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Contact poisons are useful against all soft-bodied insects 

 which can be reached by an application of substances that 

 may come into contact with their bodies. It is obvious that 

 insects which feed by puncturing the tissues of plants and 

 sucking food from the juices within cannot be killed by the 

 application of a stomach poison on the surface of the plant. 

 Contact poisons are especially Valuable in dealing with 

 insects having this habit of feeding. Soft-bodied insects, 

 such as some caterpillars and slugs, may also be killed by 

 contact poisons, when this method is more convenient than 

 the use of the stomach poisons. 



Fumigants are useful for dealing with insects which 

 cannot be reached by stomach poisons or contact poisons, or 

 live under such conditions that it is possible to surround 

 them with a poisonous vapour. Insects of various feeding 

 habits may be controlled by the use of this kind of insecti- 

 cide. 



Repellents are often useful in situations where the other 

 insecticides are not applicable, or in dealing with insects 

 which are with difficulty reached by the other kinds of 

 insecticides. 



As a preliminary to the consideration of the substan- 

 ces used for the destruction of insects, it mav be useful to 

 review the structure of the mouth parts of these animals 

 and the manner in which they obtain their food. In 

 general, it may be said that insects possess mouth parts 

 adapted for sucking or for biting. 



The jaws of insects are placed at the sides of the mouth 

 and have a lateral motion. Strong mandibles are provided 

 for the purpose of biting off particles of food, while the max- 

 illae aid in chewing and in conveying it to the gullet. This is 

 the normal condition, which is found in such insects as the 

 cockroach, grasshopper, beetles, both larva and adult, and 

 the caterpillars of butterflies and moths. 



Insects which obtain their food by sucking it up in a 

 fluid form possess mouth parts which are modified for the 

 purpose. The mandibles or the maxillae, or both, are arrang- 

 ed so as to form some kind of proboscis in which is combined 

 the ability to pierce the tissues of plants or animals and to 

 suck up the juices which are reached in this way. Mouth 

 parts of this description are to be found in plant bugs, 

 aphids, scale insects and white fly attacking plants; and 

 in ticks, lice, fleas and bed-bugs, which attack animals. 



