A NEW SPECIES OF THE Odontomachus infandus SPECIES GROUP (HYMENOPTERA: FORMICIDAE) FROM PANGASINAN, PHILIPPINES, WITH NOTES ON SPECIES ECOLOGY

ARTICLE HIGLIGHTS
- New species of Odontomachus
- Distribution and ecological notes
- Updated species key

ABSTRACT
A new species of Odontomachus Latreille, 1804 named O. ampipitbaybay sp. nov. is illustrated and described from specimens collected along the coastal area of Alaminos City, Pangasinan, Luzon Island, Philippines, with notes on its species ecology. The new species is diagnosed and compared against closely related species from Luzon. A distribution map of selected Philippine Odontomachus species is provided, and a modification to the key to the Philippine Odontomachus is proposed.

INTRODUCTION

Philippine ants are of great interest due to their high richness brought by the geologic history and diverse ecological features of the archipelago. However, only about half of the estimated 1,000 species of Philippine ants have been discovered and described thus far, and much of their biology is still unknown (General & Alpert, 2012). Unfortunately, the inherent challenges brought by the country’s archipelagic setting and the few myrmecologists in the field coupled with low funding opportunities produce a poor picture of ant diversity and distribution in the Philippines (General, 2021). Being still understudied, the Philippine ant fauna offers many opportunities to researchers.

One notable group of ants, especially in rural areas, are the trap-jaw ants from the genus Odontomachus Latreille, 1804, locally termed ampipit by the Ilocano-speaking Pangasinenses. These are large ground-foraging ants that are easily recognized by their elongated mandibles, often seen wide open when threatened or ready to capture their prey. This genus comprises 74 valid extant species (AntWeb, 2024); (Bolton, 2024). General and (General & Alpert, 2012) listed 11 species in the Philippines. In (Sorger & Zettel, 2011) review of this genus, they provided a key for the Philippines that contains a different list of 11 species that includes 2 unnamed species. The most recent additions to the Philippine fauna are the new species, O. ferminae General, 2018, from Sibuyan Island (General, 2018) and O. pangantihoni (Zettel & Sorger, 2023) from Panay Island (Zettel & Sorger, 2023). At present, there is a total of 13 valid species in the Philippines.

In their review, (Sorger & Zettel, 2011) distinguished two sets within Odontomachus. The first set contains three unrelated species from three species groups, namely O. simillimus, O. malignus, and O. rixosus. The other set includes the infandus species group composed of regionally endemic and often forest-dwelling species.

Before this study, the infandus species group was represented in the Philippines by eight described species: Odontomachus alius (Sorger & Zettel, 2011), Odontomachus banksi Forel, 1910, Odontomachus ferminae General, 2018, Odontomachus infandus Smith, 1858, Odontomachus pangantihoni Zettel & Sorger, 2023, Odontomachus philippinus Emery, 1893, Odontomachus schoedli Sorger & Zettel, 2011, and Odontomachus scifictus (Sorger & Zettel, 2011) ((Bolton, 2024); (AntWeb, 2024)).

After a brief collecting trip in a coastal forest fragment in Alaminos City in the Northwestern part of Pangasinan, Luzon Island (Figure 1), several specimens of trap-jaw ants that were superficially similar to the highland O. schoedli were collected. Further examination revealed that these specimens constitute a new species, herein described, belonging to the infandus species group. This discovery is a testament to the rich diversity of Philippine ants and terrestrial arthropods in general, eliciting more research in understudied habitats, especially in threatened areas.

MATERIALS AND METHODS

The specimens described herein were collected from a small undisturbed beach adjacent to Bolo Beach, on coastal limestone aggregates with a small forest fragment near the Hundred Islands National Park. Foraging worker specimens were manually collected and pinned on cardboard mounts. Specimens were examined, measured, and imaged using an Olympus SZ61 stereomicroscope with Touptek camera attachment (Model XCAMTOP4K8MPA) and ToupView software version X64. The morphology of O. infandus and O. schoedli followed Sorger & Zettel (2011) and additional data of types were acquired from AntWeb (2024). Measurements were given from holotype followed by paratype mean (paratype m; n = 7) and the paratype minimum (min) and maximum (max) values in parenthesis: holotype (paratype m, min–max). All measurements were given in millimeters rounded to the nearest 0.01 mm. Measurements and indices followed (Sorger & Zettel, 2011) and (General, 2018), including the additional index (PtI).

