Experiments 



1. Ceuthorrhynchus assimilis. The cabbage seed pod weevil. This insect has 

 a diapause in the adult stage from august till april. The next photoperiods 

 and temperature combinations were tested on the imaginai stage. 



hours room temperature (i 20°C) 



At none of these combinations any influence on the behaviour, egg deve- 

 lopment or egg deposition could be found. 



2. Ceuthorrhynchus pleurostigma. Two races of this insect have been de- 

 scribed which show no morphological differences (Isaac, 1923). One with 

 the same type of life cycle as C. assimilis is called the spring race. The other 

 is known as the autumnal race. 



a) Spring race. Eggs of the spring race are laid in May and June, adults 

 enter diapause in summer and emerge again in spring. In an experiment these 

 insects were reared under photoperiods of 20,13 and 5 hours, respectively, 

 temperature being 17°C. The result was negative. AU the insects entered 

 diapause after 9 days. Ovaries of 30 days old females were investigated. They 

 showed no signs of developing oocytes. 



b) Autumnal race. This race has the second type of life cycle mentioned 

 above which is characterized by an aestivation period from June to August 

 and oviposition in the autumn. As soon as they emerged these weevils were 

 reared under photoperiods and temperatures mentioned in the next table. 



These figures show that the optimal temperature for egg development at 

 short photoperiods is about 21°C. Long photoperiods prevent the development 

 of eggs, the transition point being between 16—17 hours. Also the behaviour 

 of weevils reared at photoperiods < 16 hours is different from those reared 

 at long photoperiods. They are more active and their food consumption is 

 higher. The question non arises whether or not the different environment of the 

 larvae of the spring race living in June may result in a different reaction to the 

 photoperiod. 



For this purpose weevils reared at a 13 hours photoperiod were given cabba- 

 ge plants in the beginning of May (this is the time the spring race lays its eggs) 

 for egg deposition. The resulting weevils emerged in July and wex'e tested 

 for their reaction to the photoperiod with the following result. 



278 



