Platycheirus pusillus, R.Nielsen & Romig, 2010

R. Nielsen, Tore & Romig, Thomas, 2010, Two new species of the genus Platycheirus Le Peletier & Serville, 1828 and a description of female P. altotibeticus Nielsen, 2001 (Diptera, Syrphidae) from China., Norwegian Journal of Entomology 57, pp. 1-8 : 1-5

Platycheirus pusillus sp. n.

Type material: Holotype, ♂ dated “ CHINA, NW Sichuan vicinity of Tuanjie (Serxu Co.), UTM 47S 0399984 3667047, 4100–4600 m a.s.l., 10– 21.07.2002, leg. T. Romig”. Paratypes: 4♂♂ 1♀ with same data as holotype. Holotype and 1♂ 1♀ paratypes in coll. Staatliches Museum für Naturkunde Stuttgart ( SMNS), 2♂♂ paratypes in coll. T. Romig, 1♂ paratype in coll. T. R. Nielsen. Diagnosis: P. pusillus is a remarkably small species of the ambiguus group, the size of a small Melanostoma mellinum (Linnaeus, 1758) . Tergites 2–4 each with a pair of yellowish, oblique spots. The species resembles P. abruzzensis (v.d. Goot, 1969) and P. transfugus (Zetterstedt, 1838) but separates from those in e.g. a rather produced face and a broad, shining median stripe in face (see key in Table 1 ) .

Description

MALE ( Figures 2 , 3A–G )

Head: Figures 3A–D . Eye angle 100°. Frons densely covered by metallic whitish yellow dusting, lunulae shining black, the hairs are white. Antennae black, 3 rd joint short, only 1.2x longer than broad. Face rather produced, central prominence and upper mouth edge equally so. Face similarly dusted as frons along the sides, leaving a broad black stripe in the middle. Mouth edge and genae shining black. Occiput with greyish yellow dusting. Ocellar triangle anteriorly black haired, posteriorly white haired.

Thorax: Figures 3E–F . Scutum and scutellum shining black with a brassy lustre. The hairs yellowish white, about as long as maximum thickness of hind femur. Scutellum without long bristly hairs on posterior margin. Pleura black with light whitish yellow dusting. – Legs: Fore and mid femur orange, with a brown shadow in the middle. Fore femur behind with a long curled bristle near apex, followed by a row of rather soft bristles. The bristles vary in colour: the curled apical bristle is black in most specimens, but may be yellow white like the following bristles. The hairs at base of fore femur are scarcely as long as thickness of femur. Fore and mid tibia short haired, without posterolateral bristles. Apical half of all tibia and tarsi darkened, greyish black. Hind femur black, except for base and apex broadly orange. Hind basitarsus slender, about as thick as hind tibia at apex. – Wing: Stigma and veins basally yellow brown, distally darker. Calypter white, the rim yellow. Haltere yellow.

Abdomen. Figure 3G . Tergite 1 metallic black, shining. Tergites 2–4 black, each with a pair of oblique orange yellow spots. The spots are dulled by light whitish dusting. Tergite 5 and following segments shining black. The hairs along the side margins of the tergites white, the short hairs on tergite disc follow the ground colour. Sternites blackish, the hairs white.

FEMALE ( Figures 3H–J )

The female differs from the male as follows:

Head: Figures 3H–I . Frons shining black with a pair of triangular, grey dust spots which together occupy about half the width of frons. 3 rd antennal joint broader than in the male. Face with greyish yellow dusting at the sides, leaving a broad, shining black medial stripe, from upper mouth edge and above central prominence. Anterior part of vertical triangle and frons posteriorly black haired, head otherwise with whitish yellow hairs.

Thorax: The hairs on scutum and scutellum yellow and shorter than in the male, as long as half the maximum thickness of hind femur. – Legs: Fore and mid femur clearly orange. Hind femur with a broad black ring on apical half, but leaving the apex broadly orange. Fore and mid tibia with apical third, and hind tibia with about apical half black. All tarsi blackish, the extreme base of basitarsus orange.

Abdomen: Figure 3J . The spots on tergites 2–4 rounder, less dusted and more shining than in the male.

Body length: Male 5.6–6.1mm, female 5.5 mm. Wing length: Male 4.8–5 mm, female 4.9 mm.

Etymology: Pusillus (Latin) means tiny, referring to the small body size of the species.

Ecology: The species was caught within tall herbaceous vegetation along rivers, figure 1. These small patches, which are interspersed within large stretches of short grassland, provide the only microenvironment which is reasonably sheltered from the incessant strong wind. It is highly probable also to provide the habitat for the larvae.

Figures

Figure 2. Platycheirus pusillus sp. n. male.

Figure 3. Platycheirus pusillus sp. n. Male: A: head, lateral view, B: head, dorsal view, C: head seen from in front, D: antenna, E: fore femur, F: hind basitarsus, G: abdomen. – Female: H: head, lateral view, I: head, dorsal view, J: abdomen.

Tables

Table 1. Key to separate Platycheirus pusillus sp.n. from P. abruzzensis (v.d.Goot,1969) and P. transfugus (Zetterstedt, 1838), all with short haired fore tibia.

MALES
1.	Central prominence and upper mouth-edge obviously more produced than frons. Face with a broad,
	shining black medial stripe, the stripe is covering about 1/5 the width of face. Scutum and scutellum
	with a brassy tinge ........................................................................................................ pusillus sp. n.
-	Central prominence and upper mouth-edge only slightly more produced than frons. Face with a narrow
	black medial stripe, the stripe is covering only about 1/7 the widt of face. Scutum and scutellum with a
	steel blue or bluish black tinge ........................................................................................................... 2.
2.	Fore femur postero-laterally near apex with a row of rather strong bristles of uniform thickness and
	which narrow rather abruptly towards their apices. Pleurae rather heavily dulled by white dusting,
	almost as dense as that on occiput. Frons and vertical triangle at least partly white haired. Abdomen
	rather long and narrow, tergite 2 longer than wide................................. abruzzensis (v.d. Goot, 1969)
-	Fore femur postero-laterally near apex with a row of hairs which are tapering towards
	their apices. Pleurae lightly dulled by white dusting, obviously less dense than that on
	occiput. Frons and vertical triangle black haired. Abdomen rather short; tergite 2 as long as
	broad................................................................................................... transfugus (Zetterstedt, 1838)
FEMALES
1.	Central prominence and upper mouth-edge obviously more produced than frons. Face with a broad,
	shining black medial stripe, the stripe is covering about ¼ the width of fac.................... pusillus sp. n.
-	Central prominence and upper mouth-edge only slightly more produced than frons. Face with a
	narrow black medial stripe, the stripe is covering only about 1/6 the width of face............................. 2.
2.	3 rd antennal segment about 1.5 times as long as wide. Pleurae rather heavily dulled by white dusting,
	almost as dense as that on occiput. Anteromedian part of hind tibia with a row of white hairs which are
	about as long as the diameter of the tibia............................................. abruzzensis (v.d. Goot, 1969)
-	3 rd antennal segment about 1.2 times longer than wide.Pleurae lightly dulled by white dusting,obviously
	less dense than that on occiput. Anteromedian part of hind tibia with only short hairs, the hairs no
	longer than half the diameter of the tibia............................................ transfugus (Zetterstedt, 1838)