A new species of Neossos Malloch (Diptera: Heleomyzidae) from the Yukon Territory, Canada, and a revised key to the Nearctic species

Abstract Background The rarely collected genus Neossos Malloch contains three Nearctic and one western European species. Most known specimens have been collected from bird nests. Two specimens of an undescribed species of Neossos were collected by sweeping in subarctic tundra and a mesic meadow in the Yukon Territory, Canada. This represents a significant northward extension of the known Nearctic range of the genus. New information Neossos tombstonensis sp. n. is described from the Yukon Territory. This represents the fourth described Nearctic species of Neossos. Although the type specimens were collected by sweeping, the species is predicted to be associated with bird nests, based on habits of other members of the genus. A revised key to the Nearctic species of Neossos is provided.


Introduction
Neossos Malloch, 1927 is a rarely collected genus of acalyptrate Diptera primarily associated with bird nests, where the larvae are apparently saprophagous in nest material. Gilbert and Wheeler (2007) revised the Nearctic fauna of Neossos and recognized three species in the region: Neossos marylandicus Malloch, 1927 (associated with cavity-nesting passerine birds in eastern North America); N. californicus Melander, 1952 (associated with raptors in western United States and southern British Columbia); and N. atlanticus Gilbert & Wheeler, 2007 (associated with cliff-nesting seabirds in coastal eastern North America). In the course of a large-scale study of arthropod diversity and ecology in northern Canada, we collected two specimens of Neossos from the Yukon Territory, far north of the documented geographic range of Nearctic Neossos species. Those specimens are described here as Neossos tombstonensis sp. n.

Materials and methods
Field-collected specimens were preserved in 95% ethanol and subsequently chemically dried using Hexamethyldisilazane. Type specimens are deposited in the Lyman Entomological Museum, McGill University, Ste-Anne-de-Bellevue, QC, Canada (LEMQ) and have been assigned unique specimen identifiers in the format LEM0000000. Genitalic dissection of the male holotype was made by detaching the posterior part of the abdomen, and heating it in 85% lactic acid on a heating plate for 10 minutes. Cleared genitalia were transferred to glycerin for examination and drawing, then stored in glycerin in a plastic microvial pinned with the specimen.

Etymology
The species is named for the Tombstone Mountains and Tombstone Territorial Park, where the holotype specimen was collected.

Key to the Nearctic species of Neossos
Although there are species-level differences in the shape of the male surstylus and epiphallus, the former is highly dependent on angle of view and the latter is subject to variation based on the preservation of the specimen and degree of collapse of the epiphallus. Both characters should be interpreted with caution.
1 Hind tibia expanded apically in anterior view, diameter at insertion of pre-apical dorsal seta at least 1.38 times diameter at midpoint of tibia and 3 times diameter at base (Fig. 4) 2 -Hind tibia not obviously expanded apically, diameter at insertion of preapical dorsal seta 1.2 times diameter at midpoint of tibia, and less than twice diameter at base (Fig. 5) 3 Neossos atlanticus, hind tibia, anterior view.

Discussion
Because they are apparently obligate associates of bird nests, specimens of Neossos are rarely collected. Of approximately 130 specimens studied by Gilbert and Wheeler (2007), all but one were collected or reared from bird nests or birds themselves. The single Palearctic species of Neossos, N. nidicola (Frey, 1930), known from Great Britain and Finland (Pape et al. 2015) is closely associated with bird nests. Although the collection of two specimens of N. tombstonensis by sweeping was surprising, the rarity of specimens was not. Multiple years of intensive sampling at the two localities from which types were collected, as well as other similar sites in the Yukon, have not produced another specimen.
The types of N. tombstonensis were collected in two distinct habitats. The North Fork Pass site is a wet tundra meadow north of treeline in the Ogilvie Mountain ranges. Dominant vegetation includes sphagnum mosses, grasses, sedges and ericaceous shrubs over a substrate with extensive permafrost. In contrast, the Robinson Road House site is a mesic meadow with a diverse assemblage of herbaceous plants dominated by Asteraceae, Fabaceae, and Poaceae, on a sand substrate in a clearing surrounded by spruce-pineaspen forest.
Based on the known habits of the other described species of Neossos, it is likely that N. tombstonensis is also a nest associate. Each of the other three Nearctic species exploits a different microhabitat and host group: Neossos marylandicus in association with cavitynesting passerines; N. californicus in raptor nests; and N. atlanticus in nests of cliff-nesting seabirds (Gilbert and Wheeler 2007). Several species of birds, including passerines and non-passerines, nest in both regions of the Yukon in which N. tombstonensis was collected, so it is impossible to speculate on the identity or nesting habits of the hosts. Neossos marylandicus, hind tibia, anterior view.