Acropyga paleartica Menozzi, 1936

Scupola, Antonio, Durante, Antonio, Giannuzzi, Francesco & Pellizzari, Giuseppina, 2022, The coccid-tending ant genus Acropyga Roger and its obligate associated myrmecophilous scale insect genus Eumyrmococcus Silvestri new to Italy (Hymenoptera: Formicidae; Hemiptera: Xenococcidae), Fragmenta entomologica 54 (1), pp. 89-94 : 90-92

Acropyga paleartica Menozzi, 1936

The subterranean ant genus Acropyga Roger, 1862 (Formicinae: Plagiolepidini Forel, 1886) occurs in warm temperate and tropical areas, and contains about 40 species ( LaPolla 2004). An ecological and ethological characteristic of Acropyga is the obligate mutualistic relationship with root-feeding scale insects, mostly in the family Xenococcidae , genus Eumyrmococcus but also including a few Rhizoecidae species such as Ishigakicoccus shimadai Tanaka, 2016 and Williamsrhizoecus udzungwensis ( Johnson et al. 2001; LaPolla et al. 2002; Schneider & LaPolla 2011, 2020) and Ortheziidae , namely Acropygorthezia williamsi LaPolla & Miller, 2008 ( LaPolla et al. 2008).

According to Schneider & LaPolla (2011), all Xenococcidae are obligate trophobionts with Acropyga ants. The ants feed mainly on honeydew excreted by the xenococcids, and the xenococcids live in ant nests, attended by the ants and protected from their natural enemies. It is known that, during the nuptial flight, each virgin Acropyga gyne carries a female of the associated scale insect in her mandibles ( Fig. 1 ), so ensuring a source of food for the offspring in the new nest.

This behaviour, known as trophophoresy ( LaPolla et al. 2002), is attested by fossils in Dominican amber from the Miocene (dated to at least 20 Ma) of the fossil

ant † Acropyga glaesaria LaPolla, 2005 , associated with three Xenococcid species related to the genus Eumyrmococcus , described in the extinct genus †Electromyrmoccus Williams, 2001 ( Johnson et al., 2001; LaPolla 2005; Williams, 2001). This obligate ant/scale insect symbiosis is a classic case of co-evolution between two organisms that determined their survival and success ( LaPolla et al. 2002; LaPolla 2004).

Acropyga paleartica is known thus far only from SE Europe (continental and insular Greece) ( Fig. 2 ). Hitherto, it had been recorded only from Karpathos island (type locality) ( Menozzi 1936), Crete and the nearest Gavdos island ( Seifert & Heller 1999; Salata et al. 2020), Perachora (in the Corinth channel; Bushinger et al. 1987 as Plagiolepis sp ; Williams 1993), and Meteora (Thessaly) ( LaPolla 2006).

The presence of A. paleartica and its associated symbiont Eumyrmococcus corinthiacus in Salento (South Italy) is of biogeographic significance because it is the first record from outside Greece, and the first for Italy of both species. Salata et al. (2020) considered A. paleartica as an Aegean corotype but, according to recent records from mainland Greece and the present one from the Apulia region in Italy, its pertinence to Eastern Mediterranean corotype ( Vigna Taglianti et al. 1999) appears more consistent with the available data records.

The status of Acropyga paleartica as a species native to the Mediterranean region was recently questioned ( Salata et al. 2020). The genus is pan-tropical so we cannot exclude that A. paleartica comes from the tropics (Africa?); the same argument applies to the associated E. corinthiacus mealybugs. However, additional tropical or subtropical records of either species corroborating this hypothesis are so far unknown. The genus Acropyga is known to have existed since the Miocene, when (mostly in the early periods) climatic conditions were hot and humid and the vegetation was typically tropical, even in Mediterranean areas. Later (during the Messinian period), dramatic climatic changes due to evaporation of the Mediterranean Sea led to a massive extinction of the tropical fauna and its replacement by species adapted to colder and dryer climates. It is then possible that A. paleartica survived these events as a biogeographic relict.

Acropyga , and the nearest genera Lepisiota and Plagiolepis , are the only Italian Formicinae with 11 antennal segments in the females (gynes and workers) and 12 in the males; in the other Italian Formicinae, the antennae have one extra segment: 12 in females, 13 in males. Acropyga paleartica ( Figs 3-4 ) can be distinguished from species in the other two genera by having the palp formula 4:3 (vs 6: 4 in the other two genera); pilosity profuse, with short suberect setae (very sparse and adpressed in the other two genera); and 6-8 uneven mandibular teeth (5-6 in the other two genera). In addition, A. paleartica workers are characterized by a quadrangular head with compound eyes strongly reduced, almost punctiform (diameter always less the maximum width of the scape); in contrast, workers of Lepisiota and Plagiolepis each have an oval head with compound eyes well developed (diameter> scape width).

