GREEN-FLIES AND PLANT-LICE 



323 



kermes, the latter a dye manufactured in South Europe 



and used to colour wool and cloth crimson before 



cochineal reached us from Mexico. 



The Coccida include also the 



"mussel-scale" and other destruc- 

 tive diseases of fruit trees. A 



beautiful purple colour can be 



extracted from crushed masses of 



some kinds of aphides (as well 



as from Coccida), and has been 



used as a dye. The aphides have 



very generally a green colour, like 



many insects (caterpillars and leaf 



insects) which pass their lives upon 



green leaves and feed on them. 



It is often supposed that this green 



colour is merely the green colour- 

 ing matter (so-called chlorophyll) 



of the leaf, taken up by the insects 



in feeding on the leaf. But this is not so ; it is a peculiar 



substance derived in a crude state from the plant-juice, 



but digested in the stomach and completed in the insects' 



blood and tissues. 

 Then, again, the aphides 

 produce curious secre- 

 tions, often in great 

 abundance, which sur- 

 round them as the lac 

 surrounds the lac-insect. 

 The threads which are 

 produced in such 

 abundance, by the 

 woolly aphis of apple 



trees, as to look like masses of cotton wool adhering to 



the twigs of the tree, are of this nature. 



FIG. 58. Foundress or 

 stock-mother of the hop- 

 louse: the individual 

 hatched from a winter egg, 

 laid on the bark of a plum 

 tree, who produces vivi- 

 parously a wingless virgin 

 brood. That brood pro- 

 duces wing-bearing young, 

 which fly off to the hop- 

 plants. 



FIG. 59. Side view of winged viviparous 

 female of the hop-louse, b, the stabbing 

 beak. 



