612 THE FORMS OF TISSUES [ch. 



Very beautiful instances are to be found in insects' wings. In the 

 dragonfly's wing (which we have already spoken of on p. 476) we 

 see at first sight a vague assemblage of reticulate cells; but their 

 arrangement is both orderly and simple. The long narrow wing is 

 stiffened by longitudinal " veins," which in front lie near and parallel, 

 for reasons well known to the student of aerodynamics*, but become 

 remote and divergent over the rest of the wing; finer veinlets, 

 running between the veins, break up the surface into cells or 

 areolae. Where two large veins run parallel, and so near together 

 that there is only room for one row of cells between, the walls of 

 these meet the large veins at right angles, for the reason that the 



Fig. 261. Wing of "demoiselle" dragonfly (Agrion). 



tension in these latter is much greater than their own ; and this 

 happens nearly all over the delicate wings of the little dragonflies 

 called "demoiselles." But in the big dragonflies (Aeschna), and 

 in general wherever there is space enough between two strong veins 

 to hold a double row of cells, the walls of these intercalate with one 

 another at co-equal angles of 120°, while still impinging at right 

 angles on the strong longitudinal partitions. Wherever, as in the 

 hinder parts of the wing, the great veins are few, the cells numerous, 

 and their walls equally delicate, then the reticulum of cells becomes 

 an hexagonal network of all but perfect regularity. In a cicada and 

 in many others there is less contrast between great veins and small ; 

 the cells are few, the veins meet neither orthogonally nor at co-equal 

 angles, and the shape of the cells suggests a common deformation 

 under strain. In this last case, and generally in flies, bees and 

 butterflies, the few cells form a complex space-arrangement, simplified 



* Sir George Cayley was the first to shew that in a sail — or wing — set at an 

 acute angle to the wind, the centre of pressure lay near the front edge, which 

 had, therefore, to be supported or stiffened {Nicholson's Journal, xxv, 1810). 



