THE GROWTH OF GROUPS 157 



are two generations during each year, one of which hiber- 

 nates, i.e. burrows into the ground to a depth of about 

 two feet and lies dormant for over six months. They 

 retreat in September and emerge in May. It is necessary 

 to describe very briefly the normal cycle, which is very 

 simple. On leaving the ground in May, the beetles are 

 not sexually mature, but they become so in a few days, 

 and deposit their eggs in batches. These hatch out and 

 give rise to adult insects (first generation of the year) in 

 July. The beetles of the first generation soon become 

 mature and deposit their eggs, which give rise to the 

 second generation by about the end of August. Beetles 

 of the second generation postpone their reproductive 

 period until the following summer, they fly about for a 

 time but soon enter the ground to emerge six months 

 later, when they commence to repeat the cycle. The 

 life-history is essentially the same in twenty species of 

 Leptinotarsa examined, although of course the times vary 

 a little. Hence we may say that the annual occurrence 

 of two generations, one of which hibernates, is a generic 

 feature of the group, a fact which makes the abnormal 

 line, now to be described, appear in strong contrast. 



Tower found among the offspring of some of the 

 mother beetles which had been treated artificially, a 

 number of insects which appeared outwardly to be normal 

 but were abnormal in their life-history, for they hiber- 

 nated in the fifth generation, not as usual in the second. 

 They behaved in this unusual manner for three successive 

 years, until the whole race was destroyed by an accident. 

 To allow time, so to speak, for the five generations to be 

 produced within the year, the period of hibernation was 

 curtailed. They went to ground a month late and rose 



