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Herhold, Hollister W, Davis, Steven R, Degrey, Samuel P, Grimaldi, David A (2023): COMPARATIVE ANATOMY OF THE INSECT TRACHEAL SYSTEM PART 1: INTRODUCTION, APTERYGOTES, PALEOPTERA, POLYNEOPTERA. Bulletin of the American Museum of Natural History 459 (1): 1-184, DOI: 10.5531/sd.sp.55, URL: http://dx.doi.org/10.5531/sd.sp.55

Among the many inclusions from the exceptionally rich fossiliferous amber of Zhangpu, China (Middle Miocene: Langhian), stingless bees (Apinae: Meliponini) are particularly common, analogous to the merely slightly older amber sites of Mexico and the Dominican Republic. While there is a large number of workers in Zhangpu amber, only two species are represented. The systematics and morphology of the tribe Meliponini is outlined, including a revision to terms of orientation and direction when discussing surfaces and features of appendages, all in order to better discuss the traits of the fossils as well as place them into a broader context in the global systematics of stingless bees. The two amber species are representative of two Old World genera of Meliponini: Tetragonula Moure and Austroplebeia Moure. While the former is widespread across southern Asia, Malesia, Papuasia, and Australia, the latter is today known only from New Guinea and Australia. Neither genus occurs in the environs of Zhangpu today. Tetragonula (Tetragonula) florilega Engel, new species, is a generally typical species of the subgenus although it intermingles traits otherwise found in two large species groups of non-Australian Tetragonula s.str. The species of Austroplebeia from Zhangpu is sufficiently distinct to place in a new subgenus, Anteplebeina Engel. Austroplebeia (Anteplebeina) fujianica Engel, new species, shares a long list of character states with Austroplebeia s.str., including the presence of yellow maculation on the face, mesoscutum, mesoscutellum, and metepisternum, found only in this clade among Asiatic Meliponini. Nonetheless, the species differs in the length of the malar space and various features of wing venation. Keys are provided to the genera in Zhangpu amber and the subgenera of both genera. The diversity of stingless bees in amber is discussed, as well as the presence of these two fossils in the Middle Miocene of mainland Asia in relation to prior divergence time estimates, phylogenetic relationships, as well as the paleogeography of the region and potential biogeographic hypotheses. The biological association of stingless bees with resins, particularly the collection of dipterocarp resins, is discussed as Zhangpu amber is a Class II resin likely produced by an extinct species of Dipterocarpaceae (Malvales). Also discussed are the ecological preferences of modern Tetragonula and Austroplebeia relative to the reconstructed paleoenvironment of the Zhangpu amber rainforest. Emendations to the classification of Meliponini are appended, with the following new taxa proposed: Atrichotrigona Engel, new subgenus of Axestotrigona Moure; Lispotrigona V.H. Gonzalez & Engel, new subgenus of Nannotrigona Cockerell; Asperplebeia Engel, new genus; Nanoplebeia Engel, new subgenus of Plebeia Schwarz; Aphaneuropsis Engel, Koilotrigona Engel, Necrotrigona Engel, Dichrotrigona Engel, Nostotrigona Engel, Ktinotrofia Engel, all new subgenera of Trigona Jurine; Chapadapis Engel, new subgenus of Schwarziana Moure. The following higher groups are also established: Hypotrigonina Engel, new sutribe; Heterotrigonitae Engel, new infratribe; Trigoniscitae Engel, new infratribe.

The first fossil proctotrupid wasp in Lower Cretaceous Lebanese amber is described and figured. Astarteserphus grimaldii gen. et sp. nov. is distinguished from all other living and fossil Proctotrupidae and placed into its own subfamily, Astarteserphinae subfam. nov. A key is presented to the living and fossil subfamilies and tribes of Proctotrupidae, and a brief discussion is provided on the unique features and diminutive size of the fossil.

Oxygen is an important and often limiting reagent of a firefly’s bioluminescent chemical reaction. Therefore, the development of the tracheal system and its subsequent modification to support the function of firefly light organs are key to understanding this process. We employ micro-CT scanning, 3D rendering, and confocal microscopy to assess the abdominal tracheal system in Photinus pyralis from the external spiracles to the light organ’s internal tracheal brush, a feature named here for the first time. The abdominal spiracles in firefly larvae and pupae are of the biforous type, with a filter apparatus and appear to have an occlusor muscle to restrict airflow. The first abdominal spiracle in the adult firefly is enlarged and bears an occlusor muscle, and abdominal spiracles two through eight are small, with a small atrium and bilobed closing apparatus. Internal tracheal system features, including various branches, trunks, and viscerals, were homologized across life stages. In adults, the sexually dimorphic elaboration and increase in volume associated with tracheal features of luminous segments emphasizes the importance of gas exchange during the bioluminescent process.

