340 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



Amoug the Amphibia only the green water-frog {Rana esculenta L.) is 

 dangerous to fish ponds. In some places it is found in very large num- 

 bers, and does a great deal of harm, not only by depriving the fish of 

 their food in the way of insects, worms, &c., but also by devouring large 

 quantities of small fish sporting about in the shallow water. lu large 

 ponds i^ike may be employed advantageously in destroying frogs ; while 

 the spawn of frogs and tadpoles is eagerly devoured by trout and carp. 

 Spawning and raising i^onds should, if possible, be protected against 

 frogs by being fenced in with wire screens. Frogs which have hid, be- 

 fore the pond was fenced in, are easily caught in tubs placed in the 

 ground near the edge of the pond; when jumping about they fall into 

 these tubs, and are then thrown as food to the pike or trout. Frog 

 spawn found in small ponds should be carefully removed with rakes, 

 and thrown as food into ponds containing older fish. 



Among the insects, two varieties of the water-beetle, the DyUscus mar- 

 ginalis Sturm, and the Acilius sulcafus L., are jiarticularly dangerous 

 for fish, as both the beetles and their larvre completely devour fish 

 spawn and little fish measuring several inches in length, while they 

 will eat deep holes into larger fish. 



Another insect, the Notonecta glauca L., is found in some ponds in 

 incredible quantities, and with its sharp prong, with which it also 

 stings human beings so as to cause considerable pain, it kills .1 great 

 many young fish, and sucks them. These hurtful insects may occasion- 

 ally be caught in large quantities with muslin bag-nets. Pounded and 

 kneaded into a dough with fine flour, they can very suitably be employed 

 as fish-food. It is easy to destroy these insects on a large scale by scat- 

 tering quicklime over the bottom of the i^ond after the water has been 

 let off. 



A lai'ge number of crustaceans are occasionally found on fish as 

 parasites, without, however, inflicting any serious injury. There are 

 only two varieties, which, when found in large numbers, do considerable 

 damage in our ponds, namely, the Lernwocera cyprinaeea L. and the 

 Arcjulus foliaceufi L. 



When quite young, the Lernccocera cyprinacea resembles the sand-flea. 

 After spawning the female changes, when it has firmly attached itself to 

 the body of a fish, into a worm-shaped tube, 1 to 2 centimeters [about one- 

 half inch] long, at the front i)art of which four hornlike excrescences 

 develop, for adhering to the gills or penetrating between the scales ol* 

 fish, and frequently they enter deep into the body of the fish ; at the back 

 part of the dirty-green body the numerous eggs are carrie«l in two 

 small, long bags, till they are ready to issue. Lernceocera is often 

 found in enormous numbers on crucians, frequently also on carp, but 

 rarely on other fish. By moving the hard excrescences referred to 

 above, it causes gatherings in the skin of the fish, which gradually in- 

 crease in size, soon assume a very disgusting appearance, and worry 

 the fish very much. As it is impossible, in cases where the Lernceocera* 



