1 10 
Psyche 
[June 
Text-fig. 3. Apparatus used to stimulate millipedes electrically and to 
trap the hydrogen cyanide liberated. Details in text. 
Special precautions were taken to prevent millipedes from dis- 
charging prematurely before the assay. When first collected in the 
field } they were allowed to crawl into individual vials, and from the 
vials into cages, without being handled in the process. Under these 
circumstances they do not discharge, as was determined by intro- 
ducing strips of filter paper impregnated with benzidine reagent 
(Feigl, i960) into the closed quarters of newly captured millipedes. 
The strips always remained white instead of turning blue, indicating 
absence of cyanogenesis. Likewise, in the laboratory, when the 
millipedes were transferred from cages into the apparatus used for 
the assay, the transfer was effected by way of vials without direct 
handling. 
The assay 
In principle, the assay for hydrogen cyanide is based on the fact 
(Deniges, 1895; Liebig, 1851) that a turbid colloidal suspension of 
silver iodide in aqueous ammonia can be converted into an optically 
clear solution of silver ammonium cyanide [Ag(CN) 2 NH 4 ] by addi- 
tion of hydrogen cyanide (or other sources of cyanide ion). For a 
given volume of suspension, the amount of cyanide that causes clear- 
ness is constant, and can be determined by titration. If an unknown 
quantity of cyanide is added (for instance, as hydrogen cyanide from 
