18 



INSECTS INJURIOUS TO STAPLE CROPS. 



that tobacco-dnst, lime, pj'Tethruin, and similar insecti- 

 cides kill sucking insects by ^penetrating the spiracles and 

 choking the tracheal system. Whale-oil soap, kerosene 

 emulsion, and the other "contact" insecticides, or ^' irri- 

 tants," may also stop up the spiracles and thus cause 

 death, but they act chiefly as "irritants," penetrating the 



Fig. 10. — Internal Anatomy of Silkworm. A, the upper, or dorsal, 

 body-wall seen from within; B, the back of the silkworm re- 

 moved, showing alimentary canal: C, alimentary canal removed, 

 showing nervous system and tracheal trunks; t?'., trachea; d.v., 

 dorsal vessel or heart; ph., pharynx or mouth; sti., supra- 

 oesophageal ganglion; sp. sp., spiracles or breathing-pores; n, 

 nerve-cord; ^r.^., tracheal trunk; oes., a^sophagus or throat; c/'., 

 crop; s.g.. silk-gland; pro., proventriculus or grinding-stomach ; 

 St., stomach; 7i.i., hind-intestine. (Photo, by author from Azoux 

 model.) 



skin and thus killing the insect. When insects are killed 

 by means of a gas such as carbon bisulfide or hydrocyanic 

 acid gas, they are truly asphyxiated by a substitution of 

 these gases for air, the same as are higher animals by the 

 use of anaesthetics, 



