34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 'J'] 



in from lo to 15 minutes, though, with the thicker walls of the 

 crop, the resuhs of diffusion are not apparent until nearly 24 hours 

 later. The cuticula of the crop is 5 to 8 microns in thickness, while 

 that of the intestine is but 2 microns thick. 



The cuticular walls of insect sense organs are in many cases ex- 

 tremely delicate, frequently not over half a micron in thickness, and 

 in some cases so thin that sections of them do not show a double 

 border under even the highest magnification. It is, therefore, not 

 unreasonable to suppose that they are quickly permeable by sub- 

 stances in solution. Vogel (1923) has noted that the membranous 

 cupola of a sensillum basiconicum of a wasp is colored by hematoxy- 

 lin stain, and hence is permeable by it. We may believe, therefore, 

 that sense organs in which the walls of the external part are at some 

 point reduced to a thin membrane, half a micron or less in thickness, 

 are organs capable of receiving chemical stimuli. If they are in fact 

 chemoreceptors, then either they must be penetrated directly by odor 

 or taste substances, or a liquid must exude from within them capable 

 of absorbing such substances, thus providing the means of their 

 transmission by osmosis to the ends of the sense cell processes. 



The chief objection to the idea that a liquid exudes upon the sur- 

 faces of sense organs is the lack of any observations on the presence 

 of such a liquid. Yet, the vacuole which surrounds the distal proc- 

 esses of the sense cells in many organs that have been regarded as 

 chemoreceptive, suggests a possible source of a solvent liquid. Though 

 Berlese's attempt to show that one of the elements of the insect 

 sensillum is always a gland cell has not been generally accepted, it 

 is not unreasonable to suppose that one of the cells might take on a 

 secretive function in certain organs. The cell which contains the 

 vacuole, when a vacuole is present (fig. 13 B, Vac), however, is the 

 basal enveloping cell (ECl), which is the trichogenous cell (fig. 12, 

 HrCl) and not a special gland cell. 



What takes place within the sense organ when the latter is pene- 

 trated by the stimulating force or substance is entirely unknown, but 

 it seems most probable that there must be produced some chemical 

 change in the substance of the sense cell, which, in turn, acts upon the 

 nerve and causes the latter to transmit a stimulus to the sensory center 

 of the central nervous system. Perhaps every sense cell can be 

 stimulated by a variety of stimuli, that which actually reaches it 

 being normally determined by the nature of the external part of the 

 organ ; but the fact that in certain sense organs there is no specialized 

 external part argues in favor of specificity even in the sense cells 

 themselves. 



