[ 72? ] 



weather is fine and the aphides are not tracked by ants or 

 other insects, and if the bean-stalk continues flourishing, this 

 agamogenetic propagation will go on every 4 or 5 days for 

 about 12 generations, the rate of increase being, say 10 in 

 every case. If a calculation is made it will be found that in 

 less than two months from one aphis one billion may be 

 produced, and if the rate of increase be 100 instead of 10, the 

 number comes to something enormous. The last genera- 

 tions are partly male and partly female, the intermediate 

 generations being wingless and imperfect females. The 

 male and female pair and lay eggs in the autumn or in the 

 cold weather and from these eggs come males and females of 

 the following spring. 



1.273. The Scale-insects. The Aleurodes which invade 

 rose bushes, orange trees, sugar cane plants ; the Icerya which 

 spoil various fruit trees &c.; the Dactylopius, one species of 

 which causes the disease known as tukra to mulberry trees, 

 and other coccid insects, are preyed upon in their turn by 

 lady-birds (coccinellidae) beetles. But the latter are unable 

 to cope with the insects when they become too numerous, 

 when Kerosine emulsion and other special remedies already 

 described, may be tried with success. Ants also are of great 

 benefit in devouring and killing scale-insects. 



1.274. Scale-insects do considerable damage to fruit trees 

 and other perennials, but they do not do such damage to 

 ordinary agricultural crops as the other hemipterous insects 

 (aphides) we have just described. There are several scale- 

 insects, on the other hand, which manufacture some import- 

 ant economic products. Cochineal and lac are produced 

 respectively by Coccus cacti and Coccus lacca. Manna is the 

 gummy secretion of the tamarisk tree which are punctured 

 by Coccus manniparus. The white wax of commerce is 

 produced by Erioceruspela, a Chinese scale-insect. There 

 is an Indian scale-insect also ^Ceroplastes ceriferous) which 

 yields a white wax. The females of scale-insects are always 



