Are insects capable of gliding on the air with their wings spread, similar to birds?
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: 


JM Wakeling, CP Ellington, Dragonfly flight. I. Gliding flight and steady-state aerodynamic forces. Journal of Experimental Biology 1997 200: 543-556.