genus Acentrojnts. 273 



Nothing more was done in 1872, and nothing at all in 

 the following year. But in the summer of 1874 Ritsema 

 was staying at Velp, near Arnheim, and in the latter half 

 of August he collected thirty moths, all males, and found 

 both larva3 and pupa? on Potamogeton crispus. Shortly 

 before, a fully-winged female Acentropus was captured 

 by Father Aghina, in the neighbouring Dominican con- 

 vent at Huissen; it came in the evening to the light of a 

 lamp, and flew wildly round over the table. 



Upon this Ritsema writes (p. 87): — " It is to be noted, 

 also, that in the places where I had collected numberless 

 males of nivens, I had not found a single similarly nor- 

 mally-winged female, and from larvfe from one of these 

 places had bred only rudimentarily-winged females ; thus 

 it will be seen that the circumstances were very apt to 

 corroborate my idea that the two forms of female did not 

 belong to one species, and to incite me in the following 

 year to breeding from larvre from the Kolkje, in order to 

 endeavour to obtain proof of my idea." 



Accordingly, on the 20th April and the 4th May, 1875, 

 Ritsema again went to the Kolkje, and collected some 

 five-and-twenty larvte ; on the 20th May he obtained two 

 more full-grown larvas and fifteen pupje. On the last 

 occasion he also found three dead moths, one male and 

 two females, both with rudimentary wings. During the 

 journey home, a female hatched, in the bottle in which the 

 larvffi and pupge were carried, and this, too, had only 

 rudimentary Avings. When placed in the aquarium, she 

 swam with the help of the long fringes of her middle and 

 hind legs, in an almost perpendicular position, with the 

 head upwards, pushing and jerking round, until at last 

 she fixed herself, by her fore feet and by curving her 

 abdomen, under water on the stalk of one of the plants. 

 Later in the evening, by lamp-light, this female Avas seen 

 swimming round over the surface of the water, keeping 

 the tip of her abdomen turned upwards. On the 29th 

 May, ten moths hatched, thx-ee males, and seven females, 

 all with rudimentary wings; they always emerged in the 

 evening, and the females swam round and round, both 

 on and in the water. On the 1st June, two males and 

 two females hatched, the latter with rudimentary wings; 

 and on the following morning the males were seated 

 above the surface of the water upon the stems of the 

 plants, and the females under water on the Potamogeton 



