Ecology And Diversity Wing Arrangement

The origin of wings was followed by an explosive diversification of insect orders. Many Carboniferous insects possessed wings of approximately equivalent size, shape, and aerodynamic function that were probably limited to low-amplitude flapping. Equivalently sized fore- and hind wings persist to this day in at least seven orders. However, major differences in the sizes of meso- and metathoracic wings are evident in both contemporary fauna and fossils from the Paleozoic. With the exception of the Coleoptera and Strepsiptera, enlarged hind wings are for the most part confined to extant exopterygote orders. Many endopterygote orders (Hymenoptera, Lepidop-tera, Diptera), by contrast, reduce the aerodynamic role of the hind wings. In many insects in which the hind wings provide aerodynamic force, the forewings have been modified for supplemental function. Far from isolated events, the evolutionary transformation of the forewing into either a tegmen or an elytron has occurred at least three times at the ordinal level. Elytra of the Coleoptera have much reduced aerodynamic roles relative to the hind wings and provide for greater mechanical resistance to crushing in conjunction with increased sclerotization of the body as a whole. A similar functional role may be hypothesized for tegminized forewings (e.g., Blattodea and Orthoptera) and for the hemelytra of Hemiptera. Insect wings may also serve a variety of behavioral functions unrelated to flight, including sound production and visual communication. None of these functions are mutually exclusive, although the role of aerodynamic force production remains paramount for at least one wing pair.

Flightlessness

The behavioral and ecological advantages of flight notwithstanding, flightlessness has evolved independently many times in insects. Approximately 5% of the extant insect fauna may be classified as flightless, if all forms of variable wing expression and of reduced flight musculature are included. One common feature of the otherwise diverse manifestations of flightlessness is a reduced need for locomotor mobility. Selection for maintaining flight may be weak if this capability is not required for dispersal, reproductive behavior, or predator avoidance. Even in flying species, the costly development of wings and associated musculature may not occur under all ecological conditions.

Flight Diversity and Body Size

Changes in body size represent major trends in the evolution of winged insects. Although direct paleontological evidence is not available, body lengths of the first flying insects were probably in the range of 2 to 4 cm. Substantial increases in body length appear to have occurred by the mid-Carboniferous, and gigantism relative to today's forms was typical of many late Paleozoic insects as well as of other arthropods. The most parsimonious explanation for Paleozoic gigantism is a contemporaneous increase in atmospheric oxygen concentrations, possibly to values as high as 35% relative to today's 21%. Such high oxygen concentrations, together with higher diffusion constants due to an increase in total atmospheric pressure, would have relaxed diffusional constraints on flight metabolism and thus would have permitted the evolution of giant flying forms. Increased atmospheric density would also have yielded increased augmented lift production during flight, both effects possibly being advantageous during the initial periods of wing evolution. Furthermore, geophysical evidence suggests a decline in atmospheric oxygen concentration through the mid- to end-Permian. As would be consistent with asphyxiation on a geological time scale, all giant terrestrial arthropod taxa of the late Paleozoic went extinct by the end of the era.

In sharp contrast to the late Paleozoic giants, the contemporary insect fauna is characterized by a diversity of miniaturized forms. For example, mean adult beetle body length lies between 4 and 5 mm. Much of the wealth of dipteran and hymenopteran diversity is similarly associated with small body sizes, particularly among the parasitoid and hyperparasitoid taxa. Wingbeat frequencies vary inversely proportional to body size, and today's small insects typically fly with wingbeat frequencies in excess of 100 Hz, rates achievable only with the use of asynchronous muscle. Thus, muscle type synchronous ■ ■■■■■ synchronous & asynchronous

asynchronous equivocal apterous

\ ^

FIGURE 3 Phylogenetic distribution of asynchronous flight muscle. The paraphyletic assemblage Homoptera is here represented at lower taxonomic levels of suborders and superfamilies. Equivocal branch designations indicate either an unknown (e.g., Zoraptera) or an unresolved character state.

Ephemeroptera Odonata Plecoptera Embioptera Orthoptera Phasmatodea Grylloblattodea Dermaptera Isoptera Mantodea Blattaria Zoraptera Thysanoptera Sternorrhyncha Cicadoidea Cercopoidea Cicadelloidea Archaeorrhyncha Prosorrhyncha Psocoptera

Phthiraptera

Coleoptera

Neuroptera Megaloptera Raphidioptera

Hymenoptera

Trichoptera Lepidoptera

Strepsiptera Diptera

Siphonaptera Mecoptera

FIGURE 3 Phylogenetic distribution of asynchronous flight muscle. The paraphyletic assemblage Homoptera is here represented at lower taxonomic levels of suborders and superfamilies. Equivocal branch designations indicate either an unknown (e.g., Zoraptera) or an unresolved character state.

the acquisition of asynchronous flight muscle has played a major role in morphological diversification among different insect orders. Asynchronous flight muscle is phylogenetically derived relative to synchronous precursors and has evolved repeatedly among pterygote lineages (Fig. 3). Because flight at small body sizes mandates elevated wingbeat frequencies, this repeated evolutionary acquisition of asynchronous muscle may have facilitated taxonomic radiations of small insects. For example, systematic comparison of sister insect lineages that differ in muscle type statistically demonstrates a decrease in mean body size and an increase in species number if asynchronous flight muscle is present. Three-quarters of all described insect species and three of the four largest orders (i.e., Coleoptera, Diptera, and Hymenoptera) are characterized by asynchronous muscle. Because higher wingbeat frequencies yield increased aerodynamic force, asynchronous muscle may also permit a reduced wing area relative to body mass. This effect may help to explain how one wing pair in many insect groups evolved nonaerodynamic roles.

