CHELURA TEREBRANS PHILIPPI 91 



When chelurids were present in wood collected in the harbor, adults 

 were always found inhabiting the outer tiers of the eroded wood but 

 juveniles often were found in the deepest tiers of limnoriid galleries 

 where adult chelurids were too large to penetrate. 



NEED OF CHELURIDS FOR PROTECTED NICHE 



The fact that chelurids failed to appear as original infestants of 

 freshly exposed wood led to further experiments to test their need for 

 a protected niche. Fresh blocks of wood were prepared with a series of 

 ^ inch wide furrows sawed on all sides. Each of these, along with a 

 smooth block used as a control, was immersed in the California Yacht 

 Basin and collected after an exposure of 40 days. The results of one of 

 these experiments, begun on March 23, 1951, and ended on May 3, 

 1951, are given below: 



Number of animals infesting blocks 

 Chelura terebrans Limnoria tripunctata 



Smooth block 4 165 



Grooved block 108 2063 



Repeated experiments of this kind showed that the prese.^e of fur- 

 rows on blocks freshly exposca allowed larger populations of migrant 

 borers to survive than did smooth wood. Thus, we may infer ( 1 ) that 

 migration rates are higher than indicated by the smooth block controls, 

 and (2) that few or no chelurids survive the attempt to occupy smooth 

 wood. It is possible that predators such as polychaetes (see Reish, 1954) 

 and fish are responsible for reducing the number of migrating animals. 

 Because limnoriids can excavate a protective niche in wet Douglas Fir 

 within 24 hours (demonstrated in a laboratory experiment) while 

 chelurids may take upwards of four weeks, the gribbles are more suc- 

 cessful as first infestants. 



ABILITY TO DISTINGUISH TYPES OF BORING 



Many environmental factors may affect natural populations of wood 

 borers. Some of these variables are enumerated below: 



1. The structure of the wood, depending on the species of tree, 

 softness, and orientation of the grain. 



2. Length of exposure of the wood in the water. 



3. Presence or absence of preservatives. 



