D

FIGURE 7 Paper model of right wing of Cantharis sp. arranged to demonstrate wing folding. Cross-hatched areas face ventrally in fully folded wing. In the extended wing (A) the principal veins—radius (R) and cubitus

(C)—are apart by muscular action from the wing base. When this action ceases, the wing apex automatically folds (B, C) until wing is fully folded

(D). [From Hammond, P. M. (1979). Wing-folding mechanisms of beetles, with special reference to investigations of adephagan phylogeny. In "Carabid Beetles: Their Evolution, Natural History, and Classification" (T. L. Erwin, G. E. Ball, D. R. Whitehead, and A. L. Halpern, eds.), p. 122, Fig. 1. Junk, the Hague. With kind permission of Kluwer Academic Publishers.]

direction. Beetles accomplish this increase in body volume through dorsoventral expansion of the abdomen.

In a newly eclosing beetle, the lateral reaches of the abdominal tergites lie both between and below the lateral portions of the abdominal sternites. The tergites and sternites are joined by extensive membranes, within which the spiracles are situated. These membranes may stretch, and the tergites may move dorsally relative to the stationary lateral margins of the sternites, dramatically increasing the volume of the abdomen. This volumetric expansion is accomplished without any compromise to the external armor represented

FIGURES 8-11 Hind wings of Coleoptera. (8) Omma stanleyi (Ommatidae): AA, anal anterior; AP, anal posterior; C, costa; CuA, cubitus anterior; J, jugal; MP, medial posterior; R, radius; RA, radius anterior; r-m, radial-median crossvein; RP, radius posterior; r-r, radial crossvein; Sc, subcosta. (9) Adinolepis mathesoni (Cupedidae). (10) Open hind wing of Microsporus vensensis (Microsporidae). (11) Folded wing of M. ovensensis (Images provided by copyright holder, CSIRO Entomology, Canberra, ACT, Australia.)

FIGURES 8-11 Hind wings of Coleoptera. (8) Omma stanleyi (Ommatidae): AA, anal anterior; AP, anal posterior; C, costa; CuA, cubitus anterior; J, jugal; MP, medial posterior; R, radius; RA, radius anterior; r-m, radial-median crossvein; RP, radius posterior; r-r, radial crossvein; Sc, subcosta. (9) Adinolepis mathesoni (Cupedidae). (10) Open hind wing of Microsporus vensensis (Microsporidae). (11) Folded wing of M. ovensensis (Images provided by copyright holder, CSIRO Entomology, Canberra, ACT, Australia.)

by the cuticle. The soft, flexible abdominal tergites are protected by the elytra, except when the beetle is flying. At this time the soft membranes and flexible tergites are vulnerable to attack by predators or parasites.

FIGURE 12 Wing-folding spicule patches on abdominal terga, Xylodromus concinnus (Staphylinidae) (see Fig. 7). [From Hammond, P. M. (1979). Wing-folding mechanisms of beetles, with special reference to investigations of adephagan physiology. In "Carabid Beetles: Their Evolution, Natural History, and Classification" (T. L. Erwin, G. E. Ball, D. R. Whitehead, and A. L. Halpern, eds.), p. 122, Fig. 1. Junk, the Hague. With kind permission of Kluwer Academic Publishers.]

FIGURE 12 Wing-folding spicule patches on abdominal terga, Xylodromus concinnus (Staphylinidae) (see Fig. 7). [From Hammond, P. M. (1979). Wing-folding mechanisms of beetles, with special reference to investigations of adephagan physiology. In "Carabid Beetles: Their Evolution, Natural History, and Classification" (T. L. Erwin, G. E. Ball, D. R. Whitehead, and A. L. Halpern, eds.), p. 122, Fig. 1. Junk, the Hague. With kind permission of Kluwer Academic Publishers.]

In the floricolous, day-flying Buprestidae and scarab beetles of the subfamily Cetoniinae, flight is undertaken without significant separation or lifting of the elytra, with the metatho-racic wings extended under the lateral elytral margins. In the buprestids, this posture allows the aposematic coloration of the elytra to be visible both in flight and at rest. In other polyphagans and the Archostemata, the elytra are held at an angle during flight, beating synchronously with the flight wings, and thereby providing some degree of aerodynamic lift.

Given the need to exchange oxygen and carbon dioxide at a liquid interface on the surfaces of the tracheolar cells, respiration represents the major activity through which an insect can lose water. This source of water loss is of particular importance for an animal of small body volume. The beetle respiratory system opens via large metathoracic spiracles and up to eight pairs of abdominal spiracles, all of which open onto the subelytral cavity. Thus, in addition to controlling gas exchange via the spiracular openings, a beetle can modulate respiration by the position of the abdominal venter relative to the elytra. Reduction of the elytra to the quadrate condition seen in Staphylinidae has resulted in secondary exposure of the abdominal spiracles.

Beetles have invaded freshwater aquatic habitats several times during their evolutionary history. In all instances, adult aquatic beetles retain the spiracular respiratory system of their terrestrial relatives, requiring that they regularly have access to atmospheric gases. The subelytral space provides the means to hold an air bubble while the beetle is active underwater. This bubble can be replenished by periodic surfacing of the beetle, during which the tip of the abdomen breaks the water surface, permitting exchange of gases.

Because of the makeup of our atmosphere (79% nitrogen, 21% oxygen), the subelytral air bubble serves as a compressible gill, permitting extended underwater sojourns. As the beetle uses oxygen, more oxygen diffuses into the bubble from the surrounding water. The carbon dioxide produced through the beetle's respiration, being highly soluble in water, quickly leaves the bubble. Because nitrogen dissolves slowly into the water, there is a gradual reduction in bubble size. The beetle can use up to eight times as much oxygen than was in the original bubble before being required to surface to replenish its air supply. Swimming beetles using these simple subelytral compressible gills include various Adephaga (e.g., Haliplidae and Dytiscidae).

In other families of the aquatic realm, oxygen is supplied to the subelytral bubble by a plastron composed of microfuge hairs or other columnar evaginations of the cuticle that are close together along their outer surface, excluding water by its surface-filming qualities. Oxygen diffuses into the plastron without any change in plastron gas volume, allowing the beetle to remain indefinitely below the water's surface. Nonetheless, plastron respiration can work only in highly oxygenated water, so beetles with plastrons are usually found in moving waters. Plastron breathers also are less active than the adephagan compressible gill breathers, because the plastron cannot provide the high levels of oxygen required for intense activity. This type of structure has evolved repeatedly in the order, being found in the Hydrophilidae, Dryopidae, Elmidae, and some Curculionidae.

FIGURES 13-14 (13) Abaris bigenera mature larva (Carabidae), dorsal view (larval length, 8.3 mm). (Image © F. L. Fawcett.) (14) Right larval mandible, ventral view, Platynus sp. (Carabidae). Note absence of basal mola, and presence of large retinacular tooth and serrate incisor. [From Lawrence, J. F. (1991). Order Coleoptera. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.19. Kendall/Hunt, Dubuque, IA.]

FIGURES 13-14 (13) Abaris bigenera mature larva (Carabidae), dorsal view (larval length, 8.3 mm). (Image © F. L. Fawcett.) (14) Right larval mandible, ventral view, Platynus sp. (Carabidae). Note absence of basal mola, and presence of large retinacular tooth and serrate incisor. [From Lawrence, J. F. (1991). Order Coleoptera. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.19. Kendall/Hunt, Dubuque, IA.]

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