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J. E. WODSEDALEK. 



same in the twenty-three species which he examined. It is not 

 entirely safe, however, to infer from this that the prominence of 

 Palmen's organ is essentially similar in these various species. 



The species upon which the present study is based are Hepta- 

 genia interpundata and Ecdyurus maculipennis. These two forms 

 are very closely allied, not only in matters concerning this organ, 

 but also in their natural habits and general behavior, and the 

 present paper will concern itself with nymphs of //. inter pun data, 

 unless otherwise specified. 



FIG. A. Head of H. inlerpunctata nymph. Basal joint of antennae only drawn. 

 The brain is drawn, dotted, under the three ocelli just posterior to which is shown 

 the Palmen's organ and the four tracheal tubes leading into it. 



Fig. A shows the relative position of the organs in the head 

 of a nymph. Palmen's organ together with the tracheal tubes 

 leading to it can be readily seen through the chitinous covering, 

 especially in the newly moulted specimens, by placing them under 

 a binocular. It is symmetrically located between the two large 

 compound eyes and a little posterior to the brain. Fig. I shows 

 the organ in its relation to the entire tracheal system of the head. 

 It has been the fortune of the writer to be able to make a perfect 

 dissection of the system, the first time merely through an acci- 

 dent. Upon pkicing a specimen which had been dead for some 

 time under a binocular almost the entire tracheal system of the 

 insect became visible through the transparent chitinous covering. 

 The muscles and all the other soft tissues had sufficiently de- 

 composed to form a sort of liquid mass. The thin hypodermal 

 walls surrounding the air tubes too had disintegrated, and prac- 

 tically all that remained in good condition was the exoskeleton 

 and its internal continuation, the tracheal system. The location 



