FISH ENEMIES 



101 



Back Swimmers may be recognized by the fact that the back is down 

 and the legs up while swimming, and also that they breathe at the surface 

 through the end of the body. As a matter of practice it is easier to re- 

 move all such insects rather than try to determine the species. Scalares 

 and other large hard-mouthed fishes like to eat water boatmen. 



Mites (Hydrachna). These odd-looking little balls of intense ver- 

 milion are sometimes placed in small tropical aquaria with fishes. This 



Fig. 84. Water Mite (Enlarged about jour times) 



is inadvisable, as they are parasitic. It is doubtful whether they actually 

 kill their host. They are common in the still water of lily ponds. 



Hydra. Although Hydra is not an insect, we include it here as 

 an important enemy of young fishes. Hydra is a polyp which attaches 

 itself to plants, stones or the sides of the aquarium. Being thread-like 

 in appearance, it is apt to be overlooked, especially by the inexperienced. 

 It is usually introduced with living food caught from pools. The spores 

 are so small as to be unrecognizable, and therefore they cannot be 

 avoided. The animal itself has a cylindrical body with from 5 to 12 

 tentacles surrounding a mouth. The shape varies so amazingly that an 

 accurate description is difficult. The two figures shown are of the same 

 individual. When alarmed the tentacles are entirely withdrawn, making 

 recognition still more difficult. The figure at the left is the usual form 

 when not alarmed. In this condition it reaches from % to % inch in 

 length, but the body may be considerably thinner if food is scarce. 



Fig. 85. Hydra (Greatly enlarged) 



Propagation is by budding, by spores and by division. If one of 

 the tentacles be cut off it will develop into a perfect hydra. 