Flight Behavior and Ecology

Flight plays a central role in the life history patterns of most pterygote insects. A partial list of important insect behaviors mediated by flight includes pollination, phytophagy, hema-tophagy, escape from predators, mate acquisition, and migration. Forces of both natural and sexual selection have demanded ever-increasing flight performance from insects through evolutionary time, whereas different selective agents are often mutually reinforcing. For example, intra- and intersexual selection often acts synergistically on maneuverability, as does escape from predation attempts by bats, birds, and other insects. Coevolutionary defensive responses among insects, including increased maneuverability and erratic flight styles, parallel the diverse radiations of insectivorous vertebrates worldwide. The morphological and behavioral mimicry among certain chemically defended insects provides wonderful testimony to the strength of such natural selection.

Another major coevolutionary theme in the terrestrial biosphere concerns relationships between flying insects and plants. Phytophagy and pollination by insects are particularly influenced by three-dimensional aerial mobility, the capacity for which dramatically increases access to nutritional resources and suitable oviposition sites. The antiquity of such interactions is well demonstrated by fossil evidence for feeding on plants in the Upper Carboniferous, whereas high rates of herbivory imposed by insects characterize most present-day floras. The evolutionary presence of flying insects has similarly influenced the reproductive biology of many plants. Contemporary angiosperms are pollinated primarily by a broad diversity of insect taxa, most of which are miniaturized forms that can hover at flowers either before or during pollination. Small body size facilitates both incidental and intentional dispersal by wind, and as a consequence tiny insects can act as long-distance pollen vectors.

Continuous aerial entrainment by winds interacts with the large individual numbers of insects worldwide to result in a transient but substantial population of insects moving at heights up to 10 km from the earth's surface. Remarkably, insects from continental faunas have been captured in the mid-Pacific far from any land mass or island. The ability to decouple the flight trajectory from ambient winds depends on the relative magnitude of insect airspeeds, which but rarely exceed typical wind speeds. Thus, directed movement is likely only a few meters from the ground or within canopies of vegetation. Dispersal, on the other hand, is readily attained simply by flying upward into moving air masses. Even migratory flights of larger, more powerful insects (such as locusts and butterflies) are influenced by the directionality of prevailing winds.

See Also the Following Articles

Anatomy • Migration • Muscle System • Odonata • Swimming • Walking and Jumping • Wings

Further Reading

Brodsky, A. K. (1994). "The Evolution of Insect Flight." Oxford University Press, Oxford.

Dalton, S. (1975). "Borne on the Wind: The Extraordinary World of Insects in Flight." Reader's Digest Press, New York.

Dickinson, M. H. (2001). Solving the mystery of insect flight. Sci. Am. 284, 34-41.

Dickinson, M., Lehmann, F.-O., and Sane, S. (2001). Wing rotation and the aerodynamic basis of insect flight. Science 284, 1881-2044.

Dudley, R. (2000). "The Biomechanics of Insect Flight: Form, Function, Evolution." Princeton University Press, Princeton, NJ.

Ellington, C. P. (1999). The novel aerodynamics of insect flight: Applications to micro-air vehicles. J. Exp. Biol. 202, 3439-3448.

Ellington, C. P., Van den Berg, C., Willmot, A. P., and Thomas, A. L. R. (1996). Leading edge vortices in insect flight. Nature 384, 626-630.

Harrison, J. F., and Roberts, S. P. (2000). Flight respiration and energetics. Annu. Rev. Physiol. 62, 179-205.

Heinrich, B. (1993). "The Hot-Blooded Insects: Strategies and Mechanisms of Thermoregulation." Harvard University Press, Cambridge, MA.

Josephson, R., Malamud, J. G., and Stokes, D. R. (2000). Asynchronous muscle: A primer. J. Exp. Biol. 203, 2713-2722.

Marden, J. H. (2000). Variability in the size, composition, and function of insect flight muscles. Annu. Rev. Physiol. 62, 157-178.

Nachtigall, W. (1974). "Insects in Flight: A Glimpse behind the Scenes in Biophysical Research." McGraw-Hill, New York.

Wootton, R. J. (1992). Functional morphology of insect wings. Annu. Rev. Entomol. 37, 113-140.

Young, D., and Simmons, P. (1999). "Nerve Cells and Animal Behavior," 2nd edition. Cambridge University Press, Cambridge, UK.

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