Measurements:

TL Total length. Length of the entire ant measured in dorsal view with head stretched out, from anterior margin of the mandible to apex of gaster, excluding sting.

HL Head length. Maximum length of the head in full-face view, excluding mandibles, measured from anteriormost point of clypeal margin to posterior-most point of head vertex, parallel to midline.

HW Head width. Maximum width of head in full-face view (including eyes when surpassing head outline).

MdL Mandible length. Maximum length of the mandible in frontal view of head measured from mandibular insertion to apex.

MsL Mesosoma length. Maximum length of mesosoma, measured in lateral view, diagonal from cervical shield to the posterolateral propodeal edge.

PnW Pronotum width. Maximum width of pronotum in dorsal view.

PtH Petiole height. Maximum height of petiole, measured in lateral view as a straight line from the bottom edge of the petiole, perpendicular to petiolar apex.

PtL Petiole length. Measured in lateral view along the dorsal outline of petiole from small antero-apical tooth to apex.

PtW Petiole width. Maximum width of petiole in dorsal view.

SL Scape length. Maximum length of antennal scape in dorsal view excluding basal constriction.

GL Gaster length, maximum length from base of first gastral tergite to the apex of gaster, excluding sting, measured in lateral view.

Indices:

CI Cephalic index. HW / HL × 100 MdI Mandible index. MdL / HL × 100 SI Scape index. SL / HW × 100

PtI Petiole index. PtH / PtL × 100

Depositories:

PASI Philippine Arachnological Society, Inc. - Reference Collection, Manila

PNM Philippine National Museum of Natural History, Manila

Figure 1.Distribution map of selected Odontomachus species showing records of O. ampipitbaybay sp. nov. (red), O. schoedli (green), and O. infandus (blue) based on this study and Sorger & Zettel (2011), including unverified records from AntWeb (2024) and General & Buenavente (2017) (question mark) Inset: satellite image of the type locality. Map generated by DOR Mapile using QGIS 3.16.

Materials examined:

Odontomachus ampipitbaybay sp. nov., HOLOTYPE worker, PASI hym0001: PHILIPPINES: Luzon, Pangasinan, Alaminos City, Brgy. Telbang, near Bolo Beach, 16°10'47.6" N 120°03'11.2" E, 29.vi.2023,

DC Acuña & CHV Alpez leg.; PARATYPES, 7 workers, PASI hym0002–0008: same label as holotype; PASI (to be deposited in PNM).

Odontomachus infandus Smith, 1858, 15 workers, PASI hym0009–hym0023: PHILIPPINES:

Luzon, Pangasinan, Sison, Brgy. Labayug, southwest part of Cordillera Central Range, 20

RESULTS AND DISCUSSION

Taxonomy

Family FORMICIDAE Latreille, 1809

Subfamily Ponerinae Lepeletier, 1835

Genus Odontomachus Latreille, 1804

Type species: Formica haematoda Linnaeus, 1758, by monotypy

Odontomachus ampipitbaybay sp. nov.

urn:lsid:zoobank.org:pub:2F531215-A6FF- 4FBD-9DD0-4B9C5E21584F

urn:lsid:zoobank.org:act:A93DD1B4-6CA6- 4D4B-B7E4-33D3ECA8DA44

(Figure 2, Figure 3, Figure 4a–c, & Figure 5a–c)

Type material: HOLOTYPE, worker, PASI hym0001; PARATYPES, 7 workers, PASI hym0002-0008

Etymology: The specific epithet comprises the Ilocano nouns ‘ampipit’ which means trap-jaw ant, and ‘baybay’ which means beach, forming the word ampipit-baybay which translates to ‘trap-jaw ant of the beach’; noun in apposition.

