An idea of the enlargement in the head during the seeond instar 

 may be gained l\v tlie following figures, taken from measurements 

 of living grubs : 



HEAD OF SEOOND-INSTAK GRUBS. 



All average from 23 gi-ubs varying from 18 to 2o mm. in lonstli; head 3.25 

 mm. wide. 



An average from 20 grubs varying from 26 to 30 mm. in length; head 3.42 

 mm. wide. 



The third instar. — The average duration of the third instar. fro'n 

 records of 25 grubs, was found to l)e 183 days; the maximum 266 

 days: the minimum. 78 days. Again, the maximum duration of the 

 instar is more than three times the minimum. Reduced to montlis. 

 the average length of the third instar is six months; the maximum, 

 nine months; and the minimum, two and one-half months. 



Approximately two-thirds of the larval period is spent in the 

 third or last instar, or about three-fifths of the entire life of the 

 immature insect. During more than six months of the year this 

 insect is doing great injury to crops in the soil as a grub. There 

 being no winter in Porto Rico during which the grid) is not feeding, 

 its activity as a root trinuner extends not only through five and one- 

 half months of the third instar (allowing one-half month for the quies- 

 cent, pre-pupal stage), but also through the month and a half of the 

 second instar, so that the period of injury may easily cover seven 

 months. 



At beginning of the third in.star (see Plate 111, fig. 6) the grub 

 averages about 28 nun. in length, and when full grown (see Plate III, 

 fig. 7) it reaches a length of 40 mm. to 44 mm. The average width 

 of head from 96 grubs measured in the third instar was 5.29 mm. 

 (See Plate V, fig. 3.) 



The rate of growth of the head during this instar may be seen 

 from the following table : 



HEAD OF THIRD-] XSTAR GRUBS. 



Average from 30 grubs varying from 26 to 30 mm. long; head 5.3 3 mm. 



Average from 19 grubs varying from 31 to 3.5 mm. long; head 5.26 mm. 



Average from 42 grubs varying from 36 to 40 mm. long; head 5.39 mm. 



Average from 5 grubs varying from 41 to 45 mm. long; head 5.45 mm. 



Tlie factors which influence the rate of growtli in tlie third instar 

 of the grub are : 



(1) Available food supply: i. e., contiguity of abundant living- 

 plant roots. 



(2) Humidity and texture of soil. Hardness or softness of the 



75 



