Nordus

Introduction

The genus Nordus Blackwelder, 1952, contains 38 species and was recently revised by Chatzimanolis (2004). Nordus is distributed from southern Mexico to northern Argentina, with the highest diversity of species in South America and especially in Ecuador, Peru and Bolivia. These beetles range in size from 7 to 13 mm, and most species are brightly colored. The genus was originally proposed by Nordmann in 1837 as the genus Brachydirus.

Characteristics

There are several different color patterns in Nordus; most species have (probably aposematic) golden-orange coloration on the head, pronotum and elytra. Other species have the pronotum and elytra golden-orange, and the head anteriorly black and posteriorly orange, or head, pronotum and elytra are black; while one species, N. speciosus, has the head, pronotum and elytra metallic green. Among all genera in Xanthopygina, Nordus exhibits probably the most dramatic structural modifications in secondary sexual characters (on abdominal sterna VII, VIII and IX) and in the colouration of different species. The color of the abdomen is always black (except in N. portokalis, where it is orange) with a bright yellow apex (part of segment VII and segment VIII).

Nordus can be distinguished from other xanthopygines by the combination of the following structures:

• large prominent eyes;
• head with large, dense and continuous punctures, with no micropunctures;
• anteclypeus greatly reduced;
• antennae short;
• antennal articles 5-10 rounded; antennal articles 7-10 not appearing “serrate” (anterior and posterior margins of article subequal in length);
• pronotum without uniform pattern of punctation (except in N. major), with large punctures;
• abdominal terga III-VII with variable setation, setation not uniformly distributed as in most xanthopygines;
• prominent modifications of the 7th and 8th abdominal sterna in males;
• middle and hind tarsomeres 2-4 dilated and ventrally lobed.

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From left to right, head of N. simplex showing the punctuation pattern; VIII abdominal stergum of N. terminalis and N. styloceros respectively, showing the secondary sexual structures on this segment. Images © 2003 Stylianos Chatzimanoli.

Discussion of Phylogenetic Relationships

The tree shown above is the strict consensus of four trees. The sister-group relationships of most species are supported by multiple synapomorphies and high Bremer support values. Nordus major is sister to all remaining Nordus species. The overall shape of the head, which exhibits great allometric variation in proportion to the size of the pronotum, resembles the overall shape of the head in species of the genus Xanthopygus. This species, of course, clearly is a member of Nordus because it has all the above mentioned synapomorphies, but probably it has retained the plesiomorphic general shape of the head.

The phylogeny shows that the probably aposematic golden-orange coloration evolved in the ancestor of Nordus, although it is not clear whether this happened in the immediate ancestor of Nordus, or is shared with other genera because the phylogenetic relationships among xanthopygines are unknown.

References

Chatzimanolis S. 2004. A revision of the neotropical genus Nordus (Coleoptera: Staphylinidae). Ent. Ab. 62(1):3-64.

Chatzimanolis S. 2005. Phylogeny of the neotropical rove beetle genus Nordus (Coleoptera: Staphylinidae) with a special reference to the evolution of coloration and secondary sexual characters. Sys. Ent. 30:267-280.