530 ANNUAL REPORT OF THE Off. Doc. 



cations of the insects to be treated by naming the order and family 

 and giving the scientific name of each. 



Orders and families are founded upon certain common important 

 structures, the more potent and general of which characterize all 

 insects belonging to the major group. There are nineteen orders 

 known to modern entomologists, but the insects treated in this 

 article represent only five of these. 



STAGES IN LIFE HISTORY OF INSECTS. 



Insects undergo transformations called metamorphoses. In their 

 life cycle they exist in certain forms called stages. Those treated in 

 this bulletin have either three or four stages, according to their kind 

 of metamorphosis. It may be "Incomplete," in which the insect 

 has but three stages: (1) The egg, (2) nymph or immature, and (3) 

 imago or adult. (See Fig. 1.) In this group the young resemble the 

 adult in form and generally in habits, lacking only the wings. There 

 is no worm-like existence and no pupal or quiescent stage. The 

 squash bugs and grasshoppers are good examples of insects with 

 incomplete metamorphosis. The young are called nymphs. 



The representatives of the next group, or those with complete meta- 

 morphosis, pass through four stages: (1) The egg, (2) the larva, or 

 worm-like stage, (3) the pupa or resting stage, and (4) the imago, 

 adult or mature insect. (See Fig. 2.) The young are called 'iarva?" 

 and do not at all resemble the adults. They are popularly known as 

 "worms," but this common name should not be given them, since 

 worms are independent creatures that do not transform into any other 

 form or stage. (Example, the earthworm). 



Insects grow only in the nympili or larval stages. They do not 

 become larger after having once reached the adult or winged stage. 

 They live in the latter condition but a few days or weeks, mate, lay 

 their eggs and then die. 



Some adult insects do not eat; others, like the butterflies, only 

 sip a little nectar and do not have feeding habits similar to their 

 young. Others, like the squash bugs and cucumber beetles, eat the 

 same kind of food as do their nymphs or larvce. 



The feeding habits of insects is a fundamental feature in applying 

 insecticides. Some have biting mouth-parts with strong jaws and 

 chew the leaves or tissues of the plant. (Examples, caterpillars, 

 beetles, etc.) These insects that chew can nearly always be killed by 

 poisons, which are to be taken internally, among which the arsenites 

 are prominent, and Paris green is the most valuable. (See Insecti- 

 cides, A.) The insects that do not chew their food have piercing 

 mouth-parts, as has the squash bug. (See Fig. 3.) As they are suc- 

 torial and do not suck before the bill is inserted, they are not affected 

 by poison lying on the leaf. They must be killed by contact applica- 



