50 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. "JJ 



bratory stimuli, or that they register air pressure. Experiments made 

 by Mclndoo (1914) and others appear to indicate that insects are 

 responsive, in a certain degree, to odors by organs other than those 

 of the antennas, and it is most reasonable to suppose that these or- 

 gans are the widely spread campaniform organs. Mclndoo's claim, 

 however, that the outer membrane of the- small disc-like organs is 

 perforated, and allows the distal end of the sense cell process to come 

 into direct contact with the air, has not been verified. All other re- 

 cent students of the campaniform organs, including Freiling, Vogel, 

 Pflugstaedt, Erhardt, Sihler, state that the outer membrane or the 

 outer lamella of the dome is never traversed by a pore or other open- 

 ing. The closing membrane, though, may be very thin ; Vogel says 

 that the outer lamella of the organs of the wing bases of Lepidoptera 



SCI 



Fig. 22. — A rectal sense organ of larva of Oryctes (Orlov, 1924), 

 apparently a simple organ of campaniform type. 



a, outer membranous disc; CI, cell beneath disc; d, distal process 

 of sense cell (SCI) ; Nv, nerve fiber. 



is only from 8/10 to I micron in thickness. Such organs might be 

 chemoreceptors ; but, as already noted, chemoreceptive organs in gen- 

 eral are innervated through a group of sense cells, while all campani- 

 form organs have a single sense cell, a feature characteristic of sense 

 organs responding to mechanical stimuli. The outer part of many of 

 the campaniform organs, on the other hand, is so thick as to preclude 

 any idea of a chemical sense in connection with them. 



VI. THE PLATE ORGANS 



The sense organs known as the sensilla placodea, in their typical 

 form, consist of thin chitinous plates, elongate, elliptical, or oval 

 in outline, set over large cavities or pores in the cuticula. They 

 were, therefore, designated "pore plates" by Leydig (i860) and 

 they have since commonly been known by this rather ambiguous 

 name. 



