94 THE CANADIAN ENTOMOLOGIST. 



s^ 



own observation can be due to this insect, I wish to state beforehand 

 that I have no thought of discrediting Prof. Comstock's observations, or 

 questioning the abiHty of the Thrips he describes to cause all the injury * 

 credited to it. It is evident, upon slight examination of the subject, that 

 the same appearance of the grass may be produced by very different 

 agents, provided they attack the same point in the stem. Any injury to 

 the juicy base of the terminal node that cuts off the flow of the sap to 

 the head during a certain stage of its growth must produce the withering 

 and whitening so conspicuous in affected fields. Starting with this 

 premise it is reasonable to conclude that the trouble 7nay result from a 

 number of different agents, and such, I believe, to be actually the case as a 

 result from the sum of my observations here presented. 



During the past two seasons I have examined with care a great 

 number of affected stems, usually with the aid of a hand lens. For the 

 season just past my observations in the field were interrupted, shortly 

 after the appearance of Silver-top, by a trip to Washington. But while 

 absent I had a graduate student collect as many of the whitened stalks 

 as he could and place them in alcohol, and these have been examined 

 also so as to make the observations extend through as much of the 

 season as possible. In a very few cases I have seen evidence of fungi 

 present in the shrivelled base of the withered node, but so very few and 

 in such cases so evidently a consequent of the injury that I do not think 

 it can be credited with any of the damage. 



In a very small proportion of cases I have found Thripidce present in 

 the injured part, and in so few when the greatest care was taken to get 

 stems that were but just beginning to show injury, that I feel forced to 

 abandon the view that these are the principal agents in the injury here. 

 Dipterous larvse have been still less frequent and I feel positive that only 

 an exceedingly small part of the damage for the region studied can be 

 referred to them. Moreover, I think that in fully ninety per cent, of the 

 stems examined (so many examinations have been made at odd times 

 during my walks, and in spare moments, that no exact percentage can 

 be given,) no insect of any kind was found to be within the sheath 

 of the injured stem. 



Punctures of insects have been noticed in great abundance on the 

 parts of the plant around these injured parts, and in many cases evidence 

 of the puncture of the succulent portion itself was apparent. The 

 character of these punctures agreeing closely with those known to be 



