Annual report of the Board of Regents of the Smithsonian Institution . ects, orPterygota, from paranotal lobes. ThePterygota are characterized by a specialtype of structure in the plates formingthe lateral, or pleural, walls of the thoracic segments, the pleural struc-ture being essentially the same in the wingless prothorax as in the twowing-bearing segments. In the Apterygota the corresponding platesare quite different from those of the winged insects, and differ much inthe several apterygote families. We must conclude, therefore, thatthe peculiar structure of the pleural walls of the thorac
Annual report of the Board of Regents of the Smithsonian Institution . ects, orPterygota, from paranotal lobes. ThePterygota are characterized by a specialtype of structure in the plates formingthe lateral, or pleural, walls of the thoracic segments, the pleural struc-ture being essentially the same in the wingless prothorax as in the twowing-bearing segments. In the Apterygota the corresponding platesare quite different from those of the winged insects, and differ much inthe several apterygote families. We must conclude, therefore, thatthe peculiar structure of the pleural walls of the thoracic segments inthe Pterygota, and their basic uniformity in the three segments of thethorax means that the walls of these segments once served a commonpurpose. This purpose evidently was the support of a pair of para-notal lobes on each thoracic segment. (Fig. 6.) The lack of pleuralplates in the lateral walls of the abdominal segments similar to thoseof the thorax suggests that paranotal lobes of the abdomen, if present,never reached a size of functional FiGVRE ii.—A mantis from Ecuador withl^rge lateral extensions of the back plateof the prothorax HOW INSECTS FLY SNODGRASS 389 II. STRUCTURE OF THE WINGS It is a long wny from immovable paranotal lobes to fullj devel-oped wings capable of sustained flight. In the first step toward thestatus of wings, the paranotal lobes of insects had simultaneously tobecome flexible at their bases and to acquire a motor mechanismthat would impart to them the movements necessary in organs offlight. Flight, it must be observed, is not to be accomplished by amere up-and-down flapping of a pair of flat appendages; as will beexplained later, in order to generate forward motion, wings must becapable not only of movements in a vertical direction, but also ofsome degree of anterior and posterior movement accompanied by apartial rotation on their long axes. The mechanism of an insectswings, therefore, must be such as to produce at least t
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