Diamesa tonsa

Author: (Haliday, 1856)

Exuviae 4.7-7.6mm long (n=11).
Colour: In deeply golden examples, inner half of paratergites nearly colourless, contrasting with the tergite and the lateral margin, which is about the same intensity as the golden background colour of the tergites; in pale examples, the blotchy brown colouration of the tergites is indistinct. Most of the thoracic setae and the lateral setae of the abdomen golden, hardly, if at all infuscated.
Cephalothorax: Frontal setae 340, 350µm long (n=2). Thoracic horn 340-550µm long; ThR 20-30 (n=9) (030i.jpg). Precorneal setae 1-3: 120-138; 345-360; 180-255µm long (n=3). Thorax granulate.
Abdomen: Tooth rows on on tergites II-VII; tergite II rarely with less than 7 teeth; tergites VI and VII each with 4-10 teeth (030j.jpg). Total dorsal teeth 39-76 (n=9); best developed dorsal teeth about 100µm long, golden to brown. Total ventral teeth 35-87+ (n=9). Behind the tooth rows tergite armament consists of a band of points. Tergite II with a band of dense points extending at least for half the width of the tergite and posteriorly at least as far as the anterior adhesion marks. Tergites III-VII with points spreading posteriorly outside the anterior adhesion marks, extending the point patch towards the postero-lateral corner, progressively more extensive on succeeding segments (031d.jpg). Shagreen points of tergite III 8-10µm long. Relative length segments IX:VIII:VII = 1.0:0.7,0.75:1.0,1.1 (n=2); relative width segments IX:VIII:VII = 1.0:1.2,1.3:1.4,1.7 (n=2). Segment VIII 2.8, 2.9 and segment VII 2.4 times as wide as long (n=2). Lateral setae of segments II-VII long; setae 1-4 of segment III 150-190; 160-200; -- 180-200; 95-120µm long (n=4).
Anal segment: Anal lobe ratio 1.1-1.8 (n=7). Each anal lobe with 3 macrosetae; length macrosetae 0.8, 0.85 times lobe length (n=2).

Species keys out at Page 582: Diamesinae 58 Diamesa of the Text Key.

Widespread in Europe.
(For more information see module IdentifyIt – file: Other subfamilies).

Ecological notes
Montane streams, rivers and occasionally lakes.