W. RiTciiiK 185 



Instead one usually finds all the inner bark layers converted to 

 dust by the work of parent beetles and their larvae. The best way 

 to ascertain the shape of a typical mother gallery is to dissect the 

 gallery immediately after the female has completed her egg-laying and 

 before any of the eggs have hatched. To trace the larval galleries 

 accurately it is essential in most cases to follow up the larvae as they 

 tunnel their respective galleries. 



LENGTH OF LIFE-CYCLE IN A SINGLE GENERATION. 



Many observations repeated under different conditions proved to me 

 that the period of the life-cycle from the laying of the egg to its culmina- 

 tion in the adult stage was extremely variable. The time varied according 

 to the season of the year at which the eggs w^ere laid, the quality of the 

 food material, and the position of that material with relation to the sun : 

 and consequently the environmental temperature. 



In the vicinity of Aboyne, Aberdeenshire, field observations were 

 carried out in early April, 1917, in Silver Fir woods badly infested with 

 Cryphalus abietis. Careful examination was made of all likely places for 

 females to have chosen for new brood galleries but no adults were found 

 at work. The weather up to this time had been exceptionally unfavour- 

 able. 



On the 28th September, 1916, however, I had collected some badly 

 infested material consisting of branches and twigs from these same 

 Silver Fir woods. Examination of this material at the time of collection 

 showed that Cryphalus abietis was present underneath the bark layers 

 in exceedingly large numbers both as immature fresh young adults and 

 as larvae. This badly infested material — referred to later in this paper 

 as my "stock material" — was kept in the open air throughout the 

 winter months and examined at intervals. On the 20th of April, 1917, 

 one adult issued. Several more adults issued up to 27th April, 1917. 

 Some of these were killed and dissected for the study of their repro- 

 ductive organs but 50 others were liberated in a muslin bag containing 

 a few branches of Silver Fir and Spruce. These branches had been 

 paraffined at their cut ends to keep them from becoming too dry. This 

 bag was then closed and placed in the open air. Three days later, i.e. on 

 30th April, 1917, the branches in the bag were carefully examined. 

 Some females had just commenced to bore into the Spruce and Silver 

 Fir branches and in a number of cases the male beetle was found resting 

 on the surface of the bark close by the female. 



