ANIMAL PESTS OF PLANTS 173 



pieces and use them to build nests in holes in trees and 



similar places. 



Flies. 



Distinguished from the insects already mentioned, 

 all of which have four wings (some green fly have no 

 wings), are the flies which only possess two. The 

 larvae, usually called maggots, are footless grubs with an 

 undefined head ; they are thus distinguishable from the 

 grubs of beetles. The pupae are inactive and often 

 resemble brown seeds. The adult insects have two 

 membranous wings, and the mouth is formed for suction 

 and not for biting. 



The grubs of some flies burrow in leaves, and eat 

 out tunnels in the soft tissue. Thus cineraria leaves are 

 often disfigured by meandering trails made by the grubs 

 of the leaf-miner, and the grubs of the celery-fly are 

 responsible for the blister-like patches often seen on 

 celery leaves ; in these cases the green tissue of the leaf 

 is eaten, leaving the colourless upper and lower skin. 

 The grubs or maggots can often be seen in these 

 blisters, and here they change into the pupal or resting 

 condition, whence later the perfect winged fly emerges. 



Slugs and Snails. 



Slugs and snails also cause a great deal of damage 

 in gardens. They are obviously very different from the 

 pests already noticed, and belong to another class of 

 animals altogether, the Molluscs, or Shellfish. In the 

 ordinary garden snail the shell is easily seen, but in 

 most slugs there is no visible shell, it being present only 

 as a small, hard plate embedded in the flesh of the 



