Unique extrication structure in a new megaspilid, Dendrocerus scutellaris Trietsch & Mikó (Hymenoptera: Megaspilidae)

Abstract Background A new species, Dendrocerus scutellaris Trietsch & Mikó (Hymenoptera: Megaspilidae), is described here from male and female specimens captured in Costa Rica. This species is the only known ceraphronoid wasp with a straight mandibular surface and raised dorsal projections on the scutellum, called the mesoscutellar comb. It is hypothesised that the function of the mesoscutellar comb is to aid the emergence of the adult from the host, especially since the mandibles lack a pointed surface to tear open the pupal case. The authors also provide phenotypic data in a semantic form to facilitate data integration and accessibility across taxa and provide an updated phenotype bank of morphological characters for megaspilid taxonomic treatments. In updating this phenotype bank, the authors continue to make taxonomic data accessible to future systematic efforts focusing on Ceraphronoidea. New information A new species, Dendrocerus scutellaris (Hymenoptera: Megaspilidae) Trietsch & Mikó, is described from both male and female specimens captured in Costa Rica.


Introduction
Ceraphronoidea is a relatively small superfamily of parasitoid wasps with a worldwide distribution (Johnson and Musetti 2004). The superfamily is composed of two extant families: Ceraphronidae and Megaspilidae. Dendrocerus Ratzeburg, 1852 is the second most diverse megaspilid genus with approximately 118 species described worldwide. Dendrocerus is also the most well-known megaspilid genus due to the agriculturally relevant species D. carpenteri (Curtis, 1829), which is used as a model organism to study parasitoid behaviour and ecology (Chow and Mackauer 1999, Mackauer and Chow 2015, Mackauer 2017, Nakashima et al. 2016, Schwörer et al. 1999), but much work remains to be done on the life history and taxonomy of the group.
Eclosion is the adult emergence from the pupal case in holometabolous insects. In most holometabolous insects, the tearing of the pupal case is achieved by the movement of the insect and the increased hemolymph pressure caused by muscle contractions (Gullan andCranston 2010, Zdárek andDenlinger 1992). Insects that are also protected by a cocoon or puparium may cut or push their way out with their mandibles and legs, or rely on specialized structures such as projections on the head or backward-facing spines on the dorsal surface of the abdomen (Gullan and Cranston 2010). Packard Jr (1878) reported spines called the sector coconis present at the base of the forewings in Bombyx mori (Linnaeus, 1758) and several species of Saturniidae (Lepidoptera) and observed the spine in use by an emerging Actias luna (Linnaeus, 1758). Abdominal spines and frontal protuberances serving as "cocoon-cutters" have also been reported on male Psychidae (Lepidoptera); these structures are absent from females, which do not extricate themselves from the pupal case Davis 1975.
In Hymenoptera, wood-boring families have been observed to have specialised structures for extricating themselves from the pupal chambers inside wood where they develop (Vilhelmsen and Turrisi 2011). Ibaliidae, Orussidae and Stephanidae have upward-or backward-facing cuticular processes situated anterodorsally on the head. Emerging wasps use these structures to anchor their heads while chewing through wood with their mandibles, but they could also be used to clear debris or pull themselves through the wooden galleries (Vilhelmsen and Turrisi 2011). Similar cuticular processes and spines on the mesosomata of Ibaliidae, Stephanidae, Aulacidae, the cynipoid family Liopteridae and the ichneumonid subfamily Rhyssinae may function in the same way (Turrisi and Vilhelmsen 2010, Vilhelmsen and Turrisi 2011, Van Noort and Buffington 2013, Rousse and Van Noort 2014. Along with wood-boring families, mesoscutal spines are found in Platygastroidea (Austin 1984, Johnson et al. 2008); instead of assisting in emergence from wooden galleries, these spines may assist wasps in emerging from the remains of their hosts.
Here, a new species is described of Dendrocerus captured in Costa Rica characterised by the presence of a straight mandibular edge and the mesoscutellar comb, which could aid in emergence. These two structures have never before been recorded in Ceraphronoidea and are discussed here for the first time.