The males of A. paleartica ( Figs 5-6 ) were described by LaPolla (2006) based on specimens from central Greece; Acropyga males are distinguished from those of Lepisiota and Plagiolepis by possession of the palp formula 4:3 (vs 6:4), a massive mandible with 6-8 mandibular teeth (vs 5-6), and the subgenital plate with a denticulate posterior margin ( Fig. 7 ). In addition, Acropyga males having TL>2.14 ( Tab. 1 ) distinguishes them from Plagiolepis (1.5-2 mm), and the scape protrudes from the posterior head margin by only a third of its length, whereas in Lepisota the scape protrudes by more than half of its length.

Based on the morphological characters of workers and males, A. paleartica is close to the South African A. arnoldi Santschi, 1926 and is tentatively placed in the A. arnoldi species-group. Acropyga arnoldi is a symbiont of Eumyrmococcus scorpioides (De Lotto, 1977) ( LaPolla 2006) .

The biology and ecology of A. paleartica is scarcely known. Its nests were found in pine forests ( LaPolla 2004) but also in moist soil at the bases of trees growing near riverbanks. Specimens were found in the soil (at a depth of 10 cm) near a tree growing alongside the stream Milo in Karpathos ( Menozzi 1936), while recently Salata et al. (2020) found a nest in Crete in moist soil under a rock on a dry riverbank, surrounded by phrygana vegetation. In Salento the A. paleartica alates were collected at the boundary between olive groves and a Quercus ilex wood, during their nuptial flight, in the early afternoon (about 3.00 p.m.) on October 4 th. It is interesting to note that alate A. paleartica carrying E. corynthiacus were collected near Corinth on almost the same date: October 7 th ( Williams 1993). This suggests that the swarming period occurs in early autumn. No nest has been found so far in Apulia.

Regarding the association of A. paleartica with other Hemipteran species, Menozzi (1936) reported that A. paleartica specimens were found with aphids of the root-feeding genus Forda van Heyden, 1837 ( Hemiptera : Aphididae , Fordini). Even if this observation had no further feedback, it is well known that Forda species live in Pistacia leaf galls (the primary host), then migrate to the roots of Poaceae (the secondary host) where they are attended by ants and often live in ants’ nests; so it is possible that this occasional observation by Menozzi is correct. However, the first record of Eumyrmococcus corinthiacus was with Acropyga paleartica (Williams 1983) , and the data presented herein fully confirm this association.

Figures

Fig. 1 – An Acropyga paleartica gyne carring one Eumyrmococcus corynthiacus female.

Fig. 2 – Acropyga paleartica, known distribution.

Fig. 3 – Acropyga paleartica, gyne: body, lateral view (Scale bar: 1 mm).

Fig. 4 – Acropyga paleartica, gyne: head, frontal view (Scale bar: 0,5 mm).

Fig. 5 – Acropyga paleartica, male: body, lateral view (Scale bar: 1 mm).

Fig. 6 – Acropyga paleartica, male: head, frontal view (Scale bar: 0.5 mm).

Fig. 7 – Subgenital plate of adult male Acropyga paleartica.

Tables

Tab. 1 – Comparison of measurement ranges for alates of Acropyga palearctica from Salento, with those from Greece recorded by LaPolla (2004, 2006). The data from both localities are similar, particularly in the males.

	Gyne from Salento, (n.1)	Gyne ( LaPolla 2004)	Males from Salento, (n.2)	Males ( LaPolla 2006)
CL	0.62	0.591	0.48 – 0.51	0.47 – 0.548
CW	0.60	0.577	0-44 – 0.46	0.423 – 0.501
SL	0.57	0.515	0-46 – 0.49	0.439 – 0.470
ML	0.94	0.952	0.70 – 0.76	0-782 – 0.923
EL	0.19	--	0.17 – 0.19	--
EW	0.12	--	0.12 – 0.16	--
GL	1.45	1.23	1.12 – 1.15	0.892 – 0.939
TL	3.03	2.77	2.30 – 2.42	2.14
CI	96	97.63	90 – 91	90 – 91
SI	91	89.25	104 – 106	94 – 104