162 OUR HOUSEHOLD INSECTS 



dark triangle -shown at b (Fig. 51, D) in the same line as 

 the original outer boundary of the wing, the rest will 

 fall pretty easily into its place, and become neatly spread 

 out on the card. If it does not settle on the gum quite 

 smoothly, the wrinkles may be got rid of by using the 

 needle as a sort of rolling-pin, and rolling it out from 

 the stouter towards the weaker margin of the wing. 

 This last operation requires care, as the membrane is 

 very easily torn. We have now the whole wing ex- 

 panded with its under surface uppermost. It will keep 

 any length of time in this position. 



To understand why the wing always folds in precisely 

 the same way, attention should be directed to the 

 membranous part last exposed. From the joint b 

 (Fig. 51, D) nervures will be seen radiating like the 

 framework of a fan. About half-way down its length, 

 each dilates into a minute swelling ; and as the individual 

 rays diverge more and more, other shorter ones are 

 seen to spring up alternately with them, passing out- 

 wards to the edge of the wing, but not reaching the 

 hinge aforesaid. These also have each a similar but 

 much larger swelling, the whole set forming a row 

 parallel to the hind margin of the wing. The mem- 

 brane, having been folded like a fan along the lines 

 of the radiating nervures, all these little leathery spots 

 are brought up side by .side, and the whole collection is 

 then bent across at this spot, thereby reducing the 

 membrane to half its length. A transverse nervure 

 running parallel to the hind margin, but nearer to the 

 line of bending than to it, serves to give stability to 

 the wing when fully expanded. The rays are the 

 divisions of the anal nervure, the area of which in 

 most insects forms only a small part of the wing 

 nearest the body, but in the earwig has so far 



