ZYGENTOMA

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 ZYGENTOMA

There are very few micro-CT studies of Zygentoma to date; most notably, synchrotron radiation micro-CT of Tricholepidion gertschi heads has been used to investigate Zygentoma morphology for systematic study ( Blanke et al., 2014), but not tracheae. This genus is particularly significant given the disputed placements within and near Zygentoma ( Engel, 2006; Blanke et al., 2014). Zygentoma are relevant in general as a sister group to the pterygotes, and indeed they share several tracheal synapomorphies. Three specimens from two orders were scanned here: Thermobia domestica and Lepisma saccharinum from Lepismatidae , and the relict silverfish Tricholepidion gertschi from Lepidotrichidae . Scan resolutions were within a 2.4 µm 3 /voxel range, with Lepisma at the highest resolution of 4.8 µm 3 /voxel and Thermobia with the lowest resolution at 7.2 µm 3 /voxel. Resolving the smallest tracheae at these resolutions tends to be more a function of specimen preservation rather than scanning parameters. Although the level of detail varies slightly between these scans, sufficient resolution was achieved to locate small visceral tracheae in all three specimens, indicating suitability for comparative purposes. Although the zygentoman head morphology is similar among the specimens, in the sections below Lepidotrichidae thoracic and abdominal detail are described separately due to substantial differences.

Tricholepidion appears to possess only two pairs of functional abdominal spiracles, A7-S and A8-S. The elongate, sinusoidal trunks beginning at A7-S and extending anteriorly to the thorax are not present in the two lepismatid specimens. If Tricholepidion is indeed basal to Lepismatidae , it is possible that these trunks are plesiomorphic, and a loss of these tracheae in Thermobia and Lepisma could be derived.

In both species scanned from Lepismatidae , leg tracheae branch from T2-S and T3-S in an arc leading posteriorly into the legs, each with a small branch extending dorsad. Here we interpret these as T2-AWL and T3-AWL, with T2-AWba and T3-AWba branching dorsad. An alternative pattern is to designate these dorsal branches as simply T2,3-L-DVi, but the similarity of the branching pattern with both apterous and winged taxa included herein (e.g., Grylloblatta ) strongly suggests homology with these wing base tracheae; Šulc (1927) interpreted these as possible wing tracheae.

DESCRIPTION: HEAD: H-DCT with two branches: H-Ft dorsal; H-Ant-Lbr anteriad and slightly ventral, dividing into H-Ant and H-Lbr close to base of antenna. H-VCT with three branches, dividing at approximately same location: small H-Lbm extends ventrally; H-Mx continues anteriorly before proceeding ventrad; larger H-OcMd extends anteriorly. H-VC absent in T. gertschi but present in both Lepismatidae . H-Oc branching dorsally, H-Md continues anteriorly and ventrad, following shape of mandibular sclerite.

THORAX: Tricholepidion with significant differences among Zygentoma , notably T2-DLT and T3-DLT elongate, with dorsal visceral branches similar to T2-DB and T3-DB; a complete description of the thorax of Tricholepidion thorax is below. For both Lepismatidae : T2-S with three branches: T2-CT, T2-AWL, and T2-VB. T2-CT very thick, running directly anteriad with T2-DB just anterior to T2-S; T2-CT dividing into H-DCT and H-VCT in prothorax. Thinner T2-AWL arcing posteriorly, slightly dorsad, with thin but prominent dorsad and posteriorly arcing T2-AWba. T2-ACx present with T2-VC, remainder of T2-AWL extending into midleg as T2-L. T2-DB dividing into T2-DLT, extending in posterior arc and T1-DLT, ending blind in prothorax. Small T1-Pn present just anterior of H-DCT/H-VCT split. T1-L branching ventrally from H-VCT; T1-L with T1-VC branch present; T1-ACx from T1-VC; T1-PCx from T1-L. Small T2-VB with T2-VB-Vi. T2-L two branches: prominent, dorsad and posteriorly arcing T2-L-DVi. Thermobia with T2-PCx from T2-L, note that T2-PCx is from T2-VB in Lepisma . T2-VC with connection from T3-AsymC medially, originating from left side in Thermobia and right side in L. saccharinum . T3-S with three branches: T3-DB, T3-AWL, and T3-VB. T3-DB directly dorsal with Y-shaped bifurcation to T2-DLT from anterior and T3-DLT toward A1-S. T3-AWL posteriad with T3-AWba, longer in Lepisma than Thermobia . T3-PCx from T3-L in T. domestica , T3-VC in L. saccharinum . T3-VB ventral and slightly inward with AsymC from left side of T. domestica , right side of L. saccharinum , anteriad to connect with T2-VC.

ABDOMEN: T. gertschi with significant differences to other Zygentoma , including apparent lack of functional abdominal spiracles (except A7-S and A8-S), and a sinusoidal A7-VLT extending anteriorly over several segments; a complete description of abdomen is below. For Lepismatidae : A1..8-S present, all located laterally. A n -SB present for segments in middle of abdomen but variable; see detailed descriptions. A n -S (or A n -SB) with A n -DB and A n -VB present. A n -DB running dorsad, bifurcating into Y-shaped junction with A n -DLT anteriad and posteriad; all abdominal segments connected via dorsal trunk. A n -VB running ventrad, connecting with opposite side via thin A n -VC; A1..7-VC present in T. domestica , A4..7-VC not visible in L. saccharinum scan but likely present; A8-VC absent. A n -DB-Vi and A n -VB-Vi numerous, see sections below for details. A8-DB and A8-VB large, curving medially before proceeding posteriad; A8-DB with A-TF and A-Cr in T. domestica but not visible in L. saccharinum scan (presence/ absence ambiguous).