Leeches 39 



Use of Poisons and Repellents. That leeches are excessively 

 sensitive to certain poisons is well known. For example, a trace of 

 chloroform or nicotine placed in a vessel in which they are living will 

 cause them almost immediately to fall to the bottom in a rigid mori- 

 bund condition. The thought naturally arose, therefore, that a 

 poison might be applied to the lake waters in doses lethal to the 

 leeches without injury to the other inhabitants, or that, without killing 

 th'Sm, the leeches might be repelled from the swimming places with 

 the same facility with which they are attracted. It is quite probable 

 that such a method could be worked out but there was time to 

 experiment with a few substances only. 



For several years it has been customary to treat the waters of Carr 

 Pond and other lakes with copper sulphate to destroy the minute 

 algae (water bloom) that otherwise would form extensive areas of 

 surface scum to the annoyance of bathers. On a few occasions, 

 when the application was made with insufficient care, large numbers 

 of fishes were killed. It could not be learned, however, that there 

 was any deleterious effect upon the leeches. 



A series of tests was made to determine the reactions of the leeches 

 to this salt. The experiments were made somewhat crudely as the 

 facilities of the laboratory permitted nothing more refined ; but the 

 results are sufficiently accurate for the purposes in view though hav- 

 ing no physiological value. A stock solution of one part of copper sul- 

 phate to nine hundred and ninety-nine parts of boiled and filtered lake 

 water was prepared. This was diluted for each experiment to the 

 desired strength with raw lake water. In each experiment from one 

 to three freshly caught and active leeches were placed in each of three 

 two-quart (sometimes one-quart) glass preserving jars. One was 

 filled to the top with the fluid to be tested, so that when a cover of 

 cheesecloth was tightly drawn across the top it became impossible for 

 the leeches to avoid the solution. A second was exactly similar 

 except that in place of the copper sulphate solution it was filled with 

 untreated lake water like that in which the leeches normally live. The 

 third was half-filled with the solution being tested, leaving a large air 

 space above into which the leeches could escape from the poisonous 

 solution. The three jars were placed together in a cool dark place 

 and examined at intervals of usually one or two hours, when the 

 condition of the animals, temperature, and other data were recorded. 

 The dilutions ranged from i in 1,000,000 to i in 5,000,000 of copper 

 sulphate and water and each experiment was repeated several times. 



The results were very definite. In the jars containing the copper 

 sulphate the leeches immediately showed signs of irritation and 

 excitement. They swam or less often crawled with great activity, 

 exploring all parts of the jars in their efforts to escape. In the full 

 jars this activity continued for a long time or until the leeches ap- 

 peared exhausted and fell to the bottom or hung relaxed from the 

 side of the jar. In the half-filled jars they quickly discovered the air- 

 filled upper half and came to rest usually with both suckers attached 

 to the side and the pendant body well above the irritant liquid. In 

 the check jars, after completing their explorations, they soon assumed 

 normal resting attitudes. 



