178 



E. W. SEYSTER. 



temperature is to be noted throughout the larval life. In no 

 case was there any significant effect upon the facet number of 

 the flies after the pupae had been formed. The parent flies of 

 the six different sets differed but little in facet number. In 

 experiments 14, 15 and 17 each set of parents was used in each 

 experiment. For instance, the parents of 14.1, 15.3 and 17.2 

 were identical. In experiment 18 the same parents were used 

 throughout, being changed from one bottle to another. 



Facet Change per Degree Change in Temperature. With one 

 exception, the number of facets per degree change in temperature 

 varies from 5.2 to 8.9 and is fairly constant. The following is 

 a list of facets per degree change, found by dividing the difference 

 in facet averages by the difference in degrees centigrade: 



The relation between the facet number and a ten degree difference 

 in temperature is also very interesting. Is the number of facets 

 increased two to three times per ten degrees decrease? The 

 following table gives the results, n t being the number of facets 

 at the lower temperature and n t+ io at the higher temperature: 



Experiment. 

 I, 2, 3 



4 

 5 



Average, 



2.6 



1-7 



2-5 



3-0 

 2.6 



3-5 



3-0 

 3-9 

 3-2 



3-5 



Size of Individual Facets (Plate I.). The relation of the area 

 of the eye to facet number was found to be constant for flies with 

 an intermediate number of facets. This constant, for 43 indi- 

 viduals, was found by dividing the facet number by the area of 



