196 PROC. ENT. soc. WASH., VOL. 22, NO. 8, NOV., 1920 



Fig. 1 Molted heads of the five successive instars of a Phryganidia culi- 

 fornica caterpillar. 



hold entirely true in the case of Phryganidia. The ratio between 

 the first and second instars is 0.59; the second and third, 0.78; 

 the third and fourth, 0.76; the fourth and fifth, 0.74. It may 

 be seen that the first ratio is quite different from the other three 

 and becomes more pronounced when the headwidths are cal- 

 culated from it. The head measurements actually found and 

 those computed from the ratios are listed in Table No. 2. 

 Table No. 2. Dyar's Ratio Applied to Phryganidia. 



1. Ratio 0.59. . 



2. Ratio 0.78. . 



3. Ratio 0.76. . 



4. Ratio 0.74. 



Instars . 

 Found. . 

 Calc. . . . 

 Calc. . . 

 Calc. . . 

 Calc. . 



Growth. 



The fact has been mentioned several times by others that the 

 caterpillars had enormous heads. This is not true at certain 

 periods. During the first part of each instar the head is appre- 

 ciably wider than the body, but the latter grows until at the end 

 of the instar it is about the same size or may even be wider than 

 the head. Fig. 2 gives a graphic comparison of the width of 

 the head and body. The succession of steps in the line repre- 

 senting the head widths is due to the sudden changes at molting. 

 During the somnus before each molt the width of the body de- 

 creases slightly due to the failure to take food. This is the cause 

 of the small irregularities in the body width curve. During the 

 last somnus the body increases in width in spite of the fact that 

 it does not take food, but may be accounted for by the shorten- 

 ing of the body to a great extent. 



Fig. 3 shows graphically the rate of growth in length of the 

 caterpillars during each instar. It is easily seen that this is by 

 no means constant. After each molt the growth is quite rapid. 

 Upon getting filled the caterpillar again enters the somnus 

 and in this period, loses considerable in length. During the last 



