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Sometimes birds can use the air to float on with their spread wings. Now, the wings of insects are too small to see if they can do that too. So is it possible for insects to do this also, or can't they hold their wings spread or is the air drag not enough?

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  • $\begingroup$ "Insect" is a very broad catagory. Simplistically, some can: ever watch large butterflies? Just as some birds, like hummingbirds, can't glide well, if at all. $\endgroup$ – jamesqf Aug 19 '17 at 18:58
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It's worth mentioning that insects' flight is somehow different from birds' one, mainly because of the Reynolds number.

However, there is evidence of (some) insects gliding. For instance, Walkeling and Ellington (1997) have filmed gliding in dragonflies and damselflies:

The free gliding flight of the dragonfly Sympetrum sanguineum was filmed in a large flight enclosure. Reconstruction of the glide paths showed the flights to involve accelerations. Where the acceleration could be considered constant, the lift and drag forces acting on the dragonfly were calculated. The maximum lift coefficient (CL) recorded from these glides was 0.93; however, this is not necessarily the maximum possible from the wings. Lift and drag forces were additionally measured from isolated wings and bodies of S. sanguineum and the damselfly Calopteryx splendens in a steady air flow at Reynolds numbers of 700-2400 for the wings and 2500-15 000 for the bodies. The maximum lift coefficients (CL,max) were 1.07 for S. sanguineum and 1.15 for C. splendens, which are greater than those recorded for all other insects except the locust. The drag coefficient at zero angle of attack ranged between 0.07 and 0.14, being little more than the Blassius value predicted for flat plates. Dragonfly wings thus show exceptional steady-state aerodynamic properties in comparison with the wings of other insects. A resolved-flow model was tested on the body drag data. The parasite drag is significantly affected by viscous forces normal to the longitudinal body axis. The linear dependence of drag on velocity must thus be included in models to predict the parasite drag on dragonflies at non-zero body angles. (emphasis mine)


Source:

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    $\begingroup$ Good find this article! I spend quite some time in the field observing dragonflies, and from this experience I can verify that at least some dragonflies alternate active flight with short gliding (few seconds). I observed this only in large aeshnidae, like Aeshna viridis and Anax imperator. I can't remember seeing it in Calopterix or Sympetrum, although these are very common here. But if these smaller species glide for very short times, this may be difficult to see without camera footage. $\endgroup$ – RHA Aug 19 '17 at 11:17

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