The Dark Reddish, Heavily Armored Chilean Harvestman with Hooked Long Legs—Pachylus crassus

Pachylus is a Neotropical South American genus of harvestmen, named as early as 1839 and classified in the order Opiliones, family Gonyleptidae, subfamily Pachylinae.

(Figure 1) Male Pachylus crassus（Image source：gatofans(ika)， CC BY-NC 4.0 ）

Members of the genus are now regarded as largely restricted to central Chile. Earlier records had suggested that they also occurred in Argentina and Uruguay, but those reports were later shown to be erroneous. Shared features of Pachylus include a robust carapace, a dorsal scutum covered with granules, an ocular mound armed with a single conical apophysis, and a distinctive tarsal formula, with the numbers of tarsomeres on legs I to IV given as 5:n:6:6 (Figs. 2, 3). Harvestmen do not spin webs to capture prey; instead, they usually wander across the ground or through low vegetation in search of food.

(Figure 2) The arachnid walking leg consists of seven segments: the coxa (short, gray), trochanter (short, red), femur (long, green), patella (short, blue), tibia (long, purple), metatarsus (long, brown), and tarsus (short, cyan)（Image source：Stemonitis，CC0 1.0 ）

(Figure 3) The arthropod tarsus consists of multiple tarsomeres（Image source：Nwbeeson，CC0 1.0 ）

Pachylus crassus also inhabits central Chile. For a long time, its taxonomic identity remained unstable. In 1943, it was described as a member of Pachyloidellus. Over the following decades, however, it was successively transferred to different genera, including Progyndes and Acanthopachylus, and for a period even its correct generic placement was uncertain. The main reason for this confusion was that earlier studies relied mostly on old descriptions and simplified illustrations, while few researchers actually examined the type specimens. Only after the specimens were re-examined was it confirmed that the species truly belongs to Pachylus.

In male Pachylus crassus, the dorsal scutum is about 7.7 to 8.7 mm long; in females, it measures about 7.5 to 8.6 mm. The type specimen is a male with a body length, excluding the legs, of 9.5 mm, while one examined female reaches 10.8 mm. In 70% ethanol, the general color is dark reddish brown, with the lateral and posterior areas and the free tergites lighter, often orangish or yellowish brown; the granules of the dorsal scutum are usually darker. In living animals, these color contrasts are more vivid. The male has a dark, massive dorsum and a stout fourth pair of legs bearing conspicuous armature, whereas the female is broader and more rounded. The tarsal formula of this species is 5:7:6:6.

(Figure 4) Male Pachylus crassus（Image source：Claudio Maureira， CC BY-NC 4.0 ）

The diagnostic characters of the male are concentrated in the dorsal scutum, ocular mound, and fourth leg. The posterior margin of the dorsal scutum is slightly concave, the surface granulation is conspicuous, and the sulci delimiting the scutal areas are complete and well defined. The ocular mound lies in the anterior half of the carapace region. It lacks granules but bears a large spine-like apophysis inclined frontwards. The lateral areas of the dorsal scutum carry two rows of flat granules: an outer row extending to the primary constriction of the scutum, and a more scattered inner row that gives part of the surface a rugulose appearance.

The fourth leg is the most distinctive sexually dimorphic region in the male. The prolateral apophysis of coxa IV is stout and conical, nearly horizontal, and directed backwards. Trochanter IV is sub-trapezoidal and bears several distinctive apophyses, including a large ear-like prolateral apophysis, one of the key features separating Pachylus crassus from similar congeners. Femur IV is slightly curved, but less so than in other species of Pachylus; in lateral view, the pre- and post-flexure sections form an angle of about 150°. The ventral side of the femur bears large apophyses, including a conspicuous hook-like, slightly sigmoid apical-retroventral apophysis near the distal end, giving the tip of the femur a hooked appearance. Patella IV is smooth and unarmed. Tibia IV is club-shaped, with rows of strong ventral apophyses, the three distal ones on the retroventral side being especially large. The metatarsus and tarsus are light brownish and lack conspicuous apophyses.

(Figure 5) Male Pachylus crassus（Image source：djalma2631， CC BY-NC-SA 4.0 ）

Females differ from males in having a broader, more rounded body form. The posterior margin of the dorsal scutum is straight to gently convex, rather than posteriorly depressed as in males. The armature of leg IV is also weaker. Coxa IV bears only a small conical prolateral apophysis, and although femur IV is curved and granulous throughout, its apophyses are much less strongly developed than in males.

In terms of distribution, Pachylus crassus is currently confirmed from the Maule and O'Higgins Regions of central Chile. Recorded localities include Los Queñes and Las Tablas in Curicó Province, and Tonlemo in Talca Province.

Pachylinae has long been regarded as a large and difficult-to-define subfamily, and recent studies have indicated that it may contain several unrelated lineages. Clarifying the species limits, morphological characters, and geographic distribution of Pachylus is therefore an essential step toward reconstructing the future taxonomic framework of the entire subfamily.

Author: Shui-Ye You

Reference:

Acosta LE. (2021). The identity of an elusive Chilean harvestman, Pachylus crassus (Roewer, 1943) (Opiliones: Gonyleptidae: Pachylinae), with taxonomic and distribution notes. Zootaxa.