EPHEMEROPTERA, Hyatt & Arms, 1891

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), pp. 1-184 : 1-184

ORDER EPHEMEROPTERA

Ephemeroptera are unique in having a winged subimago developmental stage; all other pterygotes have lost this molt as winged individuals. Additionally (and relevant to tracheation), Ephemeroptera , like Odonata , possess direct flight muscles, where the wing muscles insert directly at the wing bases. Mayfly tracheae were first described in detail by Swammerdam (1737; see fig. 1), and modern treatments include Needham et al. (1935), Landa (1948, 1969), and Soldán (1979). Chapman (1918) included a discussion of wing tracheae of mayflies in his early comparative analysis.

For this study, three mayfly specimens were scanned: an adult Ephemera sp. (Ephemeridae) and Neocloeon triangulifer subimago and adult ( Baetidae ). The Neocloeon specimens afforded the opportunity to compare tracheal architecture between the winged stages. Although scan resolution ranged from 13 µm for Ephemera sp. to 2 µm for the adult Neocloeon , sufficient detail was achieved to determine that all three specimens of both taxa are remarkably similar in tracheal architecture, the primary difference being the morphology of the air sac. Adult Ephemeroptera do not feed, and the alimentary canal forms a large, central air sac ( Soldán, 1979), perhaps aiding the emergence of the alate subimago from the water, with visceral trachea either reduced or displaced. While not complete until the adult stage, the subimago possesses smaller but substantial air spaces that fuse to form the single adult air sac (see fig. 21 ). As air sacs in tracheal visualizations can obscure details of smaller branches, all air sacs are omitted from the Ephemeroptera plates. Due to the size of the wings, Ephemeroptera plates are shown with the wings cropped to show as much detail as possible without scaling the image to fit the page. Other minor differences between specimens are described in their respective sections below.

T2-S is the start point of the longitudinal trunk, which arcs toward the midline of the body for a short distance before turning posteriad and proceeding all the way to the terminalia. While similar structures have been noted in the larvae of many insect orders ( Snodgrass, 1935; Whitten, 1960; Wigglesworth, 1972), the presence of a large, nearly linear longitudinal trunk in an adult that spans the length of the body appears to be unique to Ephemeroptera . This structure is likely homologous with the dorsal longitudinal trunk seen in other taxa, and individual segments of the trachea are referred as such, beginning with T2-DLT as the start. The thoracic portion of the DLT in Ephemera appears to be two tracheae, arranged dorsoventrally. This is the result of compression in the thorax—distention of the air-filled alimentary canal, compression by packing material for CT scanning, or postmortem settling of tissues can all contribute to deformation of tracheae in soft-bodied insects such as mayflies.

Noted by Chapman (1918), Ephemeroptera possess only anterior wing base tracheae (T2,3- AWba), extending into the wing as T2,3-W-c-r with the corresponding PWba and W-cu-a tracheae absent. This lack of a branch leading to the trailing-edge wing tracheae is a condition apparently unique to mayflies.

The three ephemeropteran specimens display some disparity in the origins of H-Oc branches. These appear to be developmental variations, as larger trunks may supply different regions as smaller tracheae “sprout” from them during ontogeny.

It remains unclear whether all abdominal spiracles are functional. Needham et al. (1935) indicates that abdominal spiracles may be continuously open, but close examination of the spiracular openings via CT cross section suggests that A2-S through A7-S may be closed or unused.

DESCRIPTION: BODY: Adults with long central air sac formed from modified alimentary canal, extending from head capsule to end of 7th abdominal segment; subimago with individual air sacs that fuse in the imago (see fig. 21 ).

HEAD: Tracheal morphology largely from Landa (1948), but with unified nomenclature here. Palmen’s organ (H-PO) prominent and centrally located. Just after entry into head capsule, thick H-DCT bifurcates into two tracheae; dorsal one of these proceeds anteriad a short distance before dividing into H-Ocel and dorsal branch to H-PO; ventrally H-Lbr branches anteriad shortly with H-DOc branch before remaining trachea turns directly ventrad; H-Lbr continues toward reduced mouthparts with small H-Ant branch at base of antennae. H-VCT thick, splits into three branches at base of head capsule: H-VOc, extending laterally toward eye; H-Lbm directly ventrad, and ventrally supplying H-PO.

THORAX: T2-S with three branches: T2-CT, T2-SAtr, and T2-DLT. T2-CT very short, before bifurcating into anterior H-DCT and H-VCT at posterior margin of prothorax. T1-L branching directly ventrad from T2-VCT anterior of H-DCT/H-VCT split from T2-CT. T2-SAtr dorsal, with T2-AWL posteriad and multiple T2-FM dorsad: thick T2-FM1 splitting dorsally and subdividing into smaller tracheae that supply flight muscles, thinner T2-FM2 running dorsad and slightly posteriad; T2-AWL arcing briefly dorsad and posteriad before proceeding ventrad into midleg as T2-L, with T2-AWba branching near dorsal apex of arc and continuing to T2-W-c-r. T2-PWba absent. T2-DLT very thick, beginning mediad before arcing directly posteriad and continuing to the terminalia. Small T2-FM3 extends ventrally from T2-DLT midway between T2-S and T3-S. T3-S opens into very thick T3-SAtr, with (effectively) five branches: T3-DB, T3-AWL, and three T3-FM. T3-DB runs directly mediad, linking with T2-DLT from anterior, continuing directly posteriad as T3-DLT. T3-AWL arcing briefly dorsad and then posteriad, similar to T2-AWL, with T3-Wba branching near apex of arc while T3-L continues into hindleg; T3-Wba continues as T3-W-c-r into wing. T3-PWba absent. T3-FM1 very thick, extends directly dorsad and T2-FM2 posteriad and slightly ventrad into flight muscle; T3-FM3 runs ventrad in fanlike extension into flight muscle.

ABDOMEN: A1-S, A8-S functional; unclear if A2..7-S connect through body wall. A1-S modified, with large A1-FM dorsally; T3-DLT from anterior, continuing posteriad as A1-DLT. Remaining abdominal segments similar, with A n -S linked with neighboring segments via A n - DLT extending linearly through abdomen past A8-S. A8-DLT extending into terminalia as A-Cr. All A n -S with various visceral tracheae extending dorsad and posteriad from A n -S, dorsal and ventral commissures absent.

Figures

FIGURE 21. Air sacs formed from alimentary canal in subimago and adult Ephemeroptera, shown in lateral views. A. Neocloeon triangulifer subimago; B. N. triangulifer adult; C. Ephemera sp. adult. Specimens not to same scale.