Potential phylogenetic significance of the number of functional abdominal spiracles in beetle pupae, with focus on Staphylinoidea (Coleoptera)

Abstract

The distribution of functional abdominal spiracles in pupae of Coleoptera is reviewed based on published descriptions and original observations. Aquatic Coleoptera typically have strong modifications, generally including dramatic reductions in the number of functional spiracles and often their modification into either spiracular gills or snorkels, as a response to their environment. But pupae of the great majority of Coleoptera, which are terrestrial, show broad stability across higher taxa. Most terrestrial beetles have at least the first five pairs of abdominal spiracles functional, up to and including a full set of eight pairs. However, the number is unexpectedly low in Scarabaeoidea and within Staphyliniformia, where Histeridae and all Staphylinoidea have a confirmed maximum of four pairs of spiracles. The relation between pupal size and number of functional spiracles in terrestrial pupae is explored, and it is suggested that those groups with an unexpectedly small number of functional spiracles may have passed through a “small-size bottleneck” in their ancestry. However, this hypothesis does not explain why several families of very small beetles in other groups of Coleoptera do not show a similar reduction, and little evidence was found to support a strong relation between pupal size and number of functional spiracles at lower taxonomic levels (below family). Whether pupae are exarate or obtect apparently also has little correlation with the number of functional spiracles. However, the consistency and stability of spiracular reductions in the above groups suggests that deep historical factors are involved and thus the reductions may be of phylogenetic significance. It is urged that establishing the number of functional spiracles in beetle pupae become as standard a feature of pupal descriptions as chaetotaxy and whether they are exarate or obtect.