474 P. W. CLAASSEN 
channels. As many as eight larvae have been found together in one 
channel. It is probably due to such a crowded condition that a larva 
occasionally crosses over into another channel. 
After the larvae have mined down for a distance of twenty to twenty- 
four inches, they molt in the mines and immediately afterward leave the 
mines through a little exit hole which is usually made on the inner side 
of the leaf. As soon as the larvae appear on the surface of the leaf they 
at once seek shelter, usually continuing down the stem of the plant and 
crawling behind the sheath of one of the outer leaves. 
Since the larvae later. become true stem borers, the question arises 
why they should come out of the mines of the leaf ten or fifteen inches 
away from the stem instead of remaining in the mines and working down 
the leaf until they reach the stem and can enter it directly. There are 
two plausible reasons against such behavior: first, the larvae later become 
solitary borers and after coming out of the leaf they separate and indi-| 
vidually enter the stems of different plants; second, the width of the 
head of the second-instar larva exceeds the width of the average longi- 
tudinal channels in the cat-tail leaf. Careful measurements of the molted 
heads of the first-instar larvae and measurements of the width of the 
average channel of the leaf showed that the width of the head during the 
first instar was only slightly less than the width of the channel. The 
width of the heads of the second instar was considerably wider than the 
width of the channels of the leaf. Following are the average measurements: 
Width of head of first-instar larva........... 0.597 millimeter 
Width of head of second-instar larva......... 0.90 millimeter 
Width otchannelintleaia- 2 0.62 to 0.72 millimeter 
It would therefore be impossible for the larvae to remain in the leaf after 
the first molt unless they widened the leaf by taking out the longitudinal 
fibrous partitions. 
Although the larvae ultimately become solitary stem borers, they do 
not always scatter immediately after emergence from the mines and 
bore directly into the stems of the plant. In one instance, on June 29, 1916, 
it was found that a whole contingent of larvae had migrated to the head 
of the plant, where they found shelter behind the leaves that were 
sheathing the flower spike. Here they were feeding on the staminate 
flowers. A few days later they had all descended and scattered to different 
