ELEMENTARY ENTOMOLOGY 



v 



When arsenical insecticides are applied to the food of biting 



insects, the arsenic must be in the most insoluble form, to avoid 



burning the foliage, and it is therefore not dissolved until it 



reaches the stomach, when, having been mixed 

 with the digestive juices mentioned, it becomes 

 sufficiently soluble to be absorbed by the walls 

 of the stomach and ileum. Some insects are 

 able to consume a large amount of poison before 

 an amount sufficient to kill them is dissolved 

 and absorbed. In such cases poisons are some- 

 times of no avail, because serious injury is 

 done before the pest is brought under control, 

 and other means must be employed. 



In the young 

 stages of insects 

 the digestion, 

 and consequent 

 growth, is ex- 

 tremely rapid. 

 A caterpillar 

 will frequently 

 eat and digest 



two or three times its own weight 



in a day. Thus the silkworm, 



when it hatches from the egg, 



weighs but one twentieth of a 



grain, but in 56 days, when full 



grown, it has consumed 120 oak 



leaves, weighing three fourths of 



a pound, and half an ounce of 



water, or 86,000 times its original 



. r . , . FIG. 37. Diagram to indicate the 



weight, of which food 207 grains course of the blood in the nymph of 

 have been assimilated, one fourth a dragon-fly 



Of a pound has been Voided aS a, aorta; //, heart. The arrows show the 

 pvrrpmpnr anrl fivp onnrpq have direction taken by currents of blood. (After 

 lt ' ai Kolbe, from Folsom) 



evaporated as water. 



Circulatory system. The blood vessels of an insect are exceed- 

 ingly simple, consisting of a single dorsal tube, or heart, which 



FIG. 36. Diagram 



of a portion of the 



heart of a dragon-fly 



nymph 



o, ostium ; z-, valve. 

 The arrows indicate 

 the course of the blood. 

 (After Kolbe, from 

 Folsom) 



