gonocoxite truncate in its inner apical margin. Superior volsella, large lobe-like. Inferior volsella double: dorsal lobe nose-like with smooth inner margin; ventral lobe low. Gonostylus without posterior projection, caudal margin rounded; crista dorsalis absent. Small to medium sized species. Total length (TL) 2.25 mm, wing length (WL) 1.35 mm; TL/WL = 1.67 (n =1). General colouration is ranging from pale brown to dark brown; head, thorax and antenna brown; thorax brown with dark brown mesonotal stripes; legs uniformly brown; abdomen and anal segment brown.

Ecology and geographical distribution
Only one, not associated larva of the genus Pseudorthocladius was collected in the inflow area of Lake Immez, where bryocolous and hygropetric habitats seem to represent the most favourable aquatic microhabitat for the larval populations.
The new species likely belongs to the crenophilous community of species as documented by Lindegaard (1995) as no Pseudorthocladius larvae were found during extensive sampling in the lake Immez and other small lakes in the area (Lods-Crozet unpublished data; Lods-Crozet et al. 2012). Such pristine lotic habitats are considered to be microrefugia and hotspots of diversity and therefore deserve much greater consideration, protection and preservation.
The presence of P. immezensis sp. n. in high mountain Alpine ranges in the Swiss Alps (above 2600 m a.s.l.) highlights and confirms that some cold and glacial high mountain enclaves can contain diversity previously unknown to science. Documentation and knowledge of this fauna is important to monitor diversity and community changes caused by global warming and climate change.