1. Movements of the wings and pterothorax of Aeshna spp. have been studied by stroboscopic observation combined with the cutting of certain muscles.

2. There is a constant phase difference between the contractions of the mesothoracic and metathoracic flight muscles, and a resulting phase difference between the movements of the wing-base sclerites and wings in the two pterothoracic segments.

3. As the wings in each segment move upwards and backwards, the respective notal sclerites are moving forwards and downwards. Conversely, as the wings beat forwards and downwards, the nota move backwards and upwards.

4. The anterior coxoalar elevator muscle pulls on the anterior lobe of the lateroprescutum and so, via the membrane on the posterior edge of the humeral complex. This causes extensive supination of the wing, important in hovering and backward flight, and is due to a sudden snap-like separation of the humeral and axillary complexes about a line of weakness between them when the descending wing is at 10° from its lowest position.

5. Both remotion at the end of the downstroke, and the complex propeller-like supinated wing section which develops during the last 10° of the downstroke and most of the upstroke, are made possible by the line of weakness running along the membrane between the two complexes.

6. Comparison of manipulated wingstrokes with normal wingstrokes showed that supination is actively controlled in the flying insect by muscular contraction.

7. Scutal ascension, described in the paper, was detected. At any point on the downstroke, the scuta occupy higher and more posterior positions than they do at the corresponding point on the upstroke.

8. Fore and aft movements of the nota are greater during flapping flight at low wingbeat frequency, than during full flight at higher wingbeat frequency.

9. The prescutum, scutum and scutellum move together as a unit up and down and fore and aft, throughout the wingstroke cycle.

10. Supination during the last 10° of the downstroke and the first half of the upstroke is due to the anterior coxoalar muscle, and during the second half of the upstroke to the third subalar muscle. Downstroke pronation/supination balance is controlled by the basalars and second subalar. A detailed summary of these rotation balances is given in the discussion.

11. The oblique intersegmental muscle of the metathorax is shown to be an abdomen elevator and not a wing depressor. The functions of the various flight muscles are demonstrated experimentally for the first time. The anterior coxoalar muscle is shown to be specialized, functioning in elevation and supination and assisting remotion.

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