Figure 2.Habitus of O. ampipitbaybay sp. nov. holotype worker, PASI hym0001, (a) lateral and (b) dorsal view

Diagnosis (worker): Member of the infandus species group following the treatment of Sorger and (Zettel & Sorger, 2023). Workers are different from other members of the infandus species group and similar to Odontomachus schoedli (Sorger & Zettel, 2011) and Odontomachus infandus Smith, 1858 in having posterior dorsum of head mainly striate (Figure 3a, Figure 4a) and mesopleuron with complete transverse striation (Figure 3c). Workers are also similar to O. schoedli in being unicolored head to petiole, having a reddish-brown color in life and a dark gaster (Figure 5a-5c). They can be distinguished from O. schoedli and O. infandus (i) in having a smooth area on the ocular ridge, medially (Figure 4a, arrow); (ii) in having a relatively shorter scape with SI < 134.15 (Table 1); (iii) in possessing a slightly curved and not S-shaped petiolar spine (Figure 4b) (almost straight in O. schoedli and more curved backward and S-shaped in O. infandus; Figure 4e; (Sorger & Zettel, 2011)); (iv) in having a stout subpetiolar process with base reaching less than half of petiole, ventrally (Figure 4c) (reaching more than half for the latter two species and broader on O. schoedli; Figure 4f; see (Sorger & Zettel, 2011), p. 145); (v) in having a distinctly depressed dorsum of propodeum on lateral view (Figure 3c, arrow); and (vi) in having a smaller body size with maximum TL of 13.45 (up to 16 mm on the latter two species; see Table 1). Workers further differ from O. schoedli in having a pronotum with mostly longitudinal striation (Figure 3(4)c, 3d).

They also differ from O. infandus in having a finer striation on the head and mesosoma and in possessing a lighter color in life which is reddish brown from head to petiole (Figure 5a-5c) (blackish brown in O. infandus; Figure 4d-4f; (Sorger & Zettel, 2011)).

Description (worker): Measurements: TL 13.18 (13.18, 12.84-13.45), HL 3.17 (3.20, 3.09-3.40), HW 2.19 (2.23, 2.11-2.46), MdL 1.96 (1.97, 1.85-2.11), MsL 4.97 (4.97, 4.89-5.03), PnW 1.77 (1.77, 1.75-1.80), PtH 1.24 (1.24, 1.23-1.27), PtL 1.37 (1.37, 1.34-1.39), PtW 0.40 (0.40, 0.38-0.42), SL 2.80 (2.87, 2.64-3.30), GL 3.23 (3.23, 3.19-3.26). Indices: CI 69.09 (69.65, 68.30-72.35), MdI 61.83 (61.50, 59.87-62.06), SI 127.85 (128.77, 125.12-134.15), PtI 90.51 (90.90, 91.37-91.79).

Head longer than wide, widest at eye level, posterior margin concave, and with relatively long mandible and short scape (Figure 2a-b, Figure 3a- b, Figure 4a; Table 1). Mandible with acute apical and subapical teeth (broken tip on the holotype). Dorsum of propodeum depressed in lateral view (Figure 3c, arrow). Petiole smooth, spine stout and slightly curved (Figure 4b). Subpetiolar process stout, subtriangular, pointing posteriorly, with margination, and with base reaching less than half of ventral petiole (Figure 4c). Gaster smooth with elongated pit at tergite I medially. Legs long and slender.

Sculpture: Dorsum of the head mainly striate, radiating to posterior margin, with a smooth area on ocular ridge and around median suture posteriorly, consistent in all workers examined herein (Figure 3a-b, Figure 4c). Gena and genal furrow are mainly smooth (Figure 3b, Figure 4a). Head ventrally smooth. Striation of pronotum mainly longitudinal laterally and dorsally with longitudinal ovoid loops creating transverse striae on the anterior and posterior dorsum (Figure 3c, 3d). Striation of mesonotum and propodeum entirely transverse (Figure 3c, 3d). Mesopleuron completely striate with very fine transverse striation (Figure 3c). Striation of mesosoma transverse ventrally. Overall striation is very fine.

