EXPERIMENT STATION BULLETINS. 283 



The Imago (F of Fig. 1) or perfect insect. — This is the stage in which 

 we most often recognize insects. It is short in comparison with the other 

 three stages, lasting only a day or two with some, but long enough to 

 enable them to lay eggs for a succeeding generation or bring forth their 

 young alive. Insects never grow in this stage. 



Such is said to be a complete transformation (see Fig. 1) or one where 

 each stage is entirely different from the others. Locusts, grasshoppers, 

 and bugs may be said to have an incomplete transformatioyi (see Fig. 2), 

 as their different stages of growth are quite similar after leaving the egg. 

 As such an insect grows, it becomes, at times, too large for its skin which ' 

 then bursts along the back, allowing the insect to crawl out, a new and 

 larger skin growing in its place. With the last moult comes the wings. 



HOW INSECTS BREATHE. 



Insects do not breathe by means of lungs as do the higher animals. 

 They breathe through minute openings that are scattered in various 

 places over the body. From these openings run minute tubes that carry 

 the air inside to the blood. Not only is their manner of taking air differ- 

 ent, but substances that they breathe may affect them readily, while ani- 

 mals with lungs are not affected at all. Thus it is that our pyrethrum 

 and buhach so readily affect insects, while to us it is perfectly harmless. 

 It is not the dust which affects them, but some volatile principle in the 

 oil that probably attacks the nervous system, as it throws insects into 

 spasms as soon as it is breathed. Hellebore will affect insects in the 

 same way, though it is not as likely to be fatal as the others. 



DIFFERENT METHODS IN FEEDING. 



There are two methods by which insects secure their food, by chewing 

 and by sucking. Those which chew their food masticate it in very much 

 the same way that we do, except that their jaws and other mouth parts 

 move sidewise instead of up and down. To the chewing class belong the 

 greater number of insects. All caterpillars, such as the tent caterpillar, 

 canker worm, peach tree borer, codlin moth, and bud worms; all beetles, 

 as the potato beetle, apple tree borers, grape vine flea beetle, striped 

 cucumber beetle, and plum curculio; all grasshoppers, locusts, and crick- 

 ets, and many others of less importance, belong to this class. For these 

 insects we apply poisons to the parts of the plant on which they feed and 

 they will soon eat enough of the poison to kill them. Of the poisons 

 taken by eating, the arsenites are the best, if they can be applied without 

 danger to ourselves, because they are the most deadly and certain. 

 Hellebore is also an internal poison, but milder and slower to act than 

 the arsenites. 



