NO. 9 OLFACTORY SENSE OF INSECTS McINDOO 33 



Ruland (1888), who made a thorough comparative anatomical 

 study of insect antennse, contends that only such hair structures as 

 those which are perforated at the tips can be sensitive to chemical 

 stimuli. Pegs are found in all orders of insects and, since myriapods 

 and crustaceans possess similar structures, these organs may be con- 

 sidered as the chief form of olfactory organs in the arthropods. 

 Ruland regards the pit pegs and Forel's flasks found in most insects 

 as simple pit pegs, while the compound pits, as seen in the antennae 

 of tlies and butterflies, he calls compound pit pegs. He believes that 

 all three sets of these organs are organs for the reception of stimuli 

 from certain olfactory substances. To determine whether all of the 

 hair structures are perforated at their tips, he put the antennae into 

 boiling caustic potash. After such treatment he observed that they 

 were all open at the end. In the investigations made by the author 

 it was learned that caustic potash within a short time not only de- 

 stroys all of the internal tissue but it soon dissolves thin chitin. All 

 who have studied these structures before and since 1888 assert that 

 these hairlike organs are tipped with very thin chitin through which 

 the odorous particles must pass. In the observations made by the 

 author these structures in the antennas of the honey bee have not 

 shown a single hair which is open in the slightest degree at the tip 

 and it is probable that in Ruland's treatment the caustic potash 

 dissolved the thin chitin at the tip. 



Nagel (1892, 1894, 1909, the views set forth in the first reference 

 being cited by various authors,) states that, in his opinion, the ant en- 

 ne'e are generally the olfactory organs of insects — not, however, with- 

 out exception. That insects, after amputation of the antennae, seem 

 incapable of perceiving odors is not sufificient proof that the antennae 

 are olfactory organs. He declares (1894) that organs with thick 

 chitinous walls cannot function in smelling, but he thinks that the 

 olfactory pegs, being tipped with thin chitin, are capable of receiving 

 olfactory stimuli. He asserts that these olfactory pegs are found on 

 other parts of the body besides the antennae. He (1909) does not 

 doubt that in many insects the palpi may assist in smelling. In the 

 antennae of a May beetle there are four different kinds of pitlike 

 organs (varieties of pit pegs), all of which may be olfactory in func- 

 tion. In the Hymenoptera the antennae are the only seat for their 

 highly developed olfactory sense. In some Hymenoptera both pore 

 plates and pegs, while in others only the pore plates, function in smell- 

 ing. In ants the pegs and knee-shaped bristles probably serve this 

 purpose ; in Lepidoptera the pit pegs function for smelling when the 



