28 



[NSECT PESTS OF FARM, GARDEN AND ORCHARD 



and if an insect is to be suffocated, it must be done by closing the 

 spiracles. It is in this way that tobacco-dust, lime, pyrethrum, 

 and similar insecticides kill sucking insects, by penetrating the 

 spiracles and choking the tracheal system. Whale-oil soap, 



a.v 



,-tT: 



FIG. 23. Internal anatomy of silk-worm, from photo of Azoux Model: 

 A, upper or dorsal bodywall seen from within; B, the back of the silk- 

 worm removed, showing alimentary canal; C, alimentary canal removed, 

 showing nervous system and tracheal trunks; tr., trachea; d.v., dorsal 

 vessel or heart; ph., pharynx or mouth; su., supra-oesophageal ganglion; 

 sp.sp., spiracles or breathing pores; n., nerve cord; tr.t., tracheal trunk; 

 oes., oesophagus or throat; cr., crop; s.g., silk gland; pro., proventriculus; 

 st., stomach; h.i., hind intestine. 



kerosene emulsion, and the other "contact" insecticides, or "irri- 

 tants," also stop up the spiracles and thus cause death, but they 

 may act as " irritants," penetrating the 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 asphyxiated by a substi- 

 tution of these gases for air, the same as are the higher animals. 



Though arsenical poisons are generally used as sprays for biting 

 insects, soft-bodied caterpillars and similar larvae are often killed 

 by the use of contact insecticides, which affect them the same as 

 sucking insects. 



The reader will observe that, almost without exception, the 

 remedies advised for different insect pests in the following pages 

 are determined by some peculiarity, either of structure or develop- 

 ment, of the insect to be combated. 



