752 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



ance of the series of hexagons. The accompanying diagram (hg, 39 

 shows the ideal cell arrangement.'* 



I hasten now to add that a perfectly typical arrangement of th 

 areoles throughout a single entire space in a dragon-fly wing 1 hav 

 not found, althougli it is easy to find in many of the more generalizei 



forms ample evidence of the opoi'atio 

 of the principle. The accompanyin; 

 drawing (fig. 40) of parts of actufi 

 wings will serve for illustration, 

 have made no hunt for better, for thes 

 show, also, wh}^ the typical conditio 

 is so early lost. Accessory trachea 

 branches penetrate along broken linei 

 of cross veins, bringing them into line 

 then supplements are superadded, ani 

 the havoc of the hexagons is completel 

 Enough of the typical arrangement fo 

 recognition is to be looked for only i 

 wings lacking strong accessory trachea: 

 branches and supplements. 



Tissue cells often appear hexagone 

 in section, and when crowded into sin 

 ilarly shaped areas, often behave a 

 do these areoles of dragon-flies' wingt 

 The histologist who looks over the plate 

 accompanjdng this paper will not fa 

 to see here and there groups of areok 

 showing conformation entirely compai 

 able to familiar images of tissue cell' 

 Coming now to some points whic 

 have to do probably only with win*.' 

 and with aerial navigation, we reca 

 that a form of wing broad at base an, 

 narrow and pointed at the apex, we 

 known for its efiiciency in insects get 

 erally, we have seen developed twic 

 upon two ver}' difi'erent plans, culm 

 nating in Anax and Tramea respectively. Both have adjusted tb 

 wing apex to a degree of rigidity on the costal margin, and of plianc 

 behind it, which gives the greatest resultant in forward motion f( 



«Dr. R. T. Jackson has shown (Bull. Geol. See. Amer., YIII, 1896, p. 164) that tl 

 interambulacral plates of sea urchins are potential hexagons, that the border ro\ 

 are pentagons — hexagons, with one angle eliminated, as here — and that pentagon ai 

 heptagon together initiate new rows of plates. Practical] y the only difference bet we( 

 the typical arrangement of these areoles and that of the interambulacral plates, 

 figured by Dr. Jackson, arises out of the difference in the shape of the areas to be occ 



