THE HOMOPTERA 



195 



Aphids are tiny, soft bodied insects, the largest being less than a third 

 of an inch long, generally with long legs and antennae, and are of various 

 colors, green, black, various shades of red and brown, white and gray 

 being the most usual ones. Some are more or less completely concealed 

 (Fig. 186) beneath long, white waxy threads, giving them a " woolly" 



Fio. 186. Alder twig covered by woolly plant lice, the "wool" entirely concealing their 

 bodies. Somewhat enlarged. (Original.) 



appearance; others have a sort of dust or " bloom" like that on a plum, 

 coating their bodies; but the majority (Fig. 187) are without any covering. 

 Many species of aphids have a pair of tubes called cornicles, projecting 

 upward from the top of the abdomen. These were formerly believed to 



Fio. 187. Portion of leaf showing plant lice clustered together. 



(Original.) 



Somewhat enlarged. 



be the exit ducts through which honey dew, abundantly produced by the 

 insects, escapes, but it is now known that this substance is expelled 

 through the anus, often in such quantities that when the insects are abun- 

 dant it forms a sort of fine rain which can be heard falling on the leaves 



