Dragon-flies and Damsel-flies 



97 



brown in the female. It has a slow hovering flight and often rests on the 

 tips of erect reeds with wings held perfectly horizontal. It is only on wing 

 in quiet, warm sunshine; clouds or cold breezes send them quickly into 

 hiding. Among the famihar Libellulids with unblotched wings is Aleso- 

 themis simplukoUis, an abundant species east of the Rockies. The 

 females and young males have head, thora.x, and front half of abdomen 

 green, the hinder half blackish brown. In old males the body becomes 

 grayish blue with a whitish bloom. Williamson says that sometimes two 

 males will flutter motionless, one a few inches in front of the other, when 

 suddenly the rear one will rise and pass over the other, which at the same 

 time moves in a curve downwards, backwards, and then upwards, so that 

 the former position of the two is just rever.sed. These motions kept up 



Fig. 131. — The whitetail, Plathemis lydia. (After Needham; natural size.) 



with rapidity and regularity give the observer the impression of two inter- 

 secting circles which roll along near the surface of the water. 



The whitetail, Plathemis lydia (Fig. 131), resembles the ten-spot, but 

 is one-fourth smaller. In the males also the apex of the wings is usually 

 clear, not brown. The whitetail rather likes slow-flowing brooks and 

 open ditches. WTien alight it has the habit of setting its wings aslant down- 

 ward and forward with a succession of jerks. Needham thinks that the 

 powdery whiteness of the body of the old males (in females and young males 

 the body is brown marked with yellow) must render it more easily seen by 

 its enemies, the king-birds and others, and thus be a disadvantage in the 

 struggle for existence. He says, indeed, that the whitest ones avoid rest- 