Pilosity: Setation on the head and mesosoma is short and sparse. Setation on mandibles, petiole, and gaster is short and very sparse.

Color in life: Head, antennae, mandibles, mesosoma, and petiole are reddish brown. Gaster is mainly dark brown and anteriorly brown. Legs are yellow. (Figure 5a-5c)

Gyne and male: Unknown.

Distribution: Philippine endemic, known from the type locality, Luzon Island: Alaminos City, Brgy. Telbang (adjacent to Bolo Beach, near Hundred Islands National Park).

Odontomachus ampipitbaybay sp. nov. workers were found on an undisturbed beach adjacent to Bolo Beach (Figure 6). They were discovered foraging among leaf litter at the coast (Figure 5a-5c). Nests were built on clay loam soil situated on a narrow platform of coastal limestone aggregates elevated at approximately 2 masl and 2 m away from the tidal zone (Figure 6b).

Like its congeners within the infandus species group, O. ampipitbaybay sp. nov. may present another regionally endemic species. It is currently known from the forested coastal karst area of Alaminos City, Pangasinan. Its unique ecology deviates from the usually forest-dwelling species of the infandus species group (Sorger & Zettel, 2011). Interestingly, its seaside habitat is reminiscent of the malignus species group containing O. malignus and O. litoralis that inhabit and venture into intertidal zones (Wang et al., 2020). Current observations do not support O. ampipitbaybay sp. nov. as an intertidal species, owing to the elevated coastal limestone aggregates. However, further observations are necessary.

Table 1.Total length and indices of the workers of O. ampipitbaybay sp. nov. and closely related species. Data on O. infandus and O. schoedli are acquired from Sorger & Zettel (2011)

Species	TL	CI	MdI	SI	PtI
O. ampipitbaybay sp. nov.	12.84-13.45	68.30-72.35	59.87-62.06	125.12-134.15	90.51-91.79
O. infandus	12.00-16.57	68.00-73.00	52.00-58.00	138.00-152.00	82.99-99.15
O. schoedli	12.75-16.25	72.00-73.00	55.00-59.00	137.00-146.00	96.73-105.3

Figure 3.Odontomachus ampipitbaybay sp. nov. holotype worker, PASI hym0001 (a-d). Head (a) lateral and (b) dorsal view. Mesosoma (c) lateral and (d) dorsal view

Figure 4.Head and petiole structures (a-f ). O. ampipitbaybay sp. nov. holotype worker, PASI hym0001, (a) head, dorsal view, (b) petiole and (c) subpetiolar process, lateral view. O. infandus, PASI hym0009, (d) head, dorsal view, (e) petiole, and (f ) subpetiolar process, lateral view

The small forest fragment habitat of O. ampipitbaybay sp. nov. at the beach is nearly encroached by man-made fish ponds (Figure 1 inset). Interestingly, it was absent from the nearby Bolo beach, a tourist area. The encroachment of coastal forests and disturbances along beaches due to tourism and aquaculture activities could pose conservation challenges for this species. Notably, the type locality is situated close to the Hundred Islands National Park, a group of islets legislated as a national park. Surveying the Hundred Islands National Park may reveal additional colonies of this new species and provide insights into the terrestrial arthropods inhabiting the islets. However, our ability to conduct extensive fieldwork was constrained by a lack of funding. Given the species’ potentially limited range, determining its area of occupancy and extent of occurrence would be beneficial for informing potential conservation efforts.

Closely related species of O. ampipitbaybay sp. nov., the O. infandus and O. schoedli, are known to occur on secondary to primary forest floors,

with O. schoedli notably found at higher elevations of the Central Cordillera Range (Mt. Province & Benguet; (Sorger & Zettel, 2011)). O. infandus is more widespread with records from the islands of Luzon and Mindoro, with two recent reports from Mindanao Island ((General & Buenavente, 2017); (AntWeb, 2024)). O. infandus was reported to occur in Mt. Hamiguitan, Davao Oriental, Mindanao Island (General & Buenavente, 2017) and Tinago Falls, Buru-in [=Buru-un], Iligan City, Lanao del Norte, Mindanao Island ((AntWeb, 2024); Figure 1; (General & Alpert, 2012)). However, we were unable to verify the identity of both records. We noticed that the record from Iligan City seems to be misidentified and could represent a new species. A series of images of a worker (CASENT0650353) available on (AntWeb, 2024) shows the head with a large smooth area around the median furrow and lighter body color, contrary to O. infandus which has a head almost fully striated and darker in color. The existence of other distinct morphological characters will support heterospecificity.