Angiosperms and their insect pollinators form a foundational symbiosis, evidence for which from the Cretaceous is mostly indirect, based on fossils of insect taxa that today are anthophilous, and of fossil insects and flowers that have apparent anthophilous and entomophilous specializations, respectively. We present exceptional direct evidence preserved in mid-Cretaceous Burmese amber, 100 mya, for feeding on pollen in the eudicot genus Tricolporoidites by a basal new aculeate wasp, Prosphex anthophilos, gen. et sp. nov., in the lineage that contains the ants, bees, and other stinging wasps. Plume of hundreds of pollen grains wafts from its mouth and an apparent pollen mass was detected by micro-CT in the buccal cavity: clear evidence that the wasp was foraging on the pollen. Eudicots today comprise nearly three-quarters of all angiosperm species. Prosphex feeding on Tricolporoidites supports the hypothesis that relatively small, generalized insect anthophiles were important pollinators of early angiosperms., Grimaldi et al. report on a primitive stinging wasp preserved in mid-Cretaceous amber from Myanmar, encapsulated just after it fed on the pollen of a eudicot. Micro-CT shows a pollen mass in the wasp’s mouth cavity, supporting the idea that insect anthophiles were important pollinators of early angiosperms.

Insects have long been thought to largely not require hemoglobins, with some notable exceptions like the red hemolymph of chironomid larvae. The tubular, branching network of tracheae in hexapods is traditionally considered sufficient for their respiration. Where hemoglobins do occur sporadically in plants and animals, they are believed to be either convergent, or because they are ancient in origin and their expression is lost in many clades. Our comprehensive analysis of 845 Hexapod transcriptomes, totaling over 38 Gbases, revealed the expression of hemoglobins in all 32 orders of hexapods, including the 29 recognized orders of insects. Discovery and identification of 1333 putative hemoglobins were achieved with target-gene BLAST searches of the NCBI TSA database, verifying functional residues, secondary- and tertiary-structure predictions, and localization predictions based on machine learning. While the majority of these hemoglobins are intracellular, extracellular ones were recovered in 38 species. Gene trees were constructed via multiple-sequence alignments and phylogenetic analyses. These indicate duplication events within insects and a monophyletic grouping of hemoglobins outside other globin clades, for which we propose the term insectahemoglobins. These hemoglobins are phylogenetically adjacent and appear structurally convergent with the clade of chordate myoglobins, cytoglobins, and hemoglobins. Their derivation and co-option from early neuroglobins may explain the widespread nature of hemoglobins in various kingdoms and phyla. These results will guide future work involving genome comparisons to transcriptome results, experimental investigations of gene expression, cell and tissue localization, and gas binding properties, all of which are needed to further illuminate the complex respiratory adaptations in insects.

The amber fossil record of land snails is poorly studied, with few described species relative to other invertebrate groups (e.g., insects and chelicerates). Recently discovered land snail amber fossils from Myanmar present an important opportunity to understand the tropical land snail fauna of the Cretaceous, which was a time when many terrestrial invertebrates were rapidly diversifying. We describe 12 new land snail species in 3 families from mid-Cretaceous Burmese amber: Eotrichophorus kachin gen. et sp. nov., Perissocyclos kyrtostoma gen. et sp. nov., Macropupina electricus gen. et sp. nov., Paleodiplommatina spelomphalos gen. et sp. nov., Xenostoma lophopleura gen. et sp. nov. and Assiminea striatura sp. nov. and six species of uncertain taxonomic affinity. We also discuss the fossil history of the Cyclophoroidea. We report the first mid-Cretaceous land snail species with periostracal hairs and discuss their adaptive significance, as well as the general paleoecology of Burmese mid-Cretaceous tropical land snails.

• Ceratomyrmex ellenbergeri Perrichot, et al. – an exceedingly unusual taxon described from four worker specimens of varying completeness. Retaining the same overall Cretaceous trap-jaw bauplan as other haidomyrmecines, C. ellenbergeri possesses a unique clypeal horn coated ventrally in elongate, tapered setae. The mandibles are correspondingly very long, apparently extended to pin prey between the mandibular points and lengthy clypeal projection. This configuration is a dramatic variant on the trap-jaw syndrome and would appear to confirm vertical mandibular articulation. Locality: Burmese amber.

