ELEMENTARY ENTOMOLOGY 



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 

 Diagram stages of insects 

 the digestion, 

 and consequent 

 growth, is ex- 



FIG. 36. 

 of a portion of the 

 heart of a dragon-fly 

 nymph 



(After Kolbe, from 

 Folsom) 



o, ostium ; v, valve. 



The arrows indicate tremely rapid. 



the course of the blood. j 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 



, , . , - -, . 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; /z, heart. The arrows show the 

 ^ j r r i, , direction taken by currents of blood. (After 



excrement, and nve ounces nave Koibe from Foisom) 



evaporated as water. 



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

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



