INSECTS. 479 
living, they seem to be sprinkled over with a white 
powder, which they lose as soon as the boiling water is 
poured upon them ; but preserve when killed in an oven. 
Those dried upon hot plates are the best. 
The quantity of Cochineal annually exported from 
Mexico and South America is said to be worth more than 
five hundred thousand pounds sterling, a vast sum to 
arise from so minute an insect ; and the present annual 
consumption of Cochineal in England has been esti- 
mated at about one hundred and fifty thousand pounds 
weight. The Mexicans think so highly of their trade in 
this insect, that the republic has adopted the nopal-tree 
as part of its arms. 
It is for dyeing scarlet that Cochineal is chiefly in de- 
mand ; but although a peculiarly brilliant dye is now ob- 
tained from it, this substance gave only a dull crimson 
colour, until a chemist of the name of Kuster, who, 
about the middle of the seventeenth century, Jived at 
Bow, near London, discovered the art of preparing it 
with a solution of tin. Cochineal, if kept in a dry place, 
may be preserved without injury for a great length of 
time. An instance has been mentioned of some of this 
dye, one hundred and thirty years old, having been found 
to produce the same effect as though it had been perfectly 
fresh. 
THE PLANT LOUSE, OR GREEN FLY. 
(Aphis rosce.) 
THE Aphides are sometimes viviparous, and at other 
times oviparous, according to the season of the year. 
Those of the rose-tree have been particularly noticed, and 
of ten generations produced in one spring, summer, and 
autumn, the first generation was oviparous, the eight fol- 
lowing viviparous, and the last oviparous. The first nine 
