COLOUR- PEODUCING INSECTS. 59 



actresses in Paris by the use of poisonous colours 

 containing lead, mercury, arsenic, and other toxic 

 principles. 



* * * * 



I shall now turn to gall insects, or Cynips, to 

 which we owe many useful products. 



If ink were the sole product of the insects which 

 produce the gall-nut, we should not be so much 

 indebted to them, as ink can be produced in a 

 variety of manners. But we shall see that the 

 Cynips furnish us with other substances useful to 

 mankind. Although the insect which produced the 

 gall-nuts found in commerce was not known to 

 Linnaeus or to Fabricius, it belongs to their genus 

 Cynips a genus composed of small four-winged 

 flies, and classed in the family of Hymenoptera. 

 Some of these flies are remarkably useful to the 

 Greeks in their process of caprification. A dioecious 

 fig-tree, very common in the East, would indeed be 

 comparatively useless but for their aid. By a 

 dioecious plant is meant one in which the male and 

 female flowers are found on different individuals. 

 In most plants the two sexes are united in the 

 same flower, but in others, such as the hop, the 

 nettle, some willows and figs, etc., the male flowers 

 (stamens) are found on one individual, the female 

 flowers (pistils) on the other. Now, as no fruit can 

 ripen without the concourse of these two kinds of 



