Pachyrhynchus, Germar, 1824

Rukmane-Bārbale, Anita & Cabras, Analyn Anzano, 2021, Updated Distribution Records of Pachyrhynchus Germar, 1824 (Coleoptera: Curculionidae: Entiminae) from the Philippines with Biogeographic Affinities, Baltic Journal of Coleopterology 21 (2), pp. 199-211 : 201-204

Distribution of Pachyrhynchus in the Philippines

In the monograph of Schultze (1923), he has listed 85 species and 13 subspecies. In the succeeding years until his last publication in 1934, he described numerous more species of Pachyrhynchus such as P. davaoensis and P. reicherti among others. Since Schultze’s last publication, the study on this genus remained dormant for nearly a century until recent discoveries of Yoshitake, Bollino, Sandel, Rukmane, Barševskis, Cabras, Medina described a significant number of new species and subspecies. An increase in the number of species was obtained for less explored islands such as Samar, Leyte, and Mindanao. A significant increase in the number of species was recorded for Mindanao which has an increase of 14 to 50 species as more species were described from Mt. Apo Natural Park and Bukidnon highlands and new distributional records emerged. Schultze considers Mt. Apo Natural Park and Bukidnon as the center of diversity of this genus in Mindanao. A fair number of species was added to the data of Luzon whose number of species rose from 55 to 79. Still, the island with the most number of species in the country is Luzon followed by Mindanao, Samar, and Leyte ( Fig. 1 ).

From 85 species, the number of Pachyrhynchus species now in the Philippines is 160 and more species will probably be added in the succeeding years although more studies should be conducted in species delineation as there are intraspecific variations among the population. Of the 160 species, 79 or 46% are from Luzon, 50 or 29% are from Mindanao while the rest are from other major islands such as Samar and Visayas with 19 species (11%), Mindoro with 9 species (5%), and 8 species (5%) from Panay and Lubang. A significant rise in the number of species was recorded from Mindanao (14 to 50) with the recent discovery of several species from Mt. Apo Natural Park and Bukidnon highlands. The biogeographic distribution of the genus is clearly oceanic in origin which explains why islands like Palawan and Sulu are devoid of representative species. There are no representative species from Palawan whose flora and fauna are more similar to mainland China than the Philippines and Sulu PAIC which also have unique species from mainland Mindanao. The absence of representative species in Palawan PAIC and Sulu PAIC is not astonishing since Pachyrhynchus has no population in Borneo or mainland Asia and seems to have a purely oceanic distribution. The Pleistocene Aggregate Island Complex (PAIC) is also evident in the distribution of many species in this taxon. The restricted distribution of this group can be attributed to its fused elytra making them flightless, their specific habitat preference in forested mountain ecosystems, or mountain ridges giving them a high level of endemicity. All the Philippine species of Pachyrhynchus are endemic with some species showing a muchrestricted distribution to a single island or locality.

In the monograph of Schultze, out of the 85 species, 80 or 94% are exclusive to a single island whereas only five namely P. erichsoni , P. moniliferus , P. signatus , P. speciosus , and P. venustus are widely distributed. An updated distributional record shows that 13 species have a wider distribution and are not restricted to a single island. Of the 13 species, three have a Philippine-wide distribution: P. moniliferus from Luzon PAIC (Islands of Cagayan Valley), Mindoro and Samar; P. multipunctatus from Luzon and its subspecies from Samar; P. erichsoni from various islands of the Philippines. The rest are restricted within the PAIC regions such as Greater Mindanao ( P. speciosus , P. regius , P. venustus , P. signatus ) and Greater Luzon ( P. orbifer , P. phaleratus , P. decussatus , P. apicatus , P. moellendorfi , P. rukmaneae ). The rest of the 147 species has a restricted distribution to a single island. The species with Philippine wide distribution is astonishing since long-distance dispersal of this group seems impossible with their flightless ability and restriction to mountain ecosystems. These species should be further studied with molecular data in order to know if the species from the various islands are conspecific. Species with Philippine wide distribution may have possibly originated at a much older time scale in the Miocene-Pliocene epoch and have dispersed through vicariance and diversified to islands as far as the Ryukyu and Mollucas. A further phylogenetic analysis would confirm this. It is also possible that these species may have crossed short-distance oceanic barriers during the Miocene-Pliocene epoch and have drifted to nearby islands together with some plants as Schultze (1923) mentioned that these beetles can survive floating in water for a longer period of time. Su et al. (2014) also mentioned the drifting of host plants as one of the possible mechanisms of cross-island dispersal to facilitate passive transport of eggs, larvae, and adults in the case of Philippine planthoppers. Furthermore, some species may have been transferred from one place to another through the cultivation of plants as observed in the genus Metapocyrtus .

Based on the selected division in administrative regions, some of the species seems to appear in more than one region ( Table 1). While regions like Zamboanga Peninsula and Bangsamoro remain rather isolated with individual, unique species, in Caraga, Soccskargen, Davao, and Northern Mindanao species flow are possible and part of the species overlap. For example, P. anitchtchenkoi is present at all four regions of Mindanao with exception of Zamboanga and Bangsamoro. Overall Zamboanga Peninsula is r epr esen ted by th r ee species, Nor th er n Mindanao by 30 species, Davao by 18 species, Soccskargen by 23 species, Caraga by 22 species and Bangsamoro by six species making Northern Mindanao species richest region. In Luzon situation is similar as part of the species are present at more than one region. Conditionally those regions in most cases are connected by land boarders allowing species to spread and exchange. Species richest region of the Greater is Cagayan Valley with 47 species, followed by Cordillera with 39 species, Central Luzon with 25 species, Calabarzon with 20 species, Ilocos and Bicol with 8 species each and Marinduque with 6 species .

Figures

Fig. 1. Comparison of species richness by largest Philippine Islands