32 



AN ECONOMIC ENTOMOLOGY. 



of enormous distention compared with the size of the insect, and 

 this is to accommodate the honey collected by the insects to be 

 carried into the hive. 



In the use of poisons acting through the stomach, the arsenic, 

 which is usually employed, is taken in with the food and gener- 

 ally carried into the chylific ventricle before it becomes effective. 

 In the crop the food is generally too dry to cause the necessary 

 solution of the caustic properties of the arsenic, which is neces- 

 sarily applied in its least soluble form ; but by the time it has 

 passed through the gizzard and has reached the stomach, becom- 

 ing moistened and mixed with the secretions that have been 

 already mentioned, it becomes active. Some insects are able to 

 take a very large quantity of poisonous material without injury ; 

 succumbing only after two or three days to the effects of a poi- 

 sonous meal. It is probable that in such cases the digestive 

 liquids exercise a less solvent effect upon the toxic mixture. 



Ordinarily, digestion in insects is exceedingly rapid. Among 

 caterpillars, for instance, feeding is often almost continuous, and 

 twenty-four hours are sufficient to pass through the entire diges- 

 tive system food two or three times the weight of the larvae 

 themselves. 



Fig. i6. 



Heart of a stag-beetle, showing the wings and chambers : at the side, the interior of a 

 chamber, to show the valves. 



Insects have no system of arteries and veins, and only one real 

 blood-vessel, which serves also as the heart. This, as has been 



