336 PROCEEDINOS OF THE NATIONAL MUSEUM. vol.39. 



and numerous scattered spots covering the entire ventral surface, 

 as seen in figure 61, a deep purplish blue. Near the posterior margin 

 of the first thorax segment and along the median axis of the second, 

 third, and fourth segments the blue spots coalesce into a large area 

 visible to the naked eye, even from the dorsal surface. 



This will serve to distinguish the species ordinarily. The eggs 

 when ripe are a pale blue, so that the parasite shows up distinctly 

 against the red gills. 



(cseruleus, blue.) 



This parasite infests the gills of the bluegill, Lepomis pallidus, but 

 is local in its distribution. Thus far it has been obtained only from 

 the Twin Lakes in Marshall County, and Tippecanoe Lake in Kos- 

 ciusco County, Indiana. Blue-gills are plentiful in Lake Maxin- 

 kuckee, 7 miles south of Twin Lakes, but although several hundred 

 of them have been examined by the author at different times, only 

 six specimens belonging to the Ergasilidae were obtained from the 

 entire number. These were Ergasilus centrarcMdarum. and were 

 found on the outside of the gill filaments, as in every other fish which 

 they infest. 



The present species, however, is found between the two layers of 

 gill filaments, and more than a hundred specimens were obtained from 

 two small blue-gills scarcely 5 inches long. This radical change of 

 habit and their abundance suggested at once that they were a new 

 species, which was verified upon further examination. The author 

 is not acquainted with any other species of the genus Ergasilus that 

 thus habitually frequents the space between the gill filaments. The 

 blue-gill is a vegetable feeder and its mouth is filled with fragments 

 of algte and other water plants much of the time. These fragments 

 are bound to produce more or less friction over the gills themselves, 

 particularly during breathing. Ergasilus centrarcMdarum, inhabiting 

 the outside of the gill filaments, does not take kindly to these condi- 

 tions. Hence they are only rarely found upon the blue-gill or the 

 croppie, another vegetable feeder. 



The present species, however, by frequenting the space between 

 the gill filaments escapes the discomforts incident upon vegetable 

 feeding, and is thus enabled to thrive where the other species failed. 



There are two lots of specimens of this species obtained at different 

 times from the blue-gills in Twin Lakes. They are numbered, respec- 

 tively, 39550 and 39554 U.S.N.M.; the former is made the types of the 

 new species, the latter becoming cotypes. A third lot consisting of 

 half a dozen females was obtained from 10 blue-gills caught in Tip- 

 pecanoe Lake, and is numbered 39548, U.S.N.M. 