Materials and methods
Point-mounted specimens were borrowed from the Natural History Museum (NHMUK) in London, United Kingdom. Specimen data is provided in Suppl. material 1. All specimen data was also entered into a Microsoft Excel spreadsheet template from GBIF so that the data could be published on GBIF using the Integrated Publishing Toolkit (https:// www.gbif.org/news/82852/new-darwin-core-spreadsheet-templates-simplify-datapreparation-and-publishing). Specimens are deposited at the Natural History Museum in London, United Kingdom (NHMUK) and at the Frost Entomological Museum, University Park, PA, USA (PSUC).
Point-mounted and glycerine-dissected specimens were examined using an Olympus SZX16 stereomicroscope with an Olympus SDF PLAPO 1XF objective (115×) and an Olympus SDF PLAPO 2XPFC objective (230× magnification). Blue-Tac (Bostik, Inc., Wauwatosa, Wisconsin, USA) and molding clay (Sculpey, Polyform Products Company, Elk Grove Village, Illinois, USA) was used to stabilise specimens during imaging and observation. Stacks of bright field images were taken manually on an Olympus CX41 microscope with a Canon EOS 70D camera attached. Images were subsequently aligned and stacked using Zerene Stacker Version 1.04 Build T201706041920. Figures were created in Adobe Photoshop elements Version 3.1.
For the descriptions of male and female specimens, morphological characters (following Burks et al. 2016) were scored based on observations of point-mounted and glycerinestored specimens. Specimen data, OTU concepts, natural language phenotypes and images were compiled in the online database MX (http://purl.oclc.org/NET/mx-database) which was used to render the Diagnosis, Description, Material Examined and Etymology sections. Anatomical terms follow the Hymenoptera Anatomy Ontology .

Diagnosis
Dendrocerus scutellaris (Figs 1,2,3,4,5,6) belongs to the Dendrocerus halidayi species group (Dessart 1995, Dessart 1999, based on the branched male flagellomeres, bifid anteromedian projection of the metanoto-propodeo-metapectal complex and the presence of parossiculal projections with 3 parossiculal setae. This species is distinguished from all other ceraphronoid species by the presence of the mesoscutellar comb, an anatomical cluster that is composed of a row of spines medially on the mesoscutellar-axillar complex. This species is also unique amongst Ceraphronoidea in that the distal edge of mandible is flat and not pointed.

Etymology
This species is named for the presence of the mesoscutellar comb, which is unique to this species and is not found in any other known ceraphronoid species.

Distribution
This species is only known from Costa Rica.

Discussion
Dendrocerus scutellaris belongs to the halidayi species-group, which is characterised by the presences of flabellate antennae in males (Dessart 1995, Dessart 1999. This new species is one of the few Dendrocerus species that also possesses a bifurcated anteromedian projection of the metanoto-propodeo-metapecto-mesopectal complex (Fig.  1). This character is also present in D. africanus and D. australicus Dodd 1914, both members of the halidayi species-group Dessart 1995, Dessart 1999. Based on the morphological features which these species share in common, it is hypothesised that this species is the closest known living relative to Dendrocerus scutellaris.
D. scutellaris is unique amongst members of the halidayi species-group in that, while other species have up to six fully formed branches on the flagellomeres (Dessart 1999, Dessart 1995), Dendrocerus scutellaris has 7 fully formed branches with a variable eighth branch that is more developed in the holotype male specimen than in the paratype male (Fig. 4). The length of the seventh branch also varies between specimens; in the holotype, the branch is as long as the branch on the seventh flagellomere is as long as the seventh flagellomere, while in the paratype specimen, it is shorter than the seventh flagellomere. Though the number of flagellomeres with branches and branch length has been used as a character to describe new species of Dendrocerus in the past (Pezzini et al. 2014), Dendrocerus scutellaris clearly exhibits intraspecific variation in the number and length of the flagellar branches between both male type specimens. Thus, branch length and presence of apical branches should not be used exclusively to describe Dendrocerus species.
Dendrocerus scutellaris is distinguished from all other ceraphronoid species by the presence of a straight mandibular surface (Fig. 3) and the presence of the mesoscutellar comb (Fig. 1), a ridge of backward-facing dorsal projections present in both male and female specimens. Though nothing is known about the life history or host identity of this species, other species such as Dendrocerus carpenteri Curtis 1829 are known to parasitise braconid parasitoids inside of aphid mummies (Mackauer and Chow 2015). It is hypothesised that the function of the mesoscutellar comb is to aid in emergence and extrication of the adult from its host. The function of the mesoscutellar comb is further suggested by the fact that the distal surface of the mandible is straight instead of pointed, and thus presumably cannot be used for piercing or tearing of the pupal case. All other known Ceraphronoidea possess a pointed mandibular edge and lack the mesoscutellar comb; the presence of a straight mandibular edge in the only ceraphronoid with a mesoscutellar comb is not likely to be a coincidence.