Key to the species of genus Balgus Fleutiaux, 1920, adapted from Bonvouloir (1875) and Chassain & Touroult (2013)

The identification key to the species of Balgus was adapted from Bonvouloir (1875) and Chassain & Touroult (2013), with the inclusion of new characters: presence or absence of a pair of yellow pronotal tubercles capable of emitting green light (Costa, 1984); thoracic sclerites in lateral view partially visible or hidden under the elytra; shape of eye in lateral view; body format; presence or absence of the anterior pronotal tubercles; presence or absence of the longitudinal median carina on ventrite 5 and the number of longitudinal teeth; format of ventrite 5; shape of the elytral apex; and format of the median longitudinal depression in the anterior region of the head.

1. Pronotum with a pair of yellow tubercles capable of emitting green light (bioluminescence) (Costa, 1984); thoracic sclerites in lateral view partially visible; eye in lateral view with corrugated surface, dorsal margin sinuous; large size (20–26 mm). (Figure 8)................................................................................. Balgus schnusei

- Pronotum without bioluminescent tubercles; thoracic sclerites, hidden under the elytra, not visible in lateral view; eye in lateral view with spherical surface, dorsal margin not sinuous; small to medium size (7.5–18.7 mm) (Chassain & Touroult, 2013).. 2

2. Elytra covered with diverse tubercles distributed across the elytral surface......................................... 6

- Elytra covered only with an anterior suboval tubercle or without tubercles........................................ 3

3. Posterior angle of the pronotum with short oblique carina (Figure 2E)........................................... 4

- Posterior angle of the pronotum without short oblique carina.................................................. 5

4. Scutellum carinate; elytra with an anterior suboval tubercle, covered with strong rugosities........... Balgus subfasciatus

- Scutellum not carinate; elytra without tubercles. (Figure 2)..................................... Balgus albofasciatus

5. Subcylindrical body; dorsal and ventral regions covered with moderately dense pubescence on head and pronotum, much denser on bands on elytra; basal region of elytra slightly rugose, with no elevated longitudinal stria; anterior pronotal tubercles absent; longitudinal median carina of ventrite 5 with three or four longitudinal teeth. (Figure 4)........ Balgus eschscholtzi

- Body cylindrical; body covered with dense pubescense; basal region of the elytra with five elevated longitudinal stria and without tranversal rugosities; anterior pronotal tubercles present; longitudinal median carina of ventrite 5 with one longitudinal tooth. (Figure 7)......................................................................... Balgus rugosus

6. Apical third of the elytra light brown or yellow, with a large black band on the basal two thirds of the elytra............. 7

- Apical third of the elytra dark brown to black, and may have various yellow or light brown bands, apex always black...... 8

7. Obconical body; elytral apex subrounded; ventrite 5 pentagonal, without longitudinal median carina; longitudinal median anterior depression of head continues posteriorly on a longitudinal groove on the vertex, not V-shaped. (Figure 6)................................................................................................... Balgus obconicus

- Subcylindrical body; elytral apex subacuminate; ventrite 5 triangular, with longitudinal median carina; longitudinal median anterior depression of head continues posteriorly on a longitudinal groove on the vertex, V-shaped. (Figure 5)..................................................................................................... Balgus humilis

8. Ventrite 5 with sinuous margins; strongly serrate; abruptly narrowed from base to the apex, which is abruptly truncated and curved; longitudinal median carina of the ventrite 5 with two or three longitudinal teeth; pronotum not abruptly attenuate at the anterior third; elytra with subacuminate apex, not narrowed abruptly. (Figures 1, 9)................. Balgus tuberculosus

- Ventrite 5 without sinuous margins; slightly serrate; abruptly narrowed from base to the apex, which is truncated and not curved; longitudinal median carina of the ventrite 5 with two longitudinal teeth; pronotum abruptly attenuate at the anterior third; elytra with subacuminate apex, abruptly narrowed. (Figure 3)............................... Balgus eganensis