Metamorphosis and Life-history of Gnathia maxillaris. 



477 



Table 2. 



j'requency distribution per ceut in 200 males of G. maxillaris (150 small and 



50 large , as regards mandibular index. Dotted curve refers to small males, 



continuous curve to large males. 



40 

 35 

 30 

 25 

 20 



15 



10 



5 



10 



11 



12 



13 



14 



10 11 12 13 



Scale of measurement. 



14 



15 



40 

 35 



30 

 25 

 20 



15 

 10 



5 



15 



The raeasurements taken are explained in figure 13. From the 



two measurements AB and BC au index was determined for eaeh 



AB X 10 

 mandible, the index being- — ^-z^ — '■> ^^^^ g'ives the relation between 



the leng'th and breadth of the mandible. 



The dotted curve in table 2 shows the frequency distribution 

 per cent of the index in »low« males of stature below 3 mm, the 

 continuous curve refers to »hig-h« males of 5 mm and upwards. Now 

 it is Seen that the modal condition of the index taken is the same 

 for large and small males, viz. 12, but a low index oecurs much 

 more frequently among small males than among large and vice-versa, 

 the difiference in frequencies being well without the liniit of their 

 pro))al)le errors. This means that on the whole small males have 

 mandibles that are broader an-d shorter than large males: in other 

 words, there is an incipient structural dimorphism in the mandibles, 

 in correlation with the diiference in size of the males. 



Mittheilungen a, d. Zool. Station zu Neapel. Bd. 16. 32 



