A key to the known fourth instar larval stages of North American Metriocnemus van der Wulp

1. Mentum with 4 small median teeth and 5 pairs of lateral teeth (Epler 2001: page 7.86, couplet 2(1’))................................................................................................... M. knabi Coquillett, 1904

- Mentum with bifid or simple median tooth and 4 to 5 pairs of lateral teeth........................................ 2

2. Mentum with simple median tooth and 5 pairs of lateral teeth (Saether 1989: Figure 12G)...... M. ursinus (Holmgren, 1869)

- Mentum with bifid median tooth and 4 to 5 pairs of lateral teeth................................................ 3

3. Mentum with a wide bifid median tooth and 4 pairs of lateral teeth (Fig. 3h)...................... M. erthranthei sp. nov.

- Mentum with narrow bifid median tooth and 5 pairs of lateral teeth.............................................. 4

4. Median tooth of the mentum is comparatively small, sitting much lower than lateral teeth............................ 5

- Median tooth of the mentum comparatively well-developed, sitting slightly lower or higher than lateral teeth............ 6

5. Basal antennal segment no longer than wide (Saether 1989: Figure 12B). Labral lamella and SI simple (Saether 1989: Fig. 12D). Median tooth of mentum sunken (Epler 2001: page 7.86, couplet 3(2’); Orendt & Bendt 2021: page 119, couplet D7; Saether 1989: Fig. 12H). Anal tubules long and slender (Saether 1989: Fig. 12J)..................... M. fuscipes (Meigen, 1818)

- Basal antennal segment longer than wide (Fig. 7b). Labral lamella comb-like and SI trifid (Fig. 5c). Median tooth of mentum not sunken (Fig. 7e). Anal tubules short and wide (Fig. 7f)......................................... M. sp. “ Oregon ”

6. Antenna 4 segmented (Cranston & Judd 1987: Fig. 4a). Basal antennal segment L/W> 4. Median tooth of mentum stands higher than 1 st lateral teeth. Setae submenti aligned with the 3 rd lateral teeth of mentum (Cranston & Judd 1987: Fig. 4c)............................................................................ M. yaquina Cranston & Judd, 1987

- Not with the above combination of characters............................................................... 7

7. Abdominal segments with long setae, each half as long as the segment bearing it.................... M. sp. A Epler, 2001

- Abdominal segments without long setae................................................................... 8

8. Procercus as wide as long or wider than long.AR 1.7–1.8. Basal antennal segment L/W: 2.3 (Th. Bendt & H.K.M. Moller Pillot, in litt.)................................................................................ M. tristellus agg. *

- Not with the above combination of characters............................................................... 9

9. Median bifid tooth of mentum as high as the 1 st lateral teeth (Saether, 1989: Fig. 6E). Basal antennal segment L/ W 2.5 –2.6. Head length 0.44–0.45 mm ................................................................. M. brusti Saether, 1989

- Median bifid tooth of mentum clearly lower than 1 st lateral teeth. Basal antennal segment L/ W 1.1 –2.4. Head length 0.13–0.47 mm ............................................................................................... 10

10. Basal antennal segment L/ W 1.1. AR 0.67. Mandible with 3 inner teeth (Epler 2001: page 7.162). Procercus with 4 anal setae.............................................................. M. sp. (= Orthocladiinae genus E, Epler, 2001) **

- Not with the above combination of characters.............................................................. 11

11. AR 1.5–1.8. Head length 0.28-0.31 mm (Saether, 1989)............................... M. albolineatus (Meigen, 1818)

- AR 2.0–2.2. Head length 0.39–0.47 (Saether, 1989)................................ M. eurynotus (Holmgren, 1883) ***