Coelometopon, Janssens, 1972

Perkins, Philip D., 2005, A revision of the African hygropetric genus Coelometopon Janssens, and description of Oomtelecopon new genus (Coleoptera: Hydraenidae), Zootaxa 949 (1), pp. 1-103 : 1-103

Coelometopon and Oomtelecopon Relationships

Members of Oomtelecopon differ from Coelometopon in several morphological features, including: 1) the frons has a deep median fovea surrounded by an anteromedial and two lateral callosities (Fig. 53); 2) the maxillary palpi are very short and the ultomere is reduced in size (Figs. 1, 2, 54); 3) the prothoracic hypomeron is strongly setose anteriorly, and in addition to the "normal" hypomeral carina (e.g., Fig. 60) there is an accessory carina which forms an arc across the hypomeron and delimits a non­setose posterior area; 4) the adjacent area of the elytron is also modified such that the two areas form shallow depressions into which fit the "elbows" of the middle and hind legs (Fig. 55); 5) the 8th elytral interval is not raised to form a pseudoepipleuron; 6) the metasternum has a Y­ or Tshaped carina in a large lateral depressed area (Fig. 54); 7) the last tergite in females does not bear a fringe of strong spines; and 8) the legs are slender and relatively long, indicating that these are nimble insects compared to the very strongly built Coelometopon species. The markedly setose condition, the form of the frons, maxillary palpi, hypomeron and metasternum are putative derived characteristics, whereas the lack of an elytral pseudoepipleuron, and the non­spinose last tergite of the female are putative plesiomorphic conditions. The combination of these character states indicates that Oomtelecopon is the probable sister­group of Coelometopon .

Members of Coelometopon and Oomtelecopon have one of the most highly modified head shapes in the Hydraenidae . The markedly raised eyes have certainly resulted in the unusually elongate basal articles of the antenna; this elongation is necessary in order for the antennal club to retain its protected and functional position behind the eye. The proportions of the basal articles of the antenna must therefore be seen in this context. The proportions of the antennal club, however, seem not to be subject to this morphological relationship, and therefore more reliably used as an indicator of relationships. The club articles are rigidly joined to one another, and the penultimate article is relatively large, whereas the ultimate article is smaller and firmly joined to the penultimate. This configuration is most similar to some members of the Prosthetopinae , such as Nucleotops Perkins and Balfour­Browne ; also, if the last two articles of Coelometopon were completely fused then the club would be similar to that of Protosthetops Perkins and Prosthetops Waterhouse. The process of fusion of the last two antennomeres has occurred independently in several genera of Prosthetopinae ( Perkins & Balfour­Browne 1994) .

The highly modified head must be accommodated by the anterior part of the prothorax, especially the antennal pockets and anterior part of the hypomeron. The hypomeron is strongly produced anteriorly, forming a shield which protects the antenna (Fig. 60). When the head is retracted, such as in a defensive position, the antenna can be held entirely within the antennal pockets, and the long basal articles that lie in the deep antennal groove are protected by the hypomeral shield.

The deep antennal pockets of Coelometopon was one of the primary reasons that I placed the genus in the Madagastrini ( Perkins 1997) . This should be re­evaluated in the context of the greatly modified head. In Coelometopon , in contrast to members of the Prosthetopinae , the hypomeral carina is not fused with the notosternal suture. The nonfused condition is currently considered the ancestral state for the family ( Perkins 1997; Beutel et al. 2003). The presence of this condition in Coelometopon could be a result of the extreme head/hypomeron specialization. An even further step in hypomeral specialization is seen in the "accessory" hypomeral carina as described above for Oomtelecopon ; this is unique in the family.

Aquatic members of the Prosthetopinae have plastron­forming vestiture, as do members of Coelometopon . However, the form of the individual setae differ, being scale­like in Coelometopon (e.g., Figs. 64, 65) and hair­like in Prosthetopinae (Perkins & Balfour­ Browne 1994). The setose, non­plastron venter of Oomtelecopon may represent the ancestral condition for that group.

Coelometopon and Oomtelecopon are similar to Prosthetopinae in the possession of two plate­like coxites that articulate with sternite nine (Perkins, unpublished). These coxites are not present in members of Madagaster Perkins and Davidraena Jäch , the other genera currently placed in the Madagastrini .

In light of the above considerations, these two genera are placed in the Prosthetopinae , in a new tribe, the Coelometoponini (type genus Coelometopon Janssens ). An illustrated key to the genera of Prosthetopinae will be presented in a future contribution. Much work remains on the phylogeny of the family, especially the relationships of genera currently within the Hydraeninae which have the ancestral number of antennomeres (11), and which possibly form the most basal group in other subfamilies. Future work will almost certainly include molecular studies (Ribera, pers. comm.), in addition to continued morphological studies (Beutel, pers. comm.; Jäch, pers. comm.).