NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 33 



Vogel, shows two stained cells of a sensory group with their proximal 

 and distal processes clearly defined. The structure of a sense cell 

 group might, therefore, be represented diagrammatically as at B 

 of the same figure. The findings of those who have used differential 

 staining methods do not substantiate the idea of Berlese (1909) that 

 the nerve end invests the sense cell ; but it appears likely that Berlese 

 mistook for nerves the nucleated sheath which is continued over the 

 sense cell and its distal process from the neurilemma of the nerve 

 trunk. 



THE RECEPTION OF SENSORY STIMULI 



The question now^ arises whether the specific effect of the stimulus 

 aft'ecting a sense organ depends on the character of the external 

 part, or on the nature of the sense cell itself. It is certain that the 

 cuticular part of a sense organ must be adapted to receiving the spe- 

 cific stimulus to which the organ responds ; the outer part of an eye, 

 for example, must transmit light, and an auditory organ must receive 

 sound waves. The receptive part of each sensillum must be so con- 

 structed that it will let in its particular class of stimuli and keep out 

 all others. Chemical stimuli could not be supposed to penetrate a 

 thick-walled structure, which, however, if loosely articulated, might 

 respond to purely mechanical stimuli. On the other hand, an organ 

 responsive to stimuli of taste and smell, a chemoreceptor, presupposes 

 tliat the exposed part of the organ is somehow penetrable by odor or 

 taste substances. 



The earlier students of the sense organs of insects commonly as- 

 sumed that those organs which they believed to be organs of smell 

 and taste had perforations in their cuticular walls which allowed the 

 substances to be perceived to come into direct contact with the ends 

 of the sense cell processes. The presence of slits, pores, or openings 

 of any kind in the outer covering of any insect sense organ, however, 

 has been denied by all recent writers, except ]\IcIndoo (1914), and 

 the major part of opinion now favors the idea that a chemoreceptive 

 sense can be possessed only by those organs in which the cuticular 

 walls are thin enough to be penetrated by substances of taste or 

 smell or both. 



That osmosis takes place readily through thin layers of the cuti- 

 cula of insects has been demonstrated ; and transudation must be 

 assumed to take place in all glands derived from the ectoderm, the 

 interior surfaces of which are covered by a delicate, imperforate 

 cuticular intima. Eidmann (1922) has specifically shown that both 

 acids and alkalies diffuse through the intestinal walls of the cockroach 



