Key to the Azorean species of Trechus (modified from Borges et al. 2004)

1 Apical angle of elytra rounded..................................................................................................................... 2

- Apical angle of elytra pointed or sharp angled............................................................................................ 7

2 Larger species, length> 3.5 mm; lateral margins of pronotum markedly sinuated before the basis, degree of pigmentation variable.............................................................................................................................. 3

- Length 3.0– 3.2 mm; lateral margins of pronotum hardly sinuated before basis; clearly dark pigmented species ............................................................................................................................ torretassoi Jeannel

3 3rd antennomere subequal to 2nd ................................................................................................................. 4

- 3rd antennomere longer than 2nd .................................................................................................................. 5

4 Pronotum cordiform and highly transverse (index W/L = 1.35), with lateral furrow very narrow (epigean species) ...................................................................................... terrabravensis Borges, Serrano & Amorim

- Pronotum less transverse (index W/L = 1.26); lateral furrow larger (beetles occurring in entrance of caves) ................................................................................................................. montanheirorum Oromí & Borges

5 Body depigmented with a convex and ovate aspect .................................................................................... 6

- Body pigmentation dark brown, with some lighter parts; appendages less pigmented (light brown); body more depressed ........................................................................................................... isabelae new species

6 Aedeagus with apex of lamella slightly blunt; known from Pico .................................... picoensis Machado

- Aedeagus with apex of median lobe deeply curved upwards; known from Faial ......................................... .............................................................................................................. oromii Borges, Serrano & Amorim

7 Apical angle of elytra sharp (sometimes notched), forming a small protruding point; total length 3.6–4.3 mm .............................................................................................................................. terceiranus Machado

- Apical angle of elytra sharp (forming a right angle); total length <3.5 mm ............................................... 8

8 Slightly pigmented on elytra and pronotal disc; aedeagus in lateral view with apex of median lobe more or less straight and pointed; left paramere with 4 setae ........................... pereirai Borges, Serrano & Amorim

- Depigmented; aedeagus in lateral view with apex of the median lobe sharpened and slightly curved upwards; left paramere with 5 setae .................................................................. jorgensis Oromí & Borges