296 MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
tissues of the outer part of the phloem, and hibernate in this location. 
The following spring they emerge and seek material suitable for breed- 
ing purposes. 
‘Some of the most interesting habits of the family are those concerned 
with their construction of brood burrows in the bark or wood of trees 
and shrubs. The adults lay their eggs in their burrows where most 
of them spend the remainder of their lives and die. Some beetles, 
after laying a number of eggs, emerge and reattack the same or other 
host material, producing new burrows and another group of progeny. 
This may even be repeated several times. 
While the brood burrows are of a variety of different types, those 
made by the same species are usually so characteristic as to be readily 
‘identified, and often the insect responsible for certain injuries can be 
identified by the brood burrows more readily than in any other way. 
True bark beetles construct the entrance gallery diagonally through 
the bark to the surface of the sapwood. ‘There it is “either widened 
to form an irregular cavity or it is continued as one, two, or several 
egg galleries which may extend longitudinally, transversely, or 
diagonally in the cambium. Eggs are laid either loosely in a heap in 
the irregular cavity, placed singly in small conical niches in the sides 
of the egg gallery, in groups of several in larger cuplike cavities, or in 
larger groups in long egg grooves. 
Usually each larva excavates a separate mine through the bark, 
roughly at a right angle to the egg gallery, but sometimes the larvae 
work together in a common chamber. iEete ou takes place in a 
shghtly widened chamber at the end of the larval mine. The young 
adult beetles often feed in the inner bark for some time before emereg- 
ing, the feeding tunnels frequently merging to form larger, irregular 
cavities. Where a number of young adults thus feed in a group, one 
emergence hole may serve as an exit for all. Where young radults, as 
those of the hickory bark beetle and others, do not feed to any great 
extent in the old host, each emerges through a separate exit hole and 
produces the “shot-hole” effect frequently seen. The new adults of 
ambrosia beetles, however, leave by the same opening through which 
their parents originally entered the wood. 
Tn all the studies of emerging young beetles the two sexes have been 
found in approximately e equal numbers. This is by no means always 
true of the beetles found later in the newly established brood burrows, 
where, in many species, the females consider ably outnumber the males. 
In other species only females are present in the new burrows, while in 
still others the two sexes are equally represented. In explanation of 
these differences, some discussion of the reproductive habits is neces- 
sary. 
Fertilization of the female may take place in the old host in which 
the beetle has passed its immature stages, but usually in the new host. 
With certain ambrosia beetles, such as Anisandrus and Xyleborus, 
the males are much smaller and weaker than the females and their 
wings are not sufficiently developed for flight. In such species the 
females must be fertilized before leaving their parent burrows. 
Bark beetles may be either monogamous or polygamous, and in some 
cases where several males and several feriales are associated in the 
formation of one brood burrow, the relations might be described as 
promiscuous polygamy. ‘The character of the brood burrow and the 