Linguamyrmex vladi Barden & Grimaldi sp.n. http://zoobank.org/urn:lsid:zoobank.org:act:2C55A44F-974D- 4A33-BDC2-26269687577A Figs 1, 2 A, 3F���I, 5. Diagnosis. As for genus. Description. Head: measuring 0.90 mm postero-anteriorly along dorsal margin, 0.96 mm in length/depth from vertex of head to anterior margin of clypeus. Occipital foramen positioned highly dorsad, just under vertex of head. Postgena broadly depressed; postgenal suture visible, deeply furrowed. Vertex broadly rounded and glabrous with gena gradually tapered ventrally towards mandibular socket and oral opening with fine, sparse setae. Ocelli faintly visible on vertex, positioned dorsally. Eye situated high on head capsule and bulging in frontal view, ovoid, measuring 0.38 mm in length and 0.25 mm in width when viewed laterally. Three antennal segments fully preserved (scape 0.94 mm in length; pedicel 0.12 mm; flagellomere I 0.55 mm). Antennal socket approximately in line with ventral margin of eye; socket exposed and immediately flanking a medial frontal triangle (sensu Perrichot et al., 2016). Frontal triangle extends the vertex, contrasted with antennal sockets, which are present within cuticular depressions. Clypeal horn originating at both frontal triangle and clypeal stalk, both structures heavily sclerotized with cleared, membrane-like cuticle connecting from frontal triangle to stalk. Horn paddle-shaped, total length 0.64 mm, diameter 0.49 at greatest; narrow (0.04 mm) glabrous stalk 0.22 mm in length leading to setose pad; setose pad (0.42 in length) with long trigger hairs originating at pad base; dorsal margin of setose pad glabrous, underside coated in a large number of stout setae in centre and longer, more tapered setae along edges. Anterior margin of clypeus medially triangulate; distinct medial ridge extending to clypeal horn; lateral margins, beginning just above mandibular insertion, extending diagonally toward clypeal horn. Cheek-like lobes projecting anteroventrally above mandible insertion. Mandible scythe-like, comprise a linear basal margin (0.55 mm in length) and curved apical tooth (0.92 mm measured as a straight line from base to tip, ignoring curvature) meeting nearly at right angle; preserved with apical teeth in parallel, nearly touching. Basal portion of mandible with anterior flange-like expansion, concave inner margin coated with pointed setae; leading edge of anterior flange expansion smooth; apical tooth rounded broadly with slight point. Maxillary and labial palps not visible. Mesosoma. Pronotum broad, coated in fine setae; propleuron reduced, not visible in lateral view except where abutting head capsule anteriorly. Pronotal length 0.72 mm, measured along dorsal margin. Mesonotum 0.34 mm; metanotum 0.24 mm; propodeum 0.49 mm. Weber���s length 1.77 mm, mesosomal height 0.56 mm measured perpendicular to Weber���s length line at pronotum. Procoxal length 0.72 mm, max width 0.22; mesocoxal length 0.31, max width 0.21; metacoxal length 0.49, max width 0.26; protrochantal length 0.14, max width 0.11; mesotrochantal length 0.19, max width 0.11; metatrochantal length 0.19, max width 0.12; profemoral length 1.01, max width 0.13; mesofemoral length 1.09, max width 0.12; metafemoral length 1.48, max width 0.18; protibial length 1.09, max width 0.11; mesotibial length 1.30, max width 0.08; metatibial length 1.59, max width 0.08. Trochantellus present, length included in femur measurements. Three protibial spurs present, the largest approximately 2 �� length of other two; mesotibia with two spurs, the larger 2 �� the length of the smaller; two conspicuous setae of equal length positioned along anterior margin of mesotibial apex; metatibia with two spurs, the largest pectinate and ~ 3 �� the size of the smaller. Pretarsal claw with subapical tooth positioned closer to apex than to claw origin. Dorsal margin of propodeum gradually rounded, posterior margin with sheer face. Propodeal spiracle a dorsoventrally elongate slit situated medially. Metapleural gland opening gaping, present within horizontal invagination of cuticle, opening visible posteroventrally following well-developed bulla. Metasoma. Petiole 0.49 mm in length, 0.49 mm in height at greatest, pedunculate; not tergosternally fused, deep suture visible; anterior margin gradually sloping; with gradually rounded dorsal margin; broadly attached to gastral segment I (abd seg III) with striated helcium clearly visible. Lateral sulcus visible running anteroposteriorly along petiole, signifying incomplete fusion; ventral to sulcus lies thin, membraneous cuticular expansion with distinct anteriorly positioned keel. Sternite III with pointed projection on anterior margin exhibiting striations running laterally. Circumgastral constriction present between gastral segments I and II with dorsal, posteriorly extended notch. Gastral cuticle translucent, revealing heavily sclerotized and expanded apodemes of gastral sternites IV and V withdrawn into the gaster. Type material. Holotype, AMNH BuPH-01, in the Department of Invertebrate Zoology, American Museum of Natural History. Additional material examined. Although not named, three specimens, BuPH-02 (Figs 2 D, 3C, 7, Video S1), BuPH-03 (Figs 2 B, 3A, B, 4) and BuPH-04 (2C), were studied and are illustrated here. These three specimens probably represent additional species, particularly with respect to overall size and clypeal paddle composition. Moreover, these specimens possess mandibles that appear to interlock basally, joining the distal apical tooth portions of the mouthparts (Figs 3 D, E, 7, Video S1). As the inner surface of each mandible is concave, their joining forms a channel that is open dorsally near the pointed mandibular apex (Fig. 7). However, there remains some ambiguity regarding the reliability of measurements and identification of other discrete features due to incomplete preservation. We therefore delay description until more complete material is obtained. Measurements are provided here (in mm): (BuPH-02) [BuPH-03] {BuPH-04} Head length measured postero-anteriorly along dorsal margin (1.14) [0.87] {0.69}, head depth (1.48) [1.22] {1.2}, scape length (1.56) [1.36] {1.28}, paddle length (1.5) [1.52] {1.26}, paddle diameter (���) [~ 0.57] {���}, eye length (���) [���] {0.29}, eye width (���) [���] {0.19}, basal margin of mandible (0.71) [0.53] {0.39}, apical tooth (1.97) [1.57] {1.48}, pronotal length (1.08) [0.97] {0.8}, mesosomal height (~ 0.81) [~ 0.75] {0.66}, Weber���s length (2.64) [2.49] {1.99}, petiole length (0.82) [0.76] {0.60}. Unfortunately, no specimen retained all antennal segments, preventing a reliable comparison of scape length relative to all other segments. In addition, all specimens, including the type, are missing terminal abdominal segments. Etymology. In reference to Vlad III, or Vlad Dracula (c. 1429���1476), prince of a region of Romania then called Wallachia. His moniker, Vlad the Impaler, refers to his favoured and frequent method of execution, which inspired the vampirous character Count Dracula fictionalized by Bram Stoker in 1897. The patronym is in reference to the presumed impalement of prey by Linguamyrmex and its liquid diet (see later). Systematic comments. Haidomyrmecines have been recovered as monophyletic (Barden & Grimaldi, 2016) and have traditionally been placed as tribe within Sphecomyrminae (Bolton, 2003). The subfamily is defined largely by plesiomorphic features and is now suspected to be paraphyletic (Barden & Grimaldi, 2016). We tentatively follow this subfamilial assignment here but note that, following additional phylogenetic analyses, haidomyrmecines may be best served by the erection of a separate subfamily., Published as part of Phillip Barden, Hollister W. Herhold & David A. Grimaldi, 2017, A new genus of hell ants from the Cretaceous (Hymenoptera: Formicidae: Haidomyrmecini) with a novel head structure, pp. 837-846 in Systematic Entomology 42 on pages 839-842, DOI: 10.1111/syen.12253, http://zenodo.org/record/887095, {"references":["Perrichot, V., Wang, B. & Engel, M. S. (2016) Extreme morphogenesis and ecological specialization among Cretaceous basal ants. Current Biology, 26, 1468 - 1472.","Barden, P. & Grimaldi, D. A. (2016) Adaptive radiation in socially advanced stem-group ants from the Cretaceous. Current Biology, 26, 515 - 521.","Bolton, B. (2003) Synopsis and classification of Formicidae. Memoirs of the American Entomological Institute, 71, 1 - 370."]}

