PROC. ENT. SOC. WASH., VOL. 22, NO. 8, NOV., 1920 



dividuals, while many of those reared in a rather dark room are 

 yellow. 



Larval Instar Characteristics. 



Length, size of head, color, certain bands and lines, etc., differ 

 in each instar and furnish characters which can be used in sep- 

 arating them. As separate descriptions are not necessary here, 

 a table (No. 1), which indicates the chief characteristics for each 

 instar, is offered on account of being more compact and the com- 

 parison of each feature much simpler. 



The most reliable of these characters is the size of the 

 head, which remains constant during each stage. Most of the 

 other characters are constantly changing. In fact, the head is 

 the only dependable character to be used in distinguishing the 

 different stages. Even this falls down occasionally, the extreme 

 of one instar being indistinguishable from the opposite extreme 

 of the preceding or succeeding stage. In these cases there is 

 apparently no method of ascertaining the instar except by con- 

 fining the caterpillar during its whole life and watching the num- 

 ber of its molts. The extremes generally undergo an abnormal 

 number of molts. On normal individuals one can distinguish the 

 larval stages very easily with the naked eye after a certain amount 

 of practice. 



The Molt. 



The caterpillar enters a somnus or resting stage before molt- 

 ing, having first spun a slight web on the surface of a leaf, in 

 which to entangle the legs, to aid in removing the cast skin. The 

 head is curled back by the side of the body during this trans- 

 formation. A white area appears behind the head gradually 

 growing to the size of the next head and forcing the old head off 

 as it grows. Finally the skin breaks, the head dropping off over 

 the mandibles while the skin slips off over the tail. With many 

 larvae the cast head remains attached to the cast skin, but with 

 Phryganidia the head separates entirely from it. At the last 

 molt, when the pupa emerges, the head splits over the center 

 instead of coming off whole, and this time remains attached to 

 the cast skin. Fig. 1 shows the appearance of the cast heads 

 and their comparative sizes. 



Dyar's Ratio. 



In an article written by Dr. Dyar 1 a number of years ago, he 

 mentions the fact that a certain ratio exists between the head 

 widths of the different instars. For instance, if one has tin- 

 head measurements for two consecutive instars, the head widths 

 of all instars can be calculated, approximately. This does not 



1 Dyar, H. G. Psyche. Vol. 5., pp. 420-422. 1890. 



