164 



Flies and bees combine an ability for biting and for 

 lapping or sucking in the same individual ; while butterflies 

 and moths are often able to suck up the sweet juices 

 exposed in the nectaries of flowers, without possessing the 

 ability to pierce any but delicate tissues. 



Thrips have remarkable mouth parts, in that one of the 

 mandibles is well developed, while the other is much reduced 

 in size. The well-developed mandible is probably used for 

 cutting or lacerating the surface of plant tissue ; while the 

 modified maxillae form a lapping organ, by means of which the 

 soft tissues underlying the surface may be utilized for food. 



The larvae of the lace-wing flies have biting mouth 

 parts, but modified in a peculiar manner. The mandibles 

 are well developed, and grooved. The small insects on 

 which these larvae feed are captured and held by the 

 mandibles, while the body juices are sucked out by means 

 of the grooves. 



As far as the structure of the mouth parts has relation 

 to the use of insecticides, it is sufficient to classify them as 

 biting and sucking mouth parts. A knowledge of the 

 great modifications which have been brought about by the 

 adaptation to different foods and different modes of life, 

 will perhaps help readers of this book to realize the 

 necessity of exercising some thought in preparing to deal 

 with insect pests. 



STOMACH POISONS. The principal stomach poisons are 

 those derived from mineral sources, and chief of these are 

 arsenic and its several compounds. The arsenical insecti- 

 cides are useful in combating nearly all insects with biting 

 mouth parts, and may be applied to a great variety of 

 plants. Leguminous plants (peas, beans, etc.) and the 

 Cucurbitaceae (melon, cucumbers, etc.) often suffer severe 

 injury when sprayed or dusted with this class of poison, 

 and trials should be made on a small scale before treat- 

 ing large areas of these crops, in order to determine the 

 correct strength of the application under the existing 

 local conditions. 



Paris green. Paris green depends largely for its 

 effectiveness on the amount of arsenic which it contains, 

 the arsenic being united with copper which is also com- 

 bined with acetic acid. A good quality of Paris green 

 should contain not less than 56 per cent, of arsenic, of 

 which onJy about 5 per cent, should be soluble in water. 



