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

ARTICLE Potential phylogenetic significance of the number of functional abdominal spiracles in beetle pupae, with focus on Staphylinoidea (Coleoptera) Alfred Francis Newton¹ ¹ Field Museum of Natural History (FMNH), Integrative Research Center. Chicago, Illinois, United States. ORCID: http://orcid.org/0000-0001-9885-6306. E‑mail: 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. Key-Words. Body size; Functional spiracle; Immature; Phylogeny; Pupa. INTRODUCTION Beetles, an enormous group of about 400,000 described species placed in more than 190 modern families, are holometabolous insects, and thus pass through four very distinct life stages: egg, larva (usually several instars), pupa and adult. Adults are the main life stage of systematic study, and are almost always the basis for the establishment of scientific names, as well as the source of a majority of systematic characters used for classification and morphology-based phylogenetic analyses, since they include a rich character set associated with movement, feeding and reproduction. Larvae, the other active and frequently encountered life stage, also have a large suite of characters that are, to a large extent, functionally independent of those of adults. This has led to the wide use of larvae by most modern systematists, especially for phylogenetic studies. The remaining two stages, eggs and pupae, are generally inactive and short-lived compared to the other two, are less often encountered and described, and so far have seldom been included in phylogenetic studies at higher taxonomic levels. For example, the most recent and comprehensive morphological phylogenetic analysis of Coleoptera as a whole (Lawrence et al., 2011) used 516 phylogenetically informative characters in the analysis, but all of these were either of adults (344) or larvae (172), with no contribution from characters of eggs or pupae. Nevertheless, eggs and pupae do possess systematically useful characters, and have been used occasionally for systematic and phylogenetic studies within smaller groups of beetles. Eggs, for example, when included in descriptions of immature stages at all, are often described in as little as two words (e.g., “ovoid, white”), but Hinton (1981, based on unpublished work of D.C.R. Lincoln) Pap. Avulsos Zool., 2020; v.60.special-issue: e202060(s.i.).36 http://doi.org/10.11606/1807-0205/2020.60.special-issue.36 http://www.revistas.usp.br/paz http://www.scielo.br/paz Edited by: Sônia A. Casari / Gabriel Biffi Received: 12/11/2019 Accepted: 23/01/2020 Published: 04/03/2020 ISSN On-Line: 1807-0205 ISSN Printed: 0031-1049 ISNI: 0000-0004-0384-1825 Pap. Avulsos Zool., 2020; v.60.special-issue: e202060(s.i.).36 2/27 demonstrated that, within the subfamily Staphylininae of Staphylinidae, the eggs of 43 taxa could be characterized and keyed out in a way that reflected generic and higher-level relationships within this subfamily. Pupae have received much more attention, especially through the efforts of Hinton (e.g., 1946a, b, 1948, 1949, 1955a, 1971) who developed a widely accepted classification of insect pupae and drew attention to a number of pupal characters of taxonomic and phylogenetic significance for beetle pupae, such as the presence and distribution of “gin traps” and other protective devices (e.g., Hinton, 1946b, 1955a). According to his classification (Hinton, 1946a, 1949), all known Coleoptera pupae are adecticous (lack articulated mandibles that can be moved), and most are exarate (with appendages such as antennae, wings and legs free of the body, and with more or less moveable abdomen and soft cuticle), while obtect pupae (appendages molded to the body, abdomen usually immobile, and with thick cuticle; see Fig. 1) occur within several beetle groups independently. Hinton (1946a) also noted that most beetle pupae occur in protected cells or even specially constructed cocoons and are terrestrial, although some are aquatic or adapted to episodic flooding. Pupae are featured prominently within some broader descriptive reviews of Coleoptera immature stages, e.g., in Costa et al. (1988), as well as Newton, A.F.: Abdominal spiracles in beetle pupae in detailed comparative studies of pupae within some groups such as Cerambycidae (e.g., Duffy, 1953, 1957, 1960, 1963, 1968), Chrysomelidae (e.g., Cox, 1996, 1998) and Curculionoidea (e.g., Burke, 1968; May, 1994). Pupal descriptions have become not only more frequent but also more detailed and standardized over time, and now routinely include such features as the system of setae, spines or other projections that support the pupa in its cell, the presence and distribution of protective devices such as gin traps, and whether the pupa is exarate or obtect. Such pupal characters have also been used in phylogenetic studies within a few taxa, e.g., in Hydradephaga (Ruhnau, 1986), Hydrophilidae (Archangelsky, 2004b), Staphylinidae: Staphylinini (Staniec & PietrykowskaTudruj, 2019), Tenebrionidae (Bouchard & Steiner, 2004) and Chrysomelidae (Cox, 1998). The present study is not an attempt to review pupal characters in general for potential utility in systematic and ph (...truncated)