110 THE COCKROACH: 



Sense of Smell in Insects. 



c/ 



The existence of a sense of smell in Insects has probably 

 never been disputed. Many facts of common observation prove 

 that carrion-feeders, for example, are powerfully attracted 

 towards putrid animal substances placed out of sight. The 

 situation of the olfactory organs has only been ascertained by 

 varied experiments and repeated discussion. Rosenthal, in 

 1811, and Lefebvre, in 1838, indicated the antennae as the 

 organs of smell, basing their conclusions upon physiological 

 observations made upon living insects. Many entomologists of 

 that time were inclined to regard the antennae as auditory 

 organs.* Observations on the minute structure of the antennae 

 were made by many workers, but for want of good histological 

 methods and accurate information concerning the organs of 

 smell in other animals, these proved for a long time indecisive. 

 It was by observation of living insects that the point was 

 actually determined. 



/ 



Hauser's experiments, though by no means the first, are the 

 most .instructive which we possess. He found that captive 

 insects, though not alarmed by a clean glass rod cautiously 

 brought near, became agitated if the same rod had been first 

 dipped in carbolic acid, turpentine, or acetic acid. The antennae 

 performed active movements while the rod was still distant, and 

 after it was withdrawn the insect was observed to wipe its 

 antennae by drawing them through its mouth. After the 

 antennae had been extirpated or coated with parainn, the same 

 insects became indifferent to strong-smelling substances, though 

 brought quite near. Extirpation of the antennae prevented flies 

 from discovering putrid flesh, and hindered or prevented copu- 

 lation in insects known to breed in captivity. 



Following up these experiments by histological investigation 

 of many insects belonging to different orders, Hauser clearly 

 established the following points, which had been partially made 

 known before : 



The sensory elements of the antennae are lodged in grooves 

 or pits, which may be filled with fluid. The nerve-endings are 

 associated with peculiar rods, representing modified chitino- 



* References to the literature of the question are given by Hauser in Zeits. f. wiss. 

 Zool., Bd. XXXIV., and by Plateau in Bull. Soc. Zool. de France, Tom. X. 



