126 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



going into the details of his experiments, it may be said that 

 Mclndoo has given considerable evidence that the olfactory 

 receptors do occur on regions of the body other than the antennae. 

 He has not, however, advanced sufficiently convincing proof to 

 discount the accumulated evidence that the latter organs play a 

 definite part, and in many insects a predominating part, in 

 olfactory sensation. 



Olfactory Sensillae. It will be convenient at this point to 

 consider the general structure of presumed olfactory sensillae. 

 Berlese (1909) maintained that a chemoreceptor contains both 

 glandular and sensory cells. The glandular secretion serves to 

 keep the cuticular part of the sensillae moist, and therefore in a 

 condition for the reception of stimuli. It is presumed that the 

 secretion fills the cavity of the organ and reaches the exterior 

 either by filtration through the covering membrane, or by means 

 of minute pores in the latter. The belief that a liquid, or mucous 

 layer, is a necessary component of such a type of sensilla implies 

 that the particles of a volatile compound must either become 

 soluble in that liquid, or enter into some form of combination with 

 it, before any stimulus will be appreciated by the nerve-ending. 

 Obviously, such a conception is based upon analogy with the 

 vertebrate olfactory organ, but Berlese considers that what 

 evidence there is points to the same process occurring with insects. 

 He quotes observations of Erichson made in 1847 and of Saulcy 

 in 1891, that the antennae of insects are covered with a delicate 

 film of liquid. Recent investigators have advanced little support 

 to Berlese's contention, since they have found no clear evidence of 

 gland cells in connection with any type of presumed olfactory 

 sensilla. On the other hand, Snodgrass (1926), in his very clear 

 account of insect sense-organs, suggests that the vacuole which 

 surrounds the process of the sense-cell in many of these organs 

 may be a possible source of a solvent liquid. It is noteworthy, 

 as he also points out, that the cuticular walls of insect sensillae 

 are often extremely delicate, in many cases not more than half a 

 micron in thickness. They are frequently so thin that in section 

 they do not show a double border when viewed under the highest 

 magnification. There is good reason to believe that such 



