Prenolepis

The genus Prenolepis is a relatively small clade of formicine ants that includes thirteen extant species and one fossil species. While most species are found in southern China and southeastern Asia (11 of the 13 extant species), there is one very widespread species found in North America and a species in southeastern Europe. One fossil species is known from mid-Eocene (ca. 44 Ma) Baltic amber (LaPolla & Dlussky 2010). Within the Formicinae, Prenolepis belongs to a speciose clade known as the Prenolepis genus-group, which includes six other genera: Euprenolepis, Nylanderia, Paraparatrechina, Paratrechina, Pseudolasius, and Zatania (LaPolla et al. 2010a; LaPolla et al. 2012).

Biology
Williams and LaPolla (2016) - Most of what is known about the biology and natural history of Prenolepis comes from what has been observed in the widespread North American species, Prenolepis imparis (sometimes called the false honey ant). The biology of P. imparis is recounted here, but it is not known how general its biology is overall with regards to other species in the genus, and it is important to bear in mind P. imparis is a temperate species whereas most Prenolepis are found in tropical habitats. In P. imparis, colonies are polygynous, but contain relatively small numbers of workers (typically a few thousand workers in a colony) (Wheeler & Wheeler 1986; Tschinkel 1987). The nests of P. imparis are exceedingly deep with no chambers found shallower than 60 centimeters below the ground surface and going down as far as 3.6 m (Tschinkel 1987).

False honey ants are peculiar in that the workers are cold-tolerant and forage during the cooler months when most other ant species are inactive. The colony enters an estivation period and becomes inactive above ground for the warmer months, during which time eggs are laid and brood are reared. Reproductives overwinter and emerge on the first warm day of spring for their nuptial flight. One characteristic of P. imparis colonies is the presence of workers with greatly extended gasters. Corpulents were once believed to be true repletes like those of genus Myrmecocystus (Wheeler 1930a; Talbot 1943) until Tschinkel (1987) determined that their enlarged state is actually caused by hypertrophied fat bodies and not the result of crop distention from retained liquid food. Prenolepis imparis is a generalist omnivore (Wheeler 1930a).

The biology of the European false honey ant (Prenolepis nitens) is similar to that of P. imparis (Lőrinczi 2015), but it remains unclear if other Prenolepis species, all of which are found in the tropics, display similar natural histories. It is worth noting that P. imparis and P. nitens are sister taxa and they may in fact be sister to the rest of the Prenolepis (LaPolla et al. 2012; this study). Across all Prenolepis species most specimens are collected using leaf litter extraction methods such as Berlese and Winkler extractors (LaPolla et al. 2010a). Even though P. imparis and P. nitens are one of the most commonly encountered ants in the temperate North, based on their relative rarity in collections, tropical Prenolepis species appear to be far less common. It is also possible that their biology makes them less likely to be collected using standard collecting methods. For instance, some Euprenolepis and Zatania species are known to be nocturnal. Unfortunately, for most species reproductives have not been collected.

Nomenclature

 *  PRENOLEPIS [Formicinae: Plagiolepidini]
 * Prenolepis Mayr, 1861: 52. Type-species: Tapinoma nitens, by subsequent designation of Bingham, 1903: 325.
 * [Type-species not Formica imparis, unjustified subsequent designation by Emery, 1906b: 134, repeated in Wheeler, W.M. 1911f: 171 and Wheeler, W.M. 1922a: 940.]

Williams and LaPolla (2016) - For only two species are all castes known (Prenolepis imparis and Prenolepis nitens); therefore, we provide only a worker-based diagnosis for the genus. The character states for characters 12 (mesonotum with a strong mesonotal depression that gives the appearance of a mesonotal constriction) and 15 (presence of rugae that cross from the mesonotum to the mesopleuron) are assumed to be synapomorphies based on the phylogenetic analysis mentioned above.

1. Monomorphic, medium to large in size (2.4–4.9 mm in total length); ranging from pale yellow to dark brown in color.

2. The head, mesosoma, and gaster are typically covered in erect macrosetae that are thin and wispy. Pairs of erect macrosetae run medially from the clypeus to the posterior margin of the head.

3. Antennae 12-segmented; scapes vary in length among species (SI range 109–214), but they always surpass the posterior margin and most species have a scape index above 130; dense setation on scapes.

4. Eyes medium to large relative to head width (REL2 range 21–49) and placed far posterior to the midline of the head (EPI > 125); eyes convex, sometimes surpassing the lateral margin of the head in full-face view.

5. Mandibles with 5–7 teeth on the masticatory margin; apical tooth the longest, 3rd and 5th tooth from apical reduced and 6th tooth also reduced when 7 teeth are present; in many species the ectal surface of the mandibles has longitudinal striations.

6. Palp formula 6:4.

7. In profile view, the mesonotum in all Prenolepis species is curved and depressed immediately posterior to the pronotum, which gives the appearance of a mesonotal constriction; longitudinal rugae that extend from the mesonotum to the mesopleuron are present at the constriction; mesosoma can be robust as seen in P. nitens, but sometimes much more gracile as seen in Prenolepis subopaca (BLI range 122–206).

8. In profile view, the propodeum is at about the same height or slightly higher than the mesonotum; propodeum is either domed with a rounded dorsal face as seen in Prenolepis naoroji, or obtusely angled with a flat dorsal face as seen in Prenolepis angularis.

9. Mesonotal and metanotal sutures are absent or in complete and shallow.

10. In profile view, petiole is typically forward-inclined and wedge-shaped, but in some species the petiole is elongate with a more rounded dorsal apex of the scale.

11. Legs are elongate (profemur length 0.7–1.5mm).

The constriction of the mesonotum has long been used as a defining characteristic of Prenolepis. However, the mesonotal constriction observed in Prenolepis species is distinct from what has been described as a mesonotal constriction in species from other genera, including the following: Nylanderia emmae, Nylanderia opisopthalmia, Paratrechina umbra, Paraparatrechina pallida, Zatania gibberosa, and Zatania gloriosa. In profile view, all of these species have an elongated mesosoma with a flattened mesonotum. By contrast, the constriction seen in all Prenolepis species is defined by a distinct mesonotal depression immediately posterior to the pronotum and longitudinal rugae that extend from the mesonotum to the mesopleuron. The mesosoma is also not elongate in most Prenolepis species (except Prenolepis jerdoni, and P. subopaca). Understanding the difference between a true mesonotal constriction and an elongated mesosoma (which on first examination can appear to be the same thing) is essential if a species is to be placed in its proper genus. Confusion regarding this distinction has led to species being placed in Prenolepis that did not belong there. Another important morphological feature for determining proper genus placement within the Prenolepis genus-group is differences in mesosomal sutures. All species of Prenolepis, Paraparatrechina, and Zatania have shallow and incomplete mesosomal sutures. The other four genera, Nylanderia (except Nylanderia opisopthalmia, Paratrechina, Pseudolasius, and Euprenolepis, have mesosomal sutures that are deep and complete.