FIELD BOOK OF PONDS AND STREAMS 



feeding on snails or insect larvae, and again clamped to the 

 bod}' of a frog and lustily sucking its blood. As a group they 

 appear to be wavering on the edge of parasitism, but not 

 wholly committed to it, and while certain species are perma- 

 nent parasites others are only predatory. 



They are acutely sensitive to vibrations in the water and 

 to even the smallest amount of any substance dissolved in it. 

 If one presses one's finger against the bottom of a dish contain- 

 ing leeches, they will at once begin to creep around, restlessly 

 exploring the whole surface of the dish and if they happen to 

 pass over the finger print, their agitation makes it quite evi- 

 dent that they detect its odor. They are also quickly excited by 

 the movements of water in a pond. If either cattle or persons 

 wade into a pond inhabited by the American medicinal leeches, 

 Macrobdella decora (Fig. 119), they will soon swim out into the 

 open water and gather about the intruders, finally attaching 

 themselves to their legs. That they do not rely wholly upon 

 an odor of flesh is shown by the fact that they will attach 

 themselves to rubber boots almost as readily as to bare legs. 

 Wading into leech-infected ponds either with or without boots 

 is a good way to collect them. Leeches are also very sensitive 

 to light and in different species there may be one or several 

 pairs of eyes on the head segments as well as a circlet of light- 

 perceiving organs in each segment (Fig. 115). 



Leech bites. — The only American blood-sucking leeches 

 belong to the genus Macrobdella; the well-known blood-sucker 

 of ponds and swimming pools is Macrobdella decora (Fig. 119). 

 Our other blood-sucking leech is the European medicinal leech 

 Hirudo medicinalis. The latter was imported from Europe for 

 blood-letting, and has become established in some regions of 

 New York. It is about 4 inches long, usually greenish colored 

 on its dorsal side with four to six brown stripes. Although 

 most leeches are not thorough-going blood-suckers they are all 

 more or less bloodthirsty. The structures which make them 

 such successful blood-suckers are their toothed jaws and their 



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