Didea alneti (Fallén, 1817):38

Didea species are large with broad flat abdomen bearing strongly emarginate or narrowly divided yellow, yellow-green, or pale green bands; eyes with moderately long but extremely sparse pili; and metasternum pilose. Didea alneti has the basal half of cell BM microtrichose on apical two-thirds or more, and tergum 4 with a yellow fascia narrowly divided.

Adapted from Vockeroth (1992).

MALE.

Head: frons yellow with large black macula above base of each antenna. Face yellow with lower margin entirely blackish, and with distinct dark brown triangular medial vitta on lower half. Holoptic, eyes with moderately long but extremely sparse pili. Scape rather large, twice as long as broad.

Thorax: scutum black, shining or subshining. Scutellum dull yellow with entire posterior margin dark or rarely entirely dark, blak pilose with yellow pile on at most extreme anterior margin; ventral scutellar fringe short and sparse. Pleura black with densely pale-pruinose areas on upper half. Anterior anepisternum bare; dorsal and ventral katepisternal pile patches broadly separated; meron bare; metaepisternum with some pili; metasternum pilose. Wing membrane mostly microtrichose, with bare areas in anterior one-third or less of cell BM, and in some specimens in very narrow strip behind CuP. Vein R4+5 broadly and rather deeply dipped into cell r4+5. Lower calypter usually with very few fine pale erect scattered hairs on upper surface. Metacoxa with several pili at posteromedial apical angle.

Abdomen: strongly margined. Markings of terga yellow-green to pale green; fascia of tergum 4
and rarely fascia of tergum 3 divided medially; tergum 5 black.

FEMALE.

Arms of dark Y-shaped macula above antennae wider than anteromedian yellow area. Tergum 5 usually black, rarely with pair of small anterolateral yellow maculae.

Didea alneti (Fallen, 1817).

Fallen, C.F. (1816-1817) Syrphici Sveciae. Pp. 1-14 [1816.06.08], 15-22 [1816.06.08], 23-30 [1817.05.10], 31-42 [1817.05.20], 43-54 [1817.05.21], 55-62. [1817.05.22] Berlingianis, Lundae [= Lund].

Synonyms:

Scaeva alneti Fallen, 1817: 38.

Syrphus pellucidulus Wiedemann, 1822: 311.

Didea japonica Matsumura, 1917: 138.

Didea sachalinensis Matsumura, 1917: 138.

Species similar to Didea fuscipes, but the lattest differs by having scutellum yellow pilose on basal one-third or more, otherwise black pilose; basal half of cell bm microtrichose only on posterior one-third; tergum 4th with yellow fascia deeply emarginate but entire; tergum 5th at least slightly reddish anterolaterally in male, yellow with black posteromedian triangle in female (from Vockeroth 1992).

Body lenght: 10.5-13.8 mm.

In the molecular analysis of Mengual et al. (2008) Didea alneti was not included, but the genus Didea was recovered as monophyletic (with the two species studied, D. fuscipes and D. intermedia, placed together) and as sister group of the genus Megasyrphus; in congruence with larval characters (Rotheray and Gilbert 1989).

Larvae of Didea alneti have been reported feeding on several species of aphids (Rojo et al. 2003).

Holarctic species known from the Nearctic (Alaska to Labrador, south to Colorado) and Palaearctic (Northern Europe and elevated parts of Central Europe) regions.