��� Haidomyrmex cerberus Dlussky ��� the first hell ant was named 20 years ago by the prolific paleomyrmecologist Gennady Dlussky (Dlussky, 1996). Haidomyrmex cerberus was described from a single partial worker specimen, providing the basis for what are now tribal synapomorphies for the Haidomyrmecini, established by Bolton (2003) and revised by Perrichot et al. (2016): unusual L-shaped mandibles and a bulging clypeus possessing a clypeal brush comprising dense, stout setae. Locality: Burmese amber, ~ 98.8 Ma, Kachin State, Myanmar (Shi et al., 2012), Published as part of Phillip Barden, Hollister W. Herhold & David A. Grimaldi, 2017, A new genus of hell ants from the Cretaceous (Hymenoptera: Formicidae: Haidomyrmecini) with a novel head structure, pp. 837-846 in Systematic Entomology 42 on page 838, DOI: 10.1111/syen.12253, http://zenodo.org/record/887095, {"references":["Dlussky, G. M. (1996) Ants (Hymenoptera: Formicidae) from Burmese amber. Paleontologicheskii Zhurnal, 30, 449 - 454.","Bolton, B. (2003) Synopsis and classification of Formicidae. Memoirs of the American Entomological Institute, 71, 1 - 370.","Perrichot, V., Wang, B. & Engel, M. S. (2016) Extreme morphogenesis and ecological specialization among Cretaceous basal ants. Current Biology, 26, 1468 - 1472.","Shi, G., Grimaldi, D. A., Harlow, G. E. et al. (2012) Age constraint on Burmese amber based on U - Pb dating of zircons. Cretaceous Research, 37, 155 - 163."]}

