MANUAL OF GENERAL AGRICULTURE. 89 



mentary canal changes that eventually cause its death. 

 Insecticides of an entirely different kind are used for suck- 

 ing insects. These insecticides usually contain a resin, 

 alkali, oil, or a strong caustic, which corrodes 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 

 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. Observe that 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 moutli opening below. I" ig. 12. 

 The locust is especially suitable for study, not omy be- 

 cause 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 (labrum), two biting 

 jaws (mandibles), two holding jaws (maxillae singular 

 maxilla), the tongue (hypopharynx), and a lower lip (la- 

 bium.) The labrum is a movable nap closing the mouth 

 opening in front. I 1 ig. 13. The mandibles, ^ ig. 13 md, are 

 strong, toothed jaws with sharp edges which move side- 

 wise just behind the labrum and are used for cutting and 

 grinding the food. The maxillae, Fig. 13 mx, are situated 

 just behind the mandibles and like the mandibles move 

 sidewise. Each maxilla bears on its outer end two finger- 



