History and Habits of Pityogenes. 39 
reason for this doubtless is that the strongest muscles of the 
larvae are those used in burrowing and in chewing, and as 
these are located in the head, the covering of the head must 
necessarily be more rigid in order to afford a firm insertion 
for these muscles. For this reason the size of the head 
following each of the various moults is fairly constant and 
forms a reliable index by which one may ascertain the par- 
ticular instar to which a larva belongs. On the other hand, 
the width of the thorax and the total length of the larva 
varies so greatly that these dimensions are practically value- 
less in getting at the age of a larva. These facts are shown 
by the accompanying table. 
This table shows size and proportion of larve of Pityogenes hopkinsi 
Swaine at each of the various instars. As will be seen, the width of 
the head is fairly definite for each instar while other proportions are 
quite variable. 
Length of 
Width of head Width of thorax Length of larva burrows 
Instar. in mm. in m.m. in m.m. in m.m. 
tin; Uae .300—.315 .250-.320 .o18= .720 0 -3 
7700 |e es ae .345-.370 . 300—. 540 . 680-1 .20 2%- 7 
SING Oe eee 412—.450 .420—.650 .937-1.72 5 -12 
Ot, ee aaa -475—.500 .471-—.812 1.45-1.83 8 -—20 
UDMA Ee. cio s« .520—.550 .612—.938 1.50-1.98 15 -41 
Tur Larvat Burrow.— In all cases where the act of 
hatching was observed, or where conditions were studied 
soon after hatching, the larva emerged from the egg on the 
side next to the bark. This would seem to be quite important 
as its food during the early part of the larval history consists 
entirely of bark. Usually approximately half of the larval 
burrow continues upon the surface of the wood but involves 
the inner bark only, while the burrow made by the older 
larvae grooves the sapwood deeper and deeper and the bark 
less and less as the larvae approaches full growth (Figs. 27, 
28). Exceptions to this rule are found in broods occurring 
late in the fall, when the larvae show a decided tendency to 
groove the sapwood earlier and deeper than at other seasons. 
The entire larval burrow with the exception of the chamber 
in which pupation occurs, is always closely filled with frass 
