Rhopalomastix, Forel, 1900

Yong, Gordon, Jaitrong, Weeyawat & Peeters, Christian, 2021, Nest structure of the bark-digging ant Rhopalomastix (Formicidae, Myrmicinae) and notes on its mutualistic associations with diaspidids in a Thai mango plantation, Asian Myrmecology (e 013005) 13, pp. 1-10 : 6-7

Rhopalomastix nest structure

Ant tunnels were found in the cork cambium layer of the inner bark in both cultivars. Two different types of ant tunnels were observed. The first type were tunnels chewed by the ants in the cork cambium layer where no fissures in the bark were observed. The second type were tunnels found along fissures of the bark. These tunnels were formed by the covering of these fissures with frass to form a roof ( Fig. 2 ). This frass roof gives rise to the distinctive lines of frass observed on the outermost surface of bark for both cultivars. The lines of frass mostly ran parallel to the branch ( Fig. 1 ) or trunk ( Fig. 3 ).

In the Khieo Sawoei (KSW) branch, narrow lines of frass covered fine fissures to form narrow and shallow tunnels. No ant broods were found in these narrower and shallower tunnels. Ant broods were instead found in bigger and deeper chewed tunnels in sections of bark adjacent to these frass lines ( Fig. 2 ). In the Falan branch, deep fissures covered by frass were observed running parallel to the branch. Some ant brood were found in these fissures while most were found in chewed tunnels adjacent to these frass lines ( Fig. 2 ). In both branches, tunnels were observed to be chewed at different depths in the cork cambium but not extending into the secondary phloem layer. Narrow tunnels linked sections of the nest at different depths ( Fig. 2 ).

The extent of ant tunnels started at the base of the KSW tree just above the soil. Ant tunnels were observed on the main trunk of the Falan tree 4 m above the ground. Tunnels were not found in the bark of the roots of both cultivars. Moss was observed growing over parts of both branches. Tunnels were found in the cork cambium layer under the moss mat. Some frass was found mixed with the mat of moss for both cultivars. An entrance hole connecting the external environment to the tunnels in the cork cambium layer was observed in the moss area of the Falan branch. The entrance was covered by a small aggregate of frass that was attached by some silk on one end to the bark surface. The aggregate could be easily moved to expose the entrance but stayed firmly attached to the bark surface without dropping off.

Association with diaspidids

No diaspidids were found on the leaves and external surface of the branches of either tree. Diaspidids were found living inside the tunnels of both branches. Diaspidids start life as mobile crawlers that are able to disperse by walking to new areas within the nest. They then settle down and stop moving, in the process losing their legs and antennae as they develop into the second instar. In free-living diaspidids outside the ant nest, these second instars are often protected by shields which they produce ( Foldi 1990). However, diaspidids living in ant nests develop into third instar adults with only very few adults developing shields in the process (Yong et. al 2019). Some genera of diaspidids do not develop shields even in free-living species ( Fig. 4 ); these species show pupillarial development, where the adult female is entirely enclosed within enlarged and sceloritised exuvia cast by the second instar ( Takagi 2002).

Diaspidids found in the tunnels of both Falan and Khieo Sawoei (KSW) colonies were determined to be of the genus Ligaspis (Aspodiotinae, tribe Parlatoriini ) which exhibits pupillarial development. Takagi (2002) recorded two species of Ligaspis from Luzon, the Philippines. Both species were found living on different nut trees ( Semecarpus cuneiformis ). However, the association with Rhopalomastix was not recorded in that study. In our study, we found many second instar females and adult females. Sixteen slide-mounted adult females revealed 10 crawlers within the pupillarial exuvia ( Fig. 5 ). In the Falan branch, three crawlers were observed to walk into and subsequently along a tunnel formed from frass covering a deep crack in the outer bark. The crawlers entered the deep crack from connecting chewed tunnels in adjacent sections of bark ( Fig. 2 ).

Diaspidid stylets were observed to be over 2 mm long and penetrated the cork cambium into the secondary phloem layer ( Fig. 2 and Fig. 6 ). Diaspidids were present throughout all the tunnels but were more commonly found at the base of deeper tunnels just above the secondary phloem layer. In the KSW branch, aggregations of diaspidids were not found in the shallow and narrow tunnels formed from fissures in the bark. Diaspidids were only found in in the deeper chewed tunnels in adjacent regions. In the Falan branch, only a small number of diaspidids were found in tunnels formed from deep fissures, while a larger number were found in the chewed tunnels in regions adjacent to these fissures.

Ant behaviour

In the Khieo Sawoei (KSW) colony, we observed “tidying” behaviour when the tunnels were exposed in a fragment of bark. Several second instar diaspidids in the exposed regions of the nest were completely pulled out from their fixed positions by the ants, exposing their long stylets in the process. Ants struggled in the process of removal, taking roughly one minute to remove each diaspidid from their fixed spot. The ants then ‘packaged’ the diaspidids together using orally-spewed silk strands and the whole package was thrown over the edge of the fragment. Both forelegs of Rhopalomastix were observed to be used during silk spinning. The legs moved in a circular motion under their mandibles, drawing out silk strands from silk glands in the head ( Billen & Peeters 2020).

Figures

Fig. 2. – Illustration of the nest architecture of Rhopalomastix in mango bark. Thickness of cork cambium layer is uneven. Two types of tunnels are built, one chewed in cork cambium, the other being bark fissures covered by frass roofs. Tunnels are interconnected, with most diaspidids found in deepest tunnels closest to secondary phloem layer. Entrance hole covered by frass.

Fig. 1. – Branch from the Khieo Sawoei mango tree. Vertical frass lines observed along the branch. Image by Gordon Yong.

Fig. 3. – Frass covering vertical fissures in the bark of the main trunk of Falan mango tree. Image by Christian Peeters.

Fig. 4. – (A & B) Diaspidids start their life as a mobile crawler that are able to disperse by walking to new areas within the nest. They then settle down and stop moving, in the process losing their legs and antennae as they develop into the second instar. (C) In free-living diaspidids outside the ant nest, these second instar are often protected by shields. (D) However, in the ant nest, the diaspidids develop into naked third instar adults with only very few adults developing shields in the process. (E) Pupillarial diaspidids like Ligaspis do not develop shields in the free-living form or in ant nests, growing within the pupillarium of the second instar adult instead.

Fig. 5. – Mounted Ligaspis sp. Unlike most Diaspididae, ‘pupillarial’ adult females develop and lay eggs within the enlarged and hardened exuvia of the second instar (L2).A crawler (L1) had just emerged from the adult female (L2). Unlike the adult female (L2) the crawler has legs and is able to walk, allowing it to disperse. The tentorium of both crawler and adult are visible. The long, coiled stylets of the adult is visible. Image by Christian Peeters.

Fig. 6. – Aggregation of freshly moulted second instar Ligaspis sp inside a tunnel of the Rhopalomastix rothneyi species group from the Khieo Sawoei branch. Circle shows a single long stylet of a diaspidid that had pierced into the secondary phloem layer. Image by Christian Peeters.