Tegmen

A tegmen (plural: tegmina; from the Latin tegmen, meaning "covering" or "mat," derived from tegere, "to cover") is the modified, parchment-like or leathery (coriaceous) mesothoracic forewing characteristic of several polyneopteran orders of insects, most notably Orthoptera (grasshoppers, locusts, katydids, and crickets), Blattodea (cockroaches), Mantodea (praying mantises), and Phasmatodea (stick and leaf insects), as well as certain Auchenorrhyncha within Hemiptera (such as froghoppers and planthoppers). Structurally intermediate between a delicate membranous wing and the heavily armored, veinless elytron of Coleoptera (or the bipartite hemelytron of heteropteran true bugs), a tegmen is moderately thickened across its entire blade via quinone cross-linking of the insect cuticle yet retains the complete ancestral pterygote network of hollow longitudinal veins (costa, subcosta, radius, media, cubitus, and anal veins) interconnected by dense rectilinear or reticulate cross-veins. At rest, the left and right tegmina fold roof-like or overlap flat across the dorsal abdomen to shield the broad, fan-folded metathoracic hindwings; during active behavior, they execute diverse biomechanical roles ranging from cambered leading-edge lift generation in migratory locust swarms to resonant acoustic stridulation and leaf-mimicking camouflage (Wootton, 1992; Chapman et al., 2013).

Retention of the Polyneopteran Venation Scaffold

Unlike coleopteran elytra—in which the primary wing veins are largely obliterated and replaced by longitudinal rows of cuticular pits (striae) and vertical trabecular pillars—the polyneopteran tegmen preserves all six principal longitudinal vein trunks radiating from the basal axillary sclerites (Snodgrass, 1935; Béthoux & Nel, 2002):

1. Costal (C) and Subcostal (Sc) Fields: Along the anterior leading margin of the tegmen, the costa and subcosta form thick, sclerotized tubular spars perfused by hemolymph and tracheae, conferring high flexural stiffness against bending moments during the downstroke of insect flight.
2. Radial (R) and Medial (M) Sectors: Running down the central axis of the wing blade, the radial and medial veins branch dichotomously toward the apical margin, separated by rectangular or polygonal cells (areolae) whose cuticular membrane is thickened to 8–25 μm (compared to 1–3 μm in the metathoracic hindwing).
3. Cubital (Cu) and Anal (A) Fields and the Claval Fold: Separated from the remigium (the anterior flight-active zone) by a deep longitudinal flexion line called the claval furrow (running just behind vein CuP), the posterior anal fan (clavus) folds flat over the mesonotum and metanotum when the insect is at rest. At the intersections between cross-veins and longitudinal veins, microscopic patches of the elastomeric protein resilin act as compliant micro-joints that absorb impact loads and allow reversible chordwise cambering without permanent plastic deformation.

Comparative Functional Specializations of Tegmina Across Polyneoptera

One of the most sophisticated evolutionary exaptations of the insect tegmen occurs in the order Orthoptera, where the sclerotized veins of the forewing serve as coupled mechanical oscillators and acoustic radiators for species-specific mate attraction and territorial rivalry (Bennet-Clark, 1989; Montealegre-Z et al., 2012):

Tegmino-Tegminal Stridulation in Crickets and Katydids (Ensifera)

In male field crickets (Gryllidae, such as Gryllus bimaculatus) and bush-crickets/katydids (Tettigoniidae), acoustic songs are produced by rubbing the bases of the two overlapping tegmina together (tegmino-tegminal stridulation), driven by rhythmic central pattern generators in the mesothoracic ganglion modulated by the central complex and subesophageal ganglion:

• The File (Pars Stridens) and Scraper (Plectrum): On the ventral surface of one tegmen, a modified transverse segment of the posterior cubital vein (CuP / Cu₂) carries a row of 50 to 300 microscopic sclerotized cuticular teeth forming the stridulatory file. On the inner anal margin of the opposing tegmen lies a thickened, upturned cuticular ridge called the scraper (plectrum).
• The Harp and Mirror Acoustic Resonators: As the wings close, the scraper catches and releases successive teeth on the file at a tooth-strike rate matching the carrier frequency of the song (~4.5–5.0 kHz in Gryllus; 10 to >100 kHz ultrasonic calls in neotropical katydids). These periodic mechanical impulses excite two thin, taut, triangular or circular membrane cells framed by modified cubital veins—the harp and the mirror—which vibrate as coupled acoustic dipoles to radiate narrow-band sound waves at sound pressure levels reaching 90 to 110 dB SPL at 10 cm.

Tegmino-Femoral Stridulation in Grasshoppers (Caelifera)

In short-horned grasshoppers (Acrididae: Gomphocerinae and Oedipodinae), the stridulatory mechanism is inverted between the leg and the tegmen (tegmino-femoral stridulation): the inner surface of the enlarged saltatorial hind femur bears a longitudinal row of 80 to 120 articulated peg-like sensilla (stridulatory pegs) that are scraped up and down across a raised, intercalary radial or medial vein on the lateral surface of the closed tegmen.

Although the broad, fan-like metathoracic hindwings of the desert locust (Schistocerca gregaria) provide ~70–75% of total lift and thrust during long-distance migratory flight powered by corpora cardiaca adipokinetic hormone mobilization, high-speed cinematic and wind-tunnel particle image velocimetry studies by R. J. Wootton and J. M. Wakeling demonstrated that the mesothoracic tegmina are essential aerodynamic airfoils (Wootton, 1992; Henningsson et al., 2014). Beating at ~20 Hz with a phase lag behind the hindwings, each tegmen undergoes automatic umbrella-effect cambering during the downstroke: positive air pressure beneath the wing depresses the posterior anal lobe along the claval furrow while the rigid costal-radial leading-edge spar resists torsion, forming a high-lift cambered profile (Z-profile in cross-section) that generates the remaining 25–30% of aerodynamic lift and sheds a wake vortex that enhances the aerodynamic efficiency of the trailing hindwing.

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