22 



The Bulletin. 



Fig. 8 —Life-history of Southern Horn Worm. (Same as Frontispiece.) (a) Adult (natural 

 size) ; (6) egg (enlarged); (c) larva (natural size) ; (d) pupa (slightly enlarged). 

 (From photographs by the author.) 



Groups of Insects. — All insects are divided into groups according 

 to their structure. The structure of the mouth parts is of vital im- 

 portance in arranging insects under their proper groups. The struc- 

 ture of the mouth parts is also of vital importance to the farmer who 

 is attempting to control the insect pests of tobacco. Fundamentally, 

 insects are divided into two classes, one of which is provided with 

 biting mouth parts and chews its food. Insects of this class usually 

 make their presence known by eating large, irregular holes in the 

 leaves, by eating away portions of the roots or by hollowing out the 

 stem. The insects of the second class are provided with piercing 

 mouth parts. Insects of this class secure their food by inserting 