Figure 5.Odontomachus species from Northern Luzon in their natural habitat (in situ). (a-c) O. ampipitbaybay sp. nov., types, in Alaminos City, Pangasinan. (d & e) O. schoedli, in La Trinidad, Benguet

Figure 6.Habitat of Odontomahcus ampipitbaybay sp. nov. (a & b) Undisturbed beach habitat near Bolo Beach Inset: Collection site on the coastal limestone aggregates with leaf litter and clay loam soil.

CONCLUSION

Odontomachus ampipitbaybay sp. nov. is unique based on its ecology and the distinct structures of its head, mesosoma, and petiole. The description of its reproductive members and genetic analysis will improve its taxonomic placement. This species is the only member of the infandus species group whose habitat is in the coastal area. This species raises the number of Odontomachus in the Philippines to 14 valid species. Extensive work on this poorly studied taxon, especially in Central Philippines, may reveal rich diversity for the Philippines. Modification on the Sorger and Zettel (2011) key, proposed herein, couplet 8 is modified to give way for the additional

couplet 11:

8 Head, mesosoma, and petiole unicolorously reddish brown. Petiole stout. 11

- Head distinctly lighter than mesosoma. Petiole less stout. 9

11 Petiole with an almost straight spine. Striation of pronotum mainly transverse. Subpetiolar process broad, with base extending more than half of petiole. Species from the mountains of northern Luzon…...............................

- Petiole with a slightly curved spine. Striation of pronotum mainly longitudinal. Ocular ridge smooth medially. Sub-petiolar process stout, with base extending less than half of petiole. Scape short. Species from the coast of north-western Pangasinan...............…………....

Odontomachus schoedli

Odontomachus ampipitbaybay sp. nov.

References

1. AntWeb. AntWeb version 8.103.2. California Academy of Science. 2024. Publisher Full Text
2. Bolton B.. An online catalog of the ants of the world. 2024. Publisher Full Text
3. General D.E.M., Alpert G.. A synoptic review of the ant genera (Hymenoptera, Formicidae) of the Philippines. ZooKeys. 2012; 200:1-111. DOI
4. General D.E.M., Buenavente P.A.C.. Checklist of the ants of Mt. Hamiguitan, Mindanao Island, Philippines (Hymenoptera: Formicidae. Halteres. 2017; 8:92-102. DOI
5. General D.E.M.. Odontomachus ferminae, a new Philippine species of the infandus species group (Hymenoptera: Formicidae. Halteres. 2018; 9:157-62. DOI
6. General D.E.M.. A preliminary checklist of the ants (Hymenoptera: Formicidae) of the Mt. Pantaron Range, Bukidnon Province, Mindanao Island, Philippines. Halteres. 2021; 12:4-14. DOI
7. Sorger D.M., Zettel H.. On the ants (Hymenoptera: Formicidae) of the Philippine Islands: V. The genus Odontomachus Latreille, 1804. Myrmecol News. 2011; 14:141-63.
8. Wang W.Y., Yamada A., Yamane S.. Maritime trap-jaw ants (Hymenoptera, Formicidae, Ponerinae) of the Indo-Australian region: Redescription of Odontomachus malignus Smith and description of a related new species from Singapore, including first descriptions of males. ZooKeys. 2020; 915:137-74. DOI
9. Zettel H., Sorger D.M.. Another new trap-jaw ant (Hymenoptera, Formicidae, Odontomachus Latreille, 1804) from the Philippines. Linzer Biologische Beiträge. 2023; 1:381-5.