Biorhiza Group. 147 



(i) The larva developes during the same year and 

 acquires its full growth ; then it rests a year, or even 

 more, in the gal) (species of genus Aphilotrix). 



(2) The larva during the first year undergoes only 

 partial development, passes the winter, and does not 

 complete its evolution until the following year. 



(3) Larval development undergoes a period of com- 

 plete suspension after the larva has left the Qgg and 

 commenced to form the gall. It remains dormant some 

 months, and only begins to grow again when the gall 

 falls to the ground {Neuroterus). 



The prolonged larva rest of certain species is very 

 curious ; and that which is especially remarkable is, that 

 we often see them remain unchanged for three years 

 before they enter the pupa state. Even among species 

 which have no alternate generation, there are some galls 

 from which the fly does not emerge until the third year. 

 From the regularity with which this occurs, it seems to 

 be perfectly clear that there are individual differences 

 in the duration of development, consequently we find that 

 in the same species some individuals will complete their 

 development in one year, while others will require two. 

 The prolongation of the larval stage is a remarkable 

 phenomenon ; one would rather have imagined that 

 a short stage would have been of greater advantage to 

 the species, since the gall would have been exposed for 

 a less time to danger from changes of temperature. It 

 is possible for the two generations to complete their 

 evolution in one year, as in the Neuroterus-Spaihegaster 

 and the Dryophanta-Spathegaster groups. It is inter- 

 esting to find among the genera which have habitually 



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