256 J. E. WODSEDALEK. 



of cross sections show that the organ is not perforated with air 

 passages but is a continuous mass of chitin in which the differ- 

 entiations are due mainly to variation in density of color. Sec- 

 tions of adult specimens weie also made and carefully examined, 

 but no difference in the structure of the organ could be detected. 

 Dr. Gross has no doubt mistaken the clearer areas or concentric 

 layers for air passages and the darker layers for separate solid 

 areas forming the scales from which the hairs protrude. 



Fig. 4 shows the external dorsal appearance of the organ and 

 its relation to the four tracheal tubes, the entire structure being 

 enveloped by the hypodermal layer; Fig. 5 is a horizontal section 

 of the same. The description of the organ can be best understood 

 by studying it in connection with its development and growth. 

 It is a well understood fact that the tracheal system in insects is 

 formed by the invagination of the ectodermal layer. As to the 

 origin of Palmen's organ I am not at all certain for embryonic 

 material has thus far in this study not been available. The 

 appearance of the structure of the central portion of the organ 

 suggests that, during the process of the development of the 

 tracheal system, the four large tubes leading to the organ (Fig. i) 

 come together at a common point; here the blunt ends of the 

 invaginated portions, the tracheae, surrounded by the hypo- 

 dermis, fuse and secrete this common center. From the various 

 cross sections of which Fig. 8 is typical, it can be inferred that the 

 two posterior tubes come together first and that a portion of the 

 center is secreted before it is met by the two anterior tubes. 



In the many sections of //. inter pnnctata and E. niacnlipennis, 

 which were examined, the center of the organ does not show any 

 ring-like structure, but is an irregular mass which is apparently 

 M-nvted Ix-forc the hr\.i c ists the firsl limii'j <>i its trachea] 

 system. At the time of this first ecdysis which is accompanied 

 by the shedding of the inner lining of the air tubes, this central 

 mass is larger than the openings in any of the four tubes and hence 

 the impossibility of its being cast out of the body. Shortly 

 after the casting of the inner lining of the trachea?, the hypo- 

 dermal cells surrounding the tubes undoubtedly begin to secrete 

 the new chitinous wall. The hypodermal layer surrounding the 

 central mass, the beginning of the Palmen's organ, is continuous 



