476 Geoffrey Smith 



(luring tlie Praniza stage wlien tlie animai is following its pavasitic 

 mode uf lite. Now we know tliat tliis mode of life is biglily irregulär, 

 for their is do particular species of tish to wbich the attentions of 

 the larvae are eonfined and they may attach themselves to the 

 gills or to any i)art of the body or fius inditfereutly. Tlien they 

 are ouly loosely attached and can be brushed off with the greatest 

 ease. 



Combining these facts with the evidence that was produeed to 

 show that the bimodality of our curve was due to the existence of 

 two criticai periods for the final transformation , the presumption is 

 strong that the size to whìch any larva may attain is chiefiy due 

 to the conditions of nutrition it meets with, and its fortune in being 

 brushed off its host at an early or late stage of growth. In this 

 way we obtain a rational explanation of the immense range of va- 

 riability in the animai, wliich would be correlated according to our 

 view with the great irregularity of the conditions met with during 

 the Praniza state. But whether the size of the adult is intluenced 

 greatly by hereditary tendencies or not, it is at any rate certain 

 that the larger adults owe their size to the length of time spent 

 and the amount of nutrition acquired during the Praniza stage, be- 

 cause the differeuce in size of the segmented larvae from variously 

 sized females is not sufficieut to account for the differences in size 

 of the adults. 



The great variability in size of the male and the bimodal fre- 

 quency of the sizes led me to suppose that we had bere an instance 

 of ;>high« and low« dimorphism, the occurrence of which lias been 

 noticed in Lamellicorn beetles, and the Earwig by Batesox & 

 Bi;iXDLEY (5) and according to Fritz Müller (6] in the Tanaidae 

 and Orchestiidae. I thereforc proceeded to make a series of mea- 

 suremeuts upou two hundred males of G. ìiiaxillaris to see if there 

 was any difference of structure in the mandibles of large and small 

 individuals. The result of this iuvestigation is set out in Table 2 

 and is of some interest. 



