22 FOREST PROTECTION 



and the mature or reproductive stage of all insects 

 is called the adult, or imago. The pupa of a butterfly 

 is very often called a chrysalis, and the silken sack 

 spun by many insects in which to pupate, is the co- 

 coon. Larvae of Diptera and of some other insects, 

 pupate within a tough outer covering commonly sup- 

 posed to be simply a pui)al skin. The true pupa is, 

 however, entirely within it, and the tough outer cover- 

 ing is distinguished by the name puparium. After 

 reaching the adult stage, the insect does not become 

 any larger, and does not molt; its only function is to 

 mate, and laj'^ eggs. Some species are unable even to 

 feed after becoming adult, and in almost all cases, the 

 larvse or nymphs are much more voracious than the 

 mature insects. In general, then, the greater part of 

 the insect damage to our forests is done before the in- 

 sects responsible become mature. The Ambrosia 

 beetles form a notable exception to this rule. 



The sum total of the stages of development of an 

 insect is termed a generation, and a given species may 

 be single-brooded, double-brooded, treble-brooded, etc., 

 according to the number of generations which occur 

 during a single year. Many insects require more than 

 a single year to complete a generation, and are then 

 called biennial, triennial, etc. A species of the Cica- 

 didse is known to have a life round of seventeen years. 



IV. Climatic and Seasonal Conditions Affecting Insect Life. 

 In general, the number of species of insect life decreases as 

 altitude or latitude increases, while at the same time, the num- 

 ber of individuals of a species becomes larger. The number 

 of generations of a given species is also affected by the climate; 

 for instance, a species which is "double-brooded" in the Mid- 

 dle States, may become "treble-brooded" in the Southern 

 States, and "single-brooded" in Canada. 



Insects spend the winter months in a resting or hibernating 

 stage which varies for the different species. That is, a given 

 species may hibernate either in the egg, larval, pupal, or adult 

 stage. They are protected against the cold either by their 

 own coverings, or by the hiding places selected by them in the 

 trees, in the bark, in the moss and leaves, in the stumps, or 

 in the ground. Extreme cold is no more likely to injure the 

 insect than it is to kill the tree itself; but sudden changes of 

 temperature and moisture, especially cold wet spells in late 

 spring, or after a premature thaw has drawn the hibernating 



