130 Notes on Chemotropism in the House- Fly 



intentionally not added to them, as moist substances always show a 

 certain attraction, when not actually repellant, on account of the water 

 contained by them. 



Experiment 4. Carbohydrates. 



Date:— September 10th-12th. 



General weather conditions: — Cool, rain. 



Duration of experiment: — 69 hours. 



Flies caught 



100 



In spite of the small numbers caught, there was an abundance of 

 flies round the traps, and the results show that carbohydrates in this 

 condition are very moderately attractive. 



The odour of banana is strongly reminiscent of amyl acetate, and 

 •minute quantities of the latter substance suggest that amyl compounds 

 might well be derived from ripening and decomposition products through 

 fermentation. 



Experiments were accordingly carried out to test the attractiveness 

 of these substances, relating to each other and to decomposing banana. 



They bear the formulae: amyl alcohol, CH 3 (CH 2 ) 4 OH; amyl acetate, 

 CH 3 (CH 2 ) 4 CH 2 COOH; valerianic aldehyde, CH 3 (CH 2 ) 3 CHO ; valerianic 

 acid, CH 3 (CH 2 ) 3 COOH. 



Though closely allied, the odours which they give off are very 

 different, the acid being especially pungent. 



Valerianic aldehyde took some time to prepare in the laboratory, 

 and was therefore not available for experiment until later. 



In the first of the two following experiments, an unripe pealed banana 

 was used: this was afterwards crushed and allowed to ferment in a 

 covered glass vessel for four days, at the end of which time it was used 

 for the second experiment. The liquids were used in a pure, undiluted 

 state in all subsequent experiments. 



This at once points to the fact that, whilst unripe and not decom- 

 posed banana is very much less attractive than amyl alcohol and amyl 

 acetate, on decomposition it becomes more attractive than amyl acetate 

 and valerianic acid. The figures in Exp. 6 give no idea of the relative 

 strengths of amyl acetate and valerianic acid, as the former substance 



