Meganeura is a genus of extinct insects from the Late Carboniferous (approximately 300 million years ago). It is a member of the extinct order Meganisoptera, which are closely related to and resemble dragonflies and damselflies (with dragonflies, damselflies and meganisopterans being part of the broader group Odonatoptera). Like other odonoapterans, they were predatory, with their diet mainly consisting of other insects. The genus belongs to the Meganeuridae, a family including other similarly giant dragonfly-like insects ranging from the Late Carboniferous to Middle Permian. With single wing length reaching 32 centimetres (13 in)[1] and a wingspan about 65–75 cm (2.13–2.46 ft),[2][3][4]M. monyi is one of the largest-known flying insect species.
Wing venation of Meganeura monyi, redrawn after Brongniart (1893, Pl. XLI)
Research on close relatives Meganeurula and Meganeurites suggest that Meganeura was adapted to open habitats, and similar in behaviour to extant hawkers. The eyes of Meganeura were likely enlarged relative to body size. Meganeura had spines on the tibia and tarsi sections of the legs, which would have functioned as a "flying trap" to capture prey.[5] An engineering examination estimated that the mass of the largest specimens with wingspans over 70 cm to be 100 to 150 grams. The analysis also suggested that Meganeura would be susceptible to overheating.[6]
Size
There has been some controversy as to how insects of the Carboniferous period were able to grow so large.
Oxygen levels and atmospheric density. The way oxygen is diffused through the insect's body via its tracheal breathing system puts an upper limit on body size, which prehistoric insects seem to have well exceeded. It was originally proposed by Harlé (1911) that Meganeura was able to fly only because the atmosphere of Earth at that time contained more oxygen than the present 20 percent. This hypothesis was initially dismissed by fellow scientists, but has found approval more recently through further study into the relationship between gigantism and oxygen availability.[7] If this hypothesis is correct, these insects would have been susceptible to falling oxygen levels and certainly could not survive in our modern atmosphere. Other research indicates that insects really do breathe, with "rapid cycles of tracheal compression and expansion".[8] Recent analysis of the flight energetics of modern insects and birds suggests that both the oxygen levels and air density provide an upper bound on size.[9] The presence of very large Meganeuridae with wing spans rivaling those of Meganeura during the Permian, when the oxygen content of the atmosphere was already much lower than in the Carboniferous, presented a problem to the oxygen-related explanations in the case of the giant dragonflies. However, despite the fact that meganeurids had the largest-known wingspans, their bodies were not very heavy, being less massive than those of several living Coleoptera; therefore, they were not true giant insects, only being giant in comparison with their living relatives.
Lack of predators. Other explanations for the large size of meganeurids compared to living relatives are warranted.[1]Bechly (2004) suggested that the lack of aerial vertebrate predators allowed pterygote insects to evolve to maximum sizes during the Carboniferous and Permian periods, perhaps accelerated by an evolutionary "arms race" for increase in body size between plant-feeding Palaeodictyoptera and Meganisoptera as their predators.
Aquatic larvae stadium. Another theory suggests that insects that developed in water before becoming terrestrial as adults grew bigger as a way to protect themselves against the high levels of oxygen.[10]
^Chapelle & Peck 1999: "Oxygen supply may also have led to insect gigantism in the Carboniferous period, because atmospheric oxygen was 30-35% (ref. 7). The demise of these insects when oxygen content fell indicates that large species may be susceptible to such change. Giant amphipods may therefore be among the first species to disappear if global temperatures are increased or global oxygen levels decline. Being close to the critical MPS limit may be seen as a specialization that makes giant species more prone to extinction over geological time.
^Westneat et al. 2003: "Insects are known to exchange respiratory gases in their system of tracheal tubes by using either diffusion or changes in internal pressure that are produced through body motion or hemolymph circulation. However, the inability to see inside living insects has limited our understanding of their respiration mechanisms. We used a synchrotron beam to obtain x-ray videos of living, breathing insects. Beetles, crickets, and ants exhibited rapid cycles of tracheal compression and expansion in the head and thorax. Body movements and hemolymph circulation cannot account for these cycles; therefore, our observations demonstrate a previously unknown mechanism of respiration in insects analogous to the inflation and deflation of vertebrate lungs.
^Dudley 1998: "Uniformitarian approaches to the evolution of terrestrial locomotor physiology and animal flight performance have generally presupposed the constancy of atmospheric composition. Recent geophysical data, as well as theoretical models, suggest that, to the contrary, both oxygen and carbon dioxide concentrations have changed dramatically during defining periods of metazoan evolution. Hyperoxia in the late Paleozoic atmosphere may have physiologically enhanced the initial evolution of tetrapod locomotor energetics; a concurrently hyperdense atmosphere would have augmented aerodynamic force production in early flying insects. Multiple historical origins of vertebrate flight also correlate temporally with geological periods of increased oxygen concentration and atmospheric density. Arthropod as well as amphibian gigantism appear to have been facilitated by a hyperoxic Carboniferous atmosphere and were subsequently eliminated by a late Permian transition to hypoxia. For extant organisms, the transient, chronic and ontogenetic effects of exposure to hyperoxic gas mixtures are poorly understood relative to the contemporary understanding of the physiology of oxygen deprivation. Experimentally, the biomechanical and physiological effects of hyperoxia on animal flight performance can be decoupled through the use of gas mixtures that vary in density and oxygen concentration. Such manipulations permit both paleophysiological simulation of ancestral locomotor performance and an analysis of maximal flight capacity in extant forms.
Bechly, G (2004). "Evolution and systematics"(PDF). In Hutchins, M.; Evans, A.V.; Garrison, R.W. & Schlager, N. (eds.). Grzimek's Animal Life Encyclopedia. Vol. Insects (2nd ed.). Farmington Hills, MI: Gale. pp. 7–16.
Harlé, Edouard (1911). "Le Vol de grands reptiles et insectes disparus semble indiquer une pression atmosphérique élevée". Extr. Du Bulletin de la Sté Géologique de France (in French). 4 (9): 118–121.
