Leeches 41 



of lime seemed less irritating than copper sulphate but the leeches 

 when free to do so avoided its effects in the same way. 



Common salt was reported to have been used successfully to 

 combat the leeches at one camp from which the most vigorous com- 

 plaints emanated. Upon investigation it was learned that whenever 

 leeches were caught they were laid on a board and nearly buried in 

 granulated rock salt, which of course proved fatal. The camp 

 directress was encouraged to experiment with placing a quantity of 

 this substance in the crib in the hope of determining if the leeches 

 could be repelled thereby. This was done in a casual manner 

 without any record being kept of the quantity of salt used. I was 

 unable to detect any reduction in the number of leeches. 



Laboratory experiments showed that Macrohdella decora is highly 

 resistant to sodium chloride solutions. When kept in solutions as 

 strong as one per cent no apparent injury was noticeable after five 

 days. In solutions of from .1 per cent to .02 per cent they exhibited 

 increased activity for a short time (one to three hours) but after 

 that appeared to be perfectly normal. In weaker solutions no effect 

 whatever was apparent, and even in the strongest solution used there 

 was no consistent attempt of the leeches to avoid it, such as was so 

 obvious in the case of copper sulphate and calcium chloride. 



Sulphuric acid was experimented with solely as an irritant and 

 repellent. The subjects were placed in a blackened open vessel of 

 water and allowed to assume resting attitudes. Pleasured quantities 

 of the sulphuric acid solutions were then allowed to flow gently over 

 various regions, generally the head, from the finely drawn out end of 

 a medicine dropper. In each case an equal number of drops of pure 

 water was applied with exactly the same precautions as a check, and 

 the time, amplitude, and nature of the resulting reactions compared. 

 The strengths of the solutions used ranged from i.o per cent to o.i 

 per cent of concentrated acid (sp.gr. unknown). The leeches proved 

 unpleasantly sensitive to all and exhibited avoiding reactions of vary- 

 ing moderate degrees of intensity. From the stronger solutions 

 they quickly withdrew the head or portion of the body affected, 

 turned directly away from the point of contact and either swam or 

 crawled to the opposite side of the vessel where they remained. 

 Even to these concentrations, however, the complete withdrawal 

 would be characterized as leisurely, requiring from two to four 

 seconds for its completion, though the initial contraction was 

 instantaneous. To solutions of from 0.5 per cent to 0.2 per cent the 

 first part of the response was similar but the leeches merely changed 

 position and only very rarely left the spot. To the weakest solutions 

 they behaved as they do to weak mechanical stimuli, such as a fine jet 

 of water, the part touched being withdrawn without affecting the rest 

 of the body and the original position being resumed after a brief 

 interval. 



Experiments with various other substances had been planned but 

 the definiteness of the avoiding reactions of the leeches to the above 

 seemed to sufficiently indicate their ability to escape the effects of any 

 poison that could be safely used. For this reason, and partly because 



