THE LEECHES OF MINNESOTA 5 



Economic Importance. 



In many places leeches are abundant enough to be of economic 

 importance. They may be of value in so far as they serve as food 

 for fishes and birds or in so far as they are scavengers. On the 

 whole, however, the leeches are rather an injurious group. They 

 may kill fishes and other animals, particularly the young by bleed- 

 ing them to death or, indirectly, by devouring the snails, worms, 

 larvae &c. which constitute the principal food of some fishes. They 

 are also injurious in so far as they serve as intermediate hosts 

 for various developmental stages of animals that during some pe- 

 riod of their lifetime are parasitic on fishes, birds and other animals. 

 There is still a great deal to be learnd about the life histories of 

 many of our leeches, their relations to other animals, particularly 

 their relations to fishes, and their influence on the character of 

 the fauna in particular bodies of water. 



The wounds made on man by the bloodsucking leeches very 

 rarely produce any serious results. Considerable, sometimes in- 

 tense, itching of the region immediately around the wound is usu- 

 ally the only noticeable effect. More serious results are probably 

 due to infection. When, however, the leeches find their way into 

 internal passages, they may produce serious disturbances. The 

 young of the bloodsucking horse leech taken in by horses and 

 cattle while drinking from ponds or lakes have been known to be- 

 come attacht to the lining of the farynx and the windpipe and cause 

 more or less serious disturbances. Fortunately # the number of 

 species that in this way practically become internal parasites is 

 very small, and the chance of their invading human beings exer- 

 cising some care is very slight. It may be well to remember, how- 

 ever, that it was a small inconspicuous leech, not thicker than a 

 horse hair, that was the cause of considerable trouble to Napoleon 

 in Egypt. His soldiers in drinking water direct from the streams 

 and lakes and pools took in small leeches which attacht themselves 

 in the back part of the mouth cavity and caused annoying blood 

 spitting and difficulty in breathing. The small land leeches so 

 much dreded in the forests of some countries (South America, 

 Australia, Japan, Ceylon and others) are not found in Minnesota. 

 What the introduction and acclimatization of such forms in our 

 forests would mean may be inferd from the following account taken 

 from Tennent's book on Ceylon :— Of all the plagues which beset 



