Dragon-flies and Damsel-flies 79 



on the wing. But of course all dragon-flies rest sometimes, and some of 

 them, especially the damsel-flies, are at rest most of the time, clinging to 

 stems or leaves by the water's edge. The larger kinds may be found 

 occasionally perched on the tips of tall swaying reeds, or on a stump or 

 projecting dead limb. From these coigns of vantage they swoop like 

 a hawk on any rash midge that ventures awing in the neighborhood. 

 Cold or cloudy weather, or a strong wind, will drive most dragon-flies to 

 shelter. 



The Odonata are unexcelled among insects for swiftness, stra'ghtness, 

 and quick angular changes in direction of flight. The successful main- 

 tenance of their predatory life depends upon this finely developed flight 

 function together with certain structural and functional body conditions 

 which might be said to be accessory or auxiliary to it. And this may be 

 an appropriate place to describe briefly a few of their salient structural 

 characteristics. 



All dragon-flies have four well-developed wings, and all show such a 

 similar general bodily make-up and appearance, that from an acquaintance- 

 ship with two or three familiar species any member of the order can be 

 recognized as really belonging to the group. The body in all is long, smooth, 

 and subcylindrical or gently tapering. This clean, slender body offers 

 little resistance to the air in flight, and serves as an effective steering-oar. 

 The wings are long and comparatively narrow, fore and hind wings being 

 much alike, almost exactly alike indeed in the damsel-flies. The venation 

 is of the general type known as net-veining (Fig. 1146), the few strong longi- 

 tudinal veins being connected by many short cross-veins. The fore wings 

 are greatly strengthened along their costal (front) margin by having the 

 first longitudinal (subcostal) vein behind the margin placed at the bottom 

 of a groove, and the cross-veins in that groove so enlarged vertically as 

 to take on the character of flat, plate-like braces or buttresses. As, in 

 the figure-of-eight movement of the wing in flight, the front margin first 

 meets the resistance of the air, it is necessary that swiftly and strongly beat- 

 ing wings should be especially strengthened along this edge, and this is just 

 what the peculiar folding and bracing of the costal region of the dragon-fly's 

 fore wing accomplishes. 



The head is unusually large and is more than two-thirds composed of 

 the pair of great compound eyes More than 30,000 facets have been 

 counted in the cornea of certain dragon-fly species, and this means that each 

 eye is made up of more than 30,000 distinct eye-elements or ommatidia, 

 each capable of seeing a small part or point of any object in range of vision. 

 Thus an image of a near-by object is made in fine mosaic, and the finer the 

 mosaic the more definite and precise is the vision by means of compound 

 eyes. These great eyes, too, have facets directed up and down and sidewise 



