74 SEVENTEENTH Report STATE ENTOMOLOGIST OF MINNESOTA—1918 
all of the coarser cereals the apparent size is much greater, as in bran or 
rolled products where it is difficult to separate the eggs from the flakes 
of cereal to which they adhere. 
The entire larva of the sixth instar is shown in Pl. IV, Fig. 4 and 
the heads of larvae of the other five instars are represented in the proper 
proportion (Pl. IV Fig. 3 a,b,c,d,e). At first the larval head measures 
only 0.1456 mm. at its widest point, and the entire larva is only 0.91 mm. 
in length. At each moult the width of the head increases to such an 
extent that one can usually determine the instar of a given larva by 
measuring the head. As the result of the measurement of a large 
number of larvae it was found that the average head measurements for 
each of the six instars were 0.1456 mm., 0.182 mm., 0.273 mm., 0.364 
mm., 0.546 mm. and 0.728 mm. Occasionally larvae were found which 
were almost intermediate between the usual measurements for two 
successive instars. In such cases it is not possible to determine with 
accuracy to which instar the larva belongs, except by following the in- 
dividual life cycle until the time of pupation. However, the records 
of such individuals have shown them to be undersized larvae which 
continue to lag behind the usual size for each succeeding instar until 
they eventually emerge as undersized adults. Therefore it is usually 
safe to assume that a larva which has a head measurement interme- 
diate between those given for two different instars belongs to the 
instar of the next larger head measurement. 
The pupae (PI. [IV Fig. 6) have been found to average about 3.858 
mm. in length and the average width of the heads, at the widest point, 
has been found to be about 0.819 mm. This size is, however, subject 
to the same variation as that of the larvae, as has been stated above. 
The sexes of the pupae have been distinguished by the external genital 
organs which may be seer on the terminal abdominal segment. The 
female possesses a pair of appendages on the terminal segment, while 
the male presents only indistinct elevations in this region. (Compare 
male and temale Plo Pic. 76>) 
The variations in the size of the larvae and the pupae result in 
corresponding variations in the size of the adults. The measurements 
at hand vary from 3.55 mm. to 4.25 mm. The beetle figured (Pl. IV) 
is of average size and drawn in proportion to the larvae and the pupa. 
Tue Lire History or Tribolium confusum 
The exact number of eggs laid by a female beetle has not been 
determined with any great accuracy. Ten pairs of adults were isolated 
upon their emergence and each pair was placed in a separate container. 
