STUDIES OF VARIATION IN INSECTS 315 



Out of the 239 individuals the hairs could be accurately deter- 

 mined on the right side in 189 cases, on the left in 193, the fre- 

 quencies being as follows : 



On the right side : 2 individuals have 3 hairs, 177 individuals 

 have 4 hairs, 10 individuals have 5 hairs. 



On the left side : 3 individuals have 3 hairs, 185 individuals 

 have 4 hairs, 4 individuals have 5 hairs, i individual has 6 

 hairs. 



The mean on the right side is 4.04, on the left side 4.01 ; 

 the standard deviation on right side .248, on left side .238 ; the 

 coefficient of variation on right side is 6.15, on left side 5.92. 



The few cases of variation from the mode, 4 hairs, were 

 scattered in various groups of individuals, letting all the para- 

 sites from an individual host constitute a group. That is, the 

 variation was not associated with isolation. From one host 

 were taken ten parasites of which one had 5 hairs on one side, 

 one 3 hairs on one side and all the others 4 on each side ; from 

 another host were taken twenty-six parasites, the number of 

 hairs on each side of all the individuals being 4 except one 

 case of 5 on one side, and so on. The twenty cases of varia- 

 tion from the mode were distributed in ten groups of parasites, 

 i. e., occurred in parasites from ten out of the thirty-one host 

 individuals. 



The correlation conditions (degree of bilateral symmetry) of 

 the variations are shown by the following summary (only 183 

 specimens out of the 239 had the hairs so intact that the exact 

 number of hairs on both sides of the metathorax could be 

 determined) : 



Of 183 individuals, 166 have 4 hairs (the mode) on each side, 

 the remaining having hairs as follows : nine have 4 on one side 

 and 5 on the other, five have 4 on one side and 3 on the other, 

 two have 5 on each side, one has 4 on one side and 6 on the 

 other, none has the combination of 3 and 5. 



In 72 specimens of Ltpeurus varius taken from 29 specimens of 

 Fulmar us glad alis vars. rodgersi ;'and gluptsckka (Bay of Mon- 

 terey, time period, two weeks), the range of variation in number 

 of metathoracic tactile hairs is 3-5 with 4 as a very dominant 

 mode, the frequencies being as follows : 



