1044 Rural School Leaflet. 



THE MOUTH-PARTS OF INSECTS 



Alex. D. MacGillivray 



Insects that injure plants are of two classes. The distinction between 

 these two classes is in the form of their mouth-parts. One class has 

 its mouth-parts fitted for biting or chewing, while the other class has 

 them fitted for sucking. Methods of destroying insects are based on 

 this difference in the structure of the mouth. Insecticides of one kind 

 are used for killing insects with a mouth fitted for biting. Such insects 

 generally feed upon the leaves of plants. Poisons of different kinds 

 are therefore sprayed or dusted upon the leaves, the poison is taken 

 up by the insect with its food, producing in its alimentary canal 

 changes that eventually cause its death. Insecticides 

 of an entirely different kind are used for sucking in- 

 sects. These insecticides are astringents, and usually 

 contain a resin, alkali, oil, or a strong caustic, which cor- 

 rodes or contracts and shrivels the body of the insect, 

 covers the breathing pores located along each side of the 

 insect's abdomen, and in this way causes its death. 

 To determine what kind of insecticide shall be used 

 h^H \^'~7'^^^^ to destroy any particular kind of insect, it is necessary 

 to determine first what kind of mouth-parts it has. To 

 be able to decide this question accurately, something must be known 

 of the more essential structures of an insect's mouth. 



Mouth- parts fitted for biting. — The head of an insect may be held 

 either horizontally or vertically; if horizontally, the mouth opening is at 

 the extreme front end of the head; if vertically, the mouth opening is 

 at its lower end on a plane with the under side of the body. The locust 

 or ordinary grasshopper, which has been selected as typical for those 

 insects with biting mouth-parts, holds its head vertically with the mouth 

 opening below. Fig. 14. The locust is especially suitable for study, 

 not only because specimens can be obtained easily, but also because 

 it is truly representative of the biting type of insect. 



The locust (grasshopper) mouth-parts consist of seven distinct portions : 

 an upper lip (lab.rum), two biting jaws (mandibles), two holding jaws 

 (maxillae — singular maxilla), the tongue (hypopharynx), and a lower 

 lip (labium). The labrum is a movable flap closing the mouth opening 

 in front. Fig. 15 1. The mandibles, Fig. 15 md, are strong, toothed 

 jaws with sharp edges which move sidewise just behind the labrum 

 and are used for cutting and grinding the food. The maxillae. Fig. 



