PITYOGENES IN SCOTTISH WOODLANDS. 1 05 



outline of the outer margin of the excavation (this has been 

 (described) and the size. P. bidentatus is always longer than 

 P. quadridens. The difference of size is, however, made less 

 conspicuous on account of the fact that the breadth of biden- 

 tatiis is greater in proportion than is the breadth of quadridens ; 

 the former is therefore heavier in appearance than the latter. 

 The average length of bidentatus male is about 2*7 mm.; 

 that of quadridens is about 2 2 mm. Less definite is the 

 difference of colour ; I have not found the differences described 

 to be very helpful. The fiat projections on the thorax are more 

 marked in bidentatus; generally quadridens is less hairy and 

 lighter in colour than bidentatus. 



Habit of Life and Host Trees of the Beetles. 



Both species of the genus are polygamous. The male beetle 

 bores into the bark of pine trees and hollows out a small 

 irregular copulatory chamber in the sapwood, this chamber being 

 well seen in the accompanying plates which are from photographs. 

 A very common point of entry into the bark is, especially in the 

 case of small twigs, below a branch whorl. Female beetles 

 follow the male at a later date and proceed to tunnel mother- 

 galleries, which radiate from the copulatory chamber. The 

 number of mother-galleries may be from 3 to 7. These 

 galleries are said to run in the sapwood when made by bidentatus, 

 and in the bast only when made by quadridens. The mother 

 beetles lay eggs in notches along the mother-galleries; from 

 these eggs the larvae hatch and tunnel outwards from the 

 mother-galleries. Both species may be found in branches 

 varying in size from J-3 inches in diameter. The freshness 

 of material required by the beetle varies within wide limits ; 

 it seems indeed to prefer material which is fairly dry. It is 

 possible that the flow of resin in fresh material is not favourable 

 to the insect. These facts are of some interest in considering 

 possible danger from these insects. The variation in the size 

 and freshness of material in which the beetles may breed may 

 determine in what region of the stem the mother-galleries are 

 cut; it does certainly determine the shape of the whole system 

 of galleries. It has been noted above that a frequent point of 

 entry is at a whorl of branches ; it may happen in such cases 

 that one or more of the galleries runs out into one of the side 

 branches. It is common to find both species in the one branch ; 



