338 EDWARD F. ADOLPH 
female, are not desposited without the presence of certain sensory 
qualities, such as touch, smell, and taste. No single quality 
produces a response at all comparable to the amount of egg- 
laying in nature; but combinations of moisture, texture, and odor 
call forth egg-laying of the same magnitude as that under opti- 
mum natural conditions. Egg-laying in its nature is a complete 
response (‘all or none’); that is, partial stimulation cannot be 
measured. A single potent factor in the chain may never lead 
to the extrusion of eggs. The mechanism is comparable to that 
of mating, which Sturtevant (’15) describes thus: ‘‘Sex recog- 
nition at a distance is by smell, but the actual process of copu- 
lattron depends upon the sense of touch.” 
The stimuli which produce the egg-laying response are not 
specific ones for this reaction. Many of the same ones lead the 
flies to food materials, and Barrows (’07) considers the latter the 
underlying factor in the tropic response to odorous substances. 
The odorous substances found to be most attractive in his ex- 
periments are also those which produce the largest amount of 
egg-laying when accompanying conditions are favorable. 
The attractiveness of organic compounds is often very specific, 
as illustrated by the work of Howlett ('15) on the odor responses 
of several species of melon flies (Bactrocera). The males of 
one species gather with great haste and accuracy to isoeugenol, 
those of a second species to methyleugenol, those of a third 
species to both compounds equally. These compounds are found 
in oil of citronella. 
It is well known that the substances attractive to Drosophila 
commonly occur in fruits. Ethyl alcohol is the largest: product 
of yeast fermentation, and acetic acid of bacterial fermentation, 
which follows yeast fermentation. Amyl acetate has been actu- 
ally identified in bananas (Kleber, ’12), and it is the character- 
istic ester of pears as well. Grapes are characterized by the 
presence of ethyl formate. It must be understood that wherever 
the esters occur, smaller amounts of the corresponding acid and 
alcohol are present. Many of the acids are not odorous, but 
all are stimulating to the human sense of taste. Malic acid is 
the only important acid in bananas (Bigelow and Dunbar, ’17), 
