The phylogenetic significance of ommatidium structure in the compound eyes of polyphagan beetles

Abstract

The structure of ommatidia in adults of more than 200 beetle species from 91 polyphagan families was surveyed. Three basic types of lens system (eucone, exocone, and acone) and two types of retinal unit (fused rhabdom, open rhabdom) are represented. The eucone (crystalline cone-containing) ommatidium is ancestral and prevails in primitive Eucinetoidea, Hydrophiloidea, and Scarabaeoidea; ommatidia of the primitive beetles Cupes and Omma as well as the Adephaga are of this type. The polyphagan founders most likely had ommatidia with small crystalline cones and narrow clear zones beneath the corneal facets. Exocone and acone eyes are derived structures, and their distribution suggests that both have evolved several times. Exocone ommatidia arose early in polyphagan evolution, possibly first in dascilloid-like founders of elateriform and bostrychiform beetles, where the exocone is commonly found. An exocone eye also evolved separately in the ancestors of several primitive scarabaeoid families; possible steps in this eucone to exocone transition may be seen in the Trogidae. The clear zones of eucone and exocone eyes are not homologous. The acone ommatidium is specialized and arose through a progressive loss of either crystalline cone or exocone. In the advanced staphylinoid beetles it is a relic of crystalline cone loss in their small ancestors. In the cucujiforms it arose likely from the loss of the exocone in their bostrychiform ancestors, associated here with a shift to an open rhabdom. Although the distribution of ommatidial types coincides with major lineages in the Polyphaga, a few anomalies remain. The Eucnemidae, Buprestidae, and Dryopidae are all eucone yet are placed in the elateriform series, in which 25 of 30 families are exocone. Scarab beetles have an extraordinary variety of lens types that presumably reflects the exceptional adaptability of the eye in this superfamily.