• Haidomyrmodes mammuthus Perrichot, et al. – described from both partial gyne and worker specimens, H. mammathus is the only alate specimen yet described (Perrichot et al., 2008). In addition, the type material is the only reported conspecific worker and putative queen caste for any Cretaceous taxon and provides important information regarding sociality in early ants. The description of multiple specimens from an additional locality confirmed beyond any doubt that the highly unusual features observed in H. cerberus were not preservational. Largely similar to H. cerberus, H. mammathus possesses ocelli, which have long been presumed to be an ancestral ant trait (Wilson et al., 1967b). Locality: Charentese amber, ~ 100 Ma, Charentes, France (Perrichot et al., 2010)

Genus Linguamyrmex Barden & Grimaldi, new genus http://zoobank.org/urn:lsid:zoobank.org:act:9F04FAEE-59DA- 4B3C-AB8B-EE0D234CC118 Figs 1���5, 7, Video S1. Diagnosis, worker. As in other haidomyrmecines (Haidomyrmex, Haidoterminus, Haidomyrmodes, Ceratomyrmex), head hypognathous-like with mandibles projecting primarily downward; mandible scythe-like, with flattened basal margin leading to a curved apical tooth that is expanded perpendicular to axial plane of head. Cephalic clypeal ���horn��� present but abbreviated, differs from Ceratomyrmex by horn stalk of Linguamyrmex being glabrous, that of Ceratomyrmex with fine, stiff setae of various lengths; clypeal horn much shorter in Linguamyrmex, less than head length/depth, stalk short, with the expanded, flat, paddle-shaped setose pad comprising>50% total horn length; clypeal pad slightly trough-shaped ventrally, covered with very short, dense velcro-like vestiture; trigger hairs originate not at base of stalk as in Ceratomyrmex but near basal margin of setose pad; ocelli present. In addition, Linguamyrmex with first and second gastral segments with deep constriction between them (a gastral constriction is figured in description of Haidomyrmodes mammuthus but is less developed). Type species. L. vladi sp.n. Etymology. From Latin ���lingua���, meaning tongue ��� in reference to the tongue-like clypeal projection ��� and the Greek ���myrmex���, meaning ant., Published as part of Phillip Barden, Hollister W. Herhold & David A. Grimaldi, 2017, A new genus of hell ants from the Cretaceous (Hymenoptera: Formicidae: Haidomyrmecini) with a novel head structure, pp. 837-846 in Systematic Entomology 42 on page 839, DOI: 10.1111/syen.12253, http://zenodo.org/record/887095

