Key to Trichodrilus species (Lumbriculidae) with two pairs of spermathecae

1 Atrial ampulla tubular (l/w =2.4–4.5)..................................................................... 2

- Atrial ampulla spherical, pear-shaped or oval (l/w <2.3)...................................................... 5

2 Long atria (can be longer than body diameter). Muscular penial bulb developed. Penis formed by the extension of atrial duct lining cells (type-2 penis), can attain 300 µm long, formed by internal atrial duct lining cells (type-2 penis). Thick atrial musculature (23–28 µm). Posterior lateral blood vessels present............................... T. leruthi Hrabě, 1937

- Atria without a muscular penial bulb..................................................................... 3

3 Posterior vasa deferentia do not penetrate into the post-atrial segment. Long atria (more than half body diameter). Atrial musculature 7 µm thick.............................................................. T. tacensis Hrabě, 1937

- Posterior vasa deferentia penetrate and form a loop into the post-atrial segment.................................... 4

4 Vasa deferentia join the atrium at its most apical region. Vas deferens with a proximal thick section with fine musculature, and a distal, narrow section (close to the atrium) with thicker musculature. Atrial musculature 9 µm thick. Posterior lateral blood vessels present................................................................ T. intermedius (Fauvel, 1903)

- Vasa deferentia enter basally to the atrial ampulla and open to the atrial lumen subapically; diameter and muscle layer not varying along length of vasa deferentia. Atrial musculature 5–6 µm thick (North America)................................................................................................ T. humptulips Fend & Rodriguez, 2024

5 Post-clitellar dorsal chaetae modified, long (hair-like) with proximal nodulus (ratio of tip to nodulus /total chaeta length 0.7– 0.8); ventral chaetae thicker than dorsals, also with proximal nodulus (0.6 ratio). Very small worms (0.13–0.16 µm body Ø)................................................................... T. capilliformis Rodriguez & Giani, 1994

- Chaetae more or less alike in shape, with distal nodulus.................................................... 6 *1

6 Vasa deferentia do not penetrate into post-atrial segment. Atrial duct spindle-shaped, with penis formed by the extension of atrial lining cells (type-2 penis). Thin atrial musculature (2–5 µm). Posterior lateral blood vessels present....................................................................................... T. cantabrigiensis (Beddard, 1908)

- Vasa deferentia penetrate into post-atrial segment, usually forming a loop. Atrial duct another shape................... 7

7 Epidermal glands developed in the dorsal region of middle to posterior segments, forming pads or bulky masses (to 68 µm high). Atria petiolate, ampulla with thin atrial musculature (c. 3 µm).............. T. angelieri Giani & Rodriguez, 1994

- Integument without glandular epidermal pads.............................................................. 8

8 Penis very long (to 140 µm), formed by cells of the inner epithelial layer of the atria extending through the male pore (type-2 penis). Atria spherical with crossed muscular fibers. Thinner chaetae at segment X.. T. longipenis Rodriguez & Giani, 1994

- Penis short or absent.................................................................................. 9

9 * 2 Atrial musculature very thin (<3 µm). Vasa deferentia join the ampulla in median to subapical position................ 10

- Atrial musculature 5–15 µm thick. Vasa deferentia join the ampulla in basal to apical position....................... 11

- Atrial musculature ≥ 20 µm thick. Vasa deferentia join the ampulla in median to subapical position................... 16

10 Atrial ampulla oval, with a short, conical duct which opens on a short porophore; type-2 penis that may extrude through the male pore to 50 µm.................................................................. T. tenuis Hrabě, 1960

- Atrial ampulla spherical, with a short, cylindrical duct, petiolate. with a short type-1 penis....... T. moravicus Hrabě, 1937

11 Posterior lateral blood vessels (3–6 pairs, simple) in middle and posterior segments. Body diameter 0.3–1.6 mm, atrial musculature 5–10 µm...................................................... T. allobrogum Claparède, 1862 * 3

- Posterior lateral blood vessels absent * 4 .................................................................. 12

12 Atrial duct gradually narrows from the ampulla to the pore................................................... 13

- Atrium petiolate, well-defined, cylindrical duct distinct from atrial ampulla * 5 .................................... 15

13 Atrial duct longer than ampulla; type-2 penis, formed by elongation of inner atrial duct cells. Vasa deferentia join the atrium subapically........................................................................ T. medius Hrabě, 1960

- Atrial duct shorter than ampulla; type-1 penis, formed by a short, simple protrusion of the atrial duct................. 14

14 Vasa deferentia join the atrium apically. Atrial ampulla oval, not laterally compressed. Atrial duct oriented backwards, conical porophore.............................................................................. T. benati sp. nov.

- Vasa deferentia join atrium subapically. Atrial ampulla laterally compressed, i.e., wider than long in a sagittal-plane section, appearing tubular in a transversal section. No porophore..................................... T. hrabei Cook, 1967

15 Basal junction of the vasa deferentia to the atrial ampulla. Atrial musculature thin, 2–8 µm (to 10 µm basally, at the vasa deferentia junction), with penis absent or short............................................. T. stygodytes sp. nov.

- Subapical junction of the vasa deferentia to the atrium.............................. T. seirei Timm, 1979 (partim * 6)

16 Posterior lateral blood vessels absent.................................................................... 17

- Posterior lateral blood vessels present.................................................................... 18

17 Atrial ampulla spherical, with musculature 23 µm thick, atrial duct not well defined............ T. claparedei Hrabě, 1937

- Atrial ampulla spherical to oval, with atrial musculature to 25 µm thick, atrial duct roughly cylindrical, well separated from the ampulla................................................................... T. seirei Timm, 1979 (partim * 6)

18 Lateral blood vessels ramified. Atrium petiolate. Atrial musculature 27–30 µm........... T. aporophorus Popčenko, 1976

- Lateral blood vessels not ramified.Atrium not petiolate, ampulla gradually tapering to the male pore.Atrial musculature 20 µm. Male pores open on a large porophore, oriented backwards......................... T. macroporophorus Hrabě, 1954

* 1 Trichodrilus montenegrinus Karaman, 1973 has a spherical atrium, but no information on atrial musculature, posterior vas deferens penetrating (or not) the postatrial segment, or the presence of posterior lateral blood vessels is given in the description. Thus the species cannot be classified in present key.

* 2 9 has three options.

* 3 Junior synonyms: T. icenorum Beddard, 1920, T. lengersdorfi Michaelsen, 1933 .

* 4 This character is unknown in T. pauper Finogenova, 1973, because the description was based on posteriorly incomplete specimens. Known details: Atrium pear-shaped, atrial musculature 8–11 µm; atrial duct short and wide, but its structure is not described. The position of this species in the key is uncertain due to lack of data., although the author stated similarity to T. hrabei .

* 5 T. rouchi Rodriguez & Giani, 1994, a species with 4 pairs of spermathecae and therefore not included in this key, would key out here. Characters: globular atrium, chaetae more or less alike, epidermis smooth, atrial duct short, penis short, atrial musculature of 6 µm, posterior lateral blood vessels absent, vasa deferentia junction apical-subapical.

* 6 T. seirei: Timm (1979) described two types: The holotype from Esna Spring (Estonia) has spherical atrial ampulla (75 µm) and thick atrial musculature (13–25 µm). Paratypes have oval atrial ampullae 100–113 x 60–97 µm and thinner atrial musculature (4–10 µm). The latter are supposed to be post-mature worms, with atria degenerating after their functioning (Timm, 1979: 162), but they could also be different species.