PROC. ENT. SOC. WASH., VOL. 22, NO. 8, NOV., 1920 199 



of them travelled quickly the distance of eighty-three inches to 

 the nearest food. Those which had not obtained food before the 

 fourth day after hatching died. It was noticed that practically 

 all larvae seemed to be more or less positively phototropic. 



The older larvae also migrate in the search of food. After 

 devouring all upon one tree, they let themselves down to the 

 ground by their silken threads and then crawl to nearby trees to 

 begin the destruction of more foliage. Fully fed caterpillars often 

 travel considerable distances in search of protected places in which 

 to pupate. Others move only a few inches away from the last 

 leaves upon which they were feeding. 



Spasmodic Occurrence. 



The spasmodic occurrence of the caterpillar is worthy of note. 

 It is surprising how very abundant it may be during one genera- 

 tion and then be so scarce during the next that hardly a moth 

 or caterpillar can be found. The older residents of localities 

 where the caterpillar occurs, claim that it is prevalent every 

 seven years. Their statements are fairly accurate, for the cater- 

 pillar is usually destructive about that often. There may be a 

 year or two when hardly a caterpillar can be located and then 

 several years when they become increasingly abundant until they 

 finally reach a maximum, destroying the foliage on oaks over 

 considerable areas. They then begin to wane and in a year or 

 two again reach the minimum. 



They are seldom abundant during more than one generation in 

 any one locality, but move a mile or more away in their depreda- 

 tions. The maximum periods are not coincident throughout the 

 whole State, but occur at different times. There is always some 

 place in their habitat where they are reported as doing much 

 damage. One year the reports may come from the counties north 

 of the San Francisco Bay, sometimes from the Berkeley and Oak- 

 land Hills, from the San Francisco Peninsula, or perhaps, from 

 Monterey or San Luis Obispo County. 



The moths are rather sluggish and can not be considered as flying 

 from one section of the State to another. They do, however, fly 

 short distances to deposit their eggs as they do not seem to like to 

 infest the same tree twice in succession. Local conditions, para- 

 sitic and predacious insects and bacterial enemies are apparently 

 the factors in the abundance of the caterpillars in each locality 

 irrespective of other localities. 



Summary. 



The number of larval instars is normally five although abnor- 

 mal individuals may undergo from four to nine molts. The 

 weather and amount of available food are the controlling factors 

 of the number of molts. 