��� Haidoterminus cippus McKellar, et al. ��� the youngest known haidomyrmecine, H. cippus is described from a single worker specimen. This species extends the temoporal and geographic range of hell ants considerably, indicating that the specialized mouthparts and probable trap-jaw behaviour were evidently successful for some time and over a wide area of Laurasia. Haidoterminus cippus is also unique among haidomyrmecines, possessing an elongate antennal scape somewhat similar to modern ant taxa. Locality: Medicine Hat amber, ~ 78.5 Ma, Alberta, Canada (McKellar et al., 2013), Published as part of Phillip Barden, Hollister W. Herhold & David A. Grimaldi, 2017, A new genus of hell ants from the Cretaceous (Hymenoptera: Formicidae: Haidomyrmecini) with a novel head structure, pp. 837-846 in Systematic Entomology 42 on page 838, DOI: 10.1111/syen.12253, http://zenodo.org/record/887095, {"references":["McKellar, R. C., Glasier, J. R. & Engel, M. S. (2013) A new trap-jawed ant (Hymenoptera: Formicidae: Haidomyrmecini) from Canadian Late Cretaceous amber. The Canadian Entomologist, 145, 454 - 465."]}

Genus Linguamyrmex Barden & Grimaldi, new genus http://zoobank.org/urn:lsid:zoobank.org:act:9F04FAEE-59DA- 4B3C-AB8B-EE0D234CC118 Figs 1–5, 7, Video S1. Diagnosis, worker. As in other haidomyrmecines (Haidomyrmex, Haidoterminus, Haidomyrmodes, Ceratomyrmex), head hypognathous-like with mandibles projecting primarily downward; mandible scythe-like, with flattened basal margin leading to a curved apical tooth that is expanded perpendicular to axial plane of head. Cephalic clypeal ‘horn’ present but abbreviated, differs from Ceratomyrmex by horn stalk of Linguamyrmex being glabrous, that of Ceratomyrmex with fine, stiff setae of various lengths; clypeal horn much shorter in Linguamyrmex, less than head length/depth, stalk short, with the expanded, flat, paddle-shaped setose pad comprising>50% total horn length; clypeal pad slightly trough-shaped ventrally, covered with very short, dense velcro-like vestiture; trigger hairs originate not at base of stalk as in Ceratomyrmex but near basal margin of setose pad; ocelli present. In addition, Linguamyrmex with first and second gastral segments with deep constriction between them (a gastral constriction is figured in description of Haidomyrmodes mammuthus but is less developed). Type species. L. vladi sp.n. Etymology. From Latin ‘lingua’, meaning tongue – in reference to the tongue-like clypeal projection – and the Greek ‘myrmex’, meaning ant.

Phillip Barden, Hollister W . Herhold, David A . Grimaldi (2017): A new genus of hell ants from the Cretaceous (Hymenoptera: Formicidae: Haidomyrmecini) with a novel head structure. Systematic Entomology 42: 837-846, DOI: 10.1111/syen.12253

��� Ceratomyrmex ellenbergeri Perrichot, et al. ��� an exceedingly unusual taxon described from four worker specimens of varying completeness. Retaining the same overall Cretaceous trap-jaw bauplan as other haidomyrmecines, C. ellenbergeri possesses a unique clypeal horn coated ventrally in elongate, tapered setae. The mandibles are correspondingly very long, apparently extended to pin prey between the mandibular points and lengthy clypeal projection. This configuration is a dramatic variant on the trap-jaw syndrome and would appear to confirm vertical mandibular articulation. Locality: Burmese amber., Published as part of Phillip Barden, Hollister W. Herhold & David A. Grimaldi, 2017, A new genus of hell ants from the Cretaceous (Hymenoptera: Formicidae: Haidomyrmecini) with a novel head structure, pp. 837-846 in Systematic Entomology 42 on page 838, DOI: 10.1111/syen.12253, http://zenodo.org/record/887095

Bees of the family Halictidae (Apoidea: Anthophila) have three pairs of thick, bundled muscles that are circular to subcircular in cross section within the first metasomal segment, as revealed by micro-CT scanning of 16 species in 15 genera of five bee families. In nonhalictids and the basal halictid subfamily Rophitinae, these muscles are planar (flat and sheetlike), typically lying between the anterior air sacs and abdominal wall. In Nomiinae and Halictinae, these muscles, especially the dorsal-ventral pair, bulge into air-sac space, partly enveloped by air-sac membrane. A possible function may be to facilitate metasomal compression and contraction, and thus air flow. The bundled shape of these derived halictid muscles is similar to that of flight muscles, but further data is needed to determine if they are fibrillar, which would suggest a completely different function.