Annual report of the Board of Regents of the Smithsonian Institution . all, the two layersof which eventually come together and formthe upper and lower surfaces of the maturewing. Semitubular thickenings of the op-posed surfaces give rise to the veins. Thecanals of the veins represent, therefore, theremnants of the original wing cavity, andthey contain the tracheae, nerves, and bloodof the wings. Among the Carboniferous insects thereare frequently found species which havea pair of small, flat lobes resembhng thedeveloping wing pads, which extend laterally from the margins of thefirst segment o
Annual report of the Board of Regents of the Smithsonian Institution . all, the two layersof which eventually come together and formthe upper and lower surfaces of the maturewing. Semitubular thickenings of the op-posed surfaces give rise to the veins. Thecanals of the veins represent, therefore, theremnants of the original wing cavity, andthey contain the tracheae, nerves, and bloodof the wings. Among the Carboniferous insects thereare frequently found species which havea pair of small, flat lobes resembhng thedeveloping wing pads, which extend laterally from the margins of thefirst segment of the thorax, or prothorax. (Fig. 2 A, B, ^^Ti/.) These pro-thoracic lobes suggest, therefore, that the wings have been evolved fromsimilar lobes on the mesothorax and meta thorax. There is no evidenceto suggest the improbable view that insects ever had three pairs offully developed wings used for flight; but we may assume that threepairs of flaps, or paranotai lobes as they have been termed, forming aseries of overlapping plates on each side of the thorax (fig. 3),. Figure 3.—A suggestion of the pos-sible structure of a primitive in-sect provided with three pairs ofglider lobes, or lateral extensionsof the back plates of the thoracicsegments HOW INSECTS FLY—SNODGRASS 387 could have served some important function. The idea that comesmost easily to the imagination is that the fanlike extensions of thebody wall constituted a glider apparatus, enabling its possessor tosail downward from elevated positions, or perhaps from one elevationto another, as do the modern flying squirrels with their parachute-hke folds of skin stretched between the legs on the sides of the body. Something of the possibilities of a sailing or gUding insect may bedemonstrated with a model cut from a piece of thin cardboard ac-cording to the pattern of Figure 4, and given a proper ballast byattaching a weight beneath. When passively released from any altitudethe cardboard model drops straight down; whengi
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Keywords: ., bookauthorsmithsonianinstitutio, bookcentury1800, bookdecade1840
