PHOCEEDINTJS OF THE THIRD ENTOMOLOGICAL MEETING 907 



the entomologist, the sanitary departments and the general public as 

 to the pisciculturist. Every one agrees that the question of public health 

 in the tropics, more so than anywhere else, is very seriously involved 

 in the discovery of efficient means for the destruction of mosquito larvae. 

 Several species of fish have been credited as being efficient agents in this 

 connection, but I am sorry to have to say that the most unsystematic 

 way in which this work has been carried on. has resulted in making the 

 problems more obscure and involved. However, we will have something 

 more to say about it further on. 



To a layman the word insect essentially conveys the idea of small 

 terrestrial sLx-legged animals that can fly by means of specially developed 

 structm'es — the wings. Undoubtedly insects are most numerous on 

 land, but then all of them cannot fly as indeed all are not terrestrial. 

 A large number are permanently aquatic and a still larger number pass 

 the earlier stages of their life-history in water. The very keen struggle 

