45 
of natural selection. The twig-girdler of the dogwood is an ex- 
ample. 
This is a small, cylindrical beetle (Fig. 48), about half an inch 
long and less than an eighth of an inch in diameter, which prepares 
a chosen twig for jthe reception of the egg by first cutting a groove 
around it a few inches from its tip in such a way that the twig 
Fig 48. Dogwood Twig-gird- 
ler, Oberea tripunctata, aault. 
About 5 limes natural size. 
Fig-. 49, Cornus twig 
girdled by Dogwood 
Twig-girdler, Cbe7-ea tri- 
punctata, and part en- 
larged, showing egg in 
position. 
presently breaks off at this point, and afterwards making a second 
girdle, not so deep as the first, and from two to four inches farther 
back. (Fig. 49.) It then makes two parallel cuts, about half an 
inch long, lengthwise thru the bark between the two girdling in- 
cisions, and at the proximal end of these makes a short transverse 
slit in a way to form an angular flap, beneath which it pushes its 
egg. The effect of all this surgery must be to stop the growth of 
that part of the branch operated on, and to check the flow of sap 
to the section in which the egg is laid. 
