XIT] Aphides and Sawflies 161 
“honey dew,” which may be found on the leaves of 
trees. It blocks up the stomata and sometimes 
becomes so dirty, owing to the growth of a saprophytic 
fungus, as to appear like a coating of soot. 
If the aphides were not checked by some natural 
means it is obvious from the rate at which they 
reproduce that they would soon cover all the plants 
on the earth. They have a large number of natural 
enemies, and nature has given them this capacity for 
reproduction in order to withstand these. After a bad 
Fig. 49. Lady-birds. An egg (highly magnified), « larva, two 
pupae and three beetles. 
attack of aphis we often find a large number of lady- 
birds on plants. These small beetles and their larvae 
devour a large number of aphides. The larvae of the 
lace-wing flies, also known as “niggers,” feed on 
plant lice and may often be found with the skins of 
their victims on their backs (see Fig. 50). They are 
usually difficult to find when at rest. 
The legless larvae of hover flies also eat a consider- 
able number of green flies. 
11 
P.F 
