100 



THE REPORT OF THE 



No. 19 



dioxide for oxygen. In the case of sucking insects, the most effective way 

 ol' killing them is to suffocate them by clogging the spiracles with such sub- 

 stances as tobacco, soap solution, kerosene emulsion, insect powder, lime, or 

 dust. 



The blood, which is nearly colorless, flows free within the cavity of the 

 cavity of the body, and bathes the various organs. The requisite amount of 

 oxygen is obtained by exchange from the tracheae which contain air. 



Reproduction and Metamorphosis. Most insects lay eggs, but a few 

 biing forth living young. As a rule, insects have great powers of reproduc- 

 tion, but seldom are the newly hatched insects exactly like the adults. In 



Sriif^aCal !' 



Fig. 22. Respiratory system of Grasshopper (after Hyatt). 



fact, most insects pass through a more or less marked change during their 

 lives. This is known as Metamorphosis, which is said to be complete (Fig. 

 23), when the insect passes through a stage of almost total rest, takes no food, 

 and moves. but little; and incom,plete (Fig 24), when the insect does not pass 

 through a stage of rest, but feeds and changes its form gradually with every 

 moult, becoming more and more like the adult. As insects grow they cast 

 off their old skin at intervals. The new skin is soft and elastic, but as it 

 gets older, it becomes hard and inelastic. This is stripped off and replaced 



Fig. 23. Complete metamorphosis ; a caterpillar ; 

 b chrysalis in a cocoon ; c moth. 



by a new one. By this m,oulting, growth is not interfered with. Among 

 these insects which undergo complete metamorphosis are : Butterflies, moths, 

 beetles, bees and wasps and flies. The different stages are known as egg, larva, 

 pupa, and adult. The larval stage of insects is the most important one 

 from an economic standpoint, for it is the feeding stage, and it is then that 

 vegetation is injured. The larvae of butterflies and moths are known as 

 caterpillars (Fig. 21), and have usually 8 pairs of legs, sometimes only 5 

 pairs; those of saw-flies have usually 10 pairs of legs. 



