Monomorium dine Cover & Rabeling, 2025

Cover, Stefan P. & Rabeling, Christian, 2025, Monomorium dine sp. nov. (Hymenoptera, Formicidae): a new inquiline social parasite ant species from North America, ZooKeys 1243, pp. 159-172 : 159-172

Monomorium dine Cover & Rabeling sp. nov.

Diagnosis.

An apparently workerless, inquiline social parasite of a free-living, hitherto undescribed Monomorium species. Monomorium dine shows morphological traits of the inquiline syndrome (Fig. 1 ). Females are miniaturized (i. e., approximately the size of host workers) and have reduced sculpturing compared with the much larger putative host queens (compare Figs 1 , 4 ; see Table 1 ). Wings are present but are fragile and probably quickly deciduous. Mouthparts are not fully visible, but palp formula appears to be reduced to 1, 2. The number of mandibular teeth is reduced to 3. Males are unknown but are probably closely similar in size and habitus to the females.

The female of M. dine may be easily distinguished from all other New World Monomorium social parasites by the abundant, long, erect setae on all body surfaces including the antennal scapes and the legs (Fig. 1 ). In M. dine , the anterior margin of the clypeus has a small central emargination, whereas the emargination is broad in M. pergandei and absent in M. inquilinum and M. talbotae . In addition, M. dine can be readily distinguished from M. pergandei by the absence of a median impression on the 1 st gastric tergite.

Description.

Holotype female: HL 0.44, HW 0.43, SL 0.50, ML 0.68, CI 97, SI 117. In full-face view, head narrowly trapezoidal, tapering towards mandibular insertion; head broadest posterior to eyes near posterior corners; dorsal margin straight with corners evenly rounded. Anterior margin of clypeus convex with shallow median impression with tiny central emargination; clypeal carinae absent, median clypeal seta present. Mandibles reduced in size, when closed fitting tightly under the clypeus; apical tooth enlarged, cutting edge with 2 denticles. Antennae with 12 segments; scapes surpassing the dorsal margin of head; with abundant long erect setae. Mouthparts not fully visible, palp formula apparently 1, 2. Mesosoma with typical modifications related to wing bearing. Wings appear functional. In lateral view, propodeum forming an evenly rounded convexity, lacking distinct dorsal and posterior surfaces. Propodeal spiracle with small, inconspicuous opening. Opening of metapleural gland tiny, barely visible. In lateral view, petiole with short peduncle and well-rounded dorsal node; postpetiole has a broadly rounded node and a broad ventral tooth. In lateral view, first gastric tergite evenly convex, no trace of median impression. Body surface, antennal scapes, and legs with abundant, long, erect pilosity. Body surfaces smooth, shiny, lacking sculpture except for numerous punctures; lacking pubescence. Color of body uniformly medium brown; appendages slightly lighter, yellowish brown. Paratype females (n = 2): HL 0.44, HW 0.41–0.43, SL 0.47–0.50, ML 0.68–0.71, CI 93–97, SI 114–117.

Etymology.

This new inquiline social parasite species was discovered on Beautiful Mountain in the Navajo Nation, New Mexico, USA. This new species is named in honor of the Diné People. The species epithet is a noun in apposition.

Type Locality.

USA • Navajo Nation, New Mexico, San Juan County, Beautiful Mountain . GPS: 36.5011°N, 108.9672°W; elevation 2544 m. Ponderosa Pine forest; dcbm 090711_ant 13, Navajo Reservation, under rock. Collected by Gary D. Alpert, 11 July 2009.

Type material.

Holotype female ( MCZENT 00670596 ) . Two paratype females ( MCZENT 00670595 & 00670597 ) . Holotype and paratypes deposited in the Museum of Comparative Zoology at Harvard University (Cambridge, MA, USA).

Discussion and biology.

Monomorium dine is only known from the single collection made at the type locality. Three partially alate females were collected from under a rock, where they were associated with workers of their free-living Monomorium host. Unfortunately, the host workers were apparently not collected along with the social parasite. Therefore, the precise identity of the host of M. dine remains unknown. However, we have several collections of an undescribed, free-living Monomorium species from similar altitudes in neighboring mountain ranges (Fig. 4 ). In the southwestern United States, Monomorium species seem to be distributed altitudinally and overlap between them is uncommon (S. P. Cover unpublished collection data). In this case, the similarity in mountain ranges, altitudes, and habitat types makes us strongly suspect that this undescribed species is also the host of M. dine . See Fig. 4 (MCZENT 673296) for an example of this probable host.

Despite the lack of direct evidence concerning the host, the identity of M. dine as an inquiline social parasite is secure. It derives from its very close similarity to the other three Nearctic Monomorium social parasites (Figs 1 – 4 ; see also DuBois 1986). All share characteristics associated with the inquiline syndrome ( Kutter 1968; Wilson 1971, 1984; Rabeling et al. 2019; Cover and Rabeling 2024). These include small body size, shiny integument, reduced mandibles, presumably reduced palp formula, the presence of a postpetiolar ventral process (i. e., “ Parasitendorn ”), the loss of the worker caste, and its exceeding rarity (Table 1 ). In addition, all four of these species, M. dine , M. inquilinum , M. pergandei , and M. talbotae have been collected only in the nests of free-living Monomorium species.

Figures

Figure 1. Morphological comparison of the Monomorium dine sp. nov. holotype (A, C, E) and paratype (B, D, F) females in lateral (A, B), dorsal (C, D), and full-face (E, F) view. The holotype (MCZENT 670596) and the paratype (MCZENT 670597) of M. dine were collected in the nest of a free-living Monomorium species in the Navajo Nation, New Mexico at Beautiful Mountain. Scale bars: 1 mm (A – D); 0.5 mm (E, F).

Figure 4. Morphological comparison of the Monomorium talbotae DuBois paratype male (A, C, E) and a Monomorium sp. female (B, D, F) in lateral (A, B), dorsal (C, D), and full-face (E, F) view. The M. talbotae paratype male (MZCENT 522080) was collected at the Edwin S. George Reserve in Michigan. The Monomorium sp. female (MCZENT 673296) was collected in the Navajo Nation, Arizona at Black Rock Dike in the Carrizo Mountains and likely represents the host of M. dine. Scale bars: 1 mm (A – D); 0.5 mm (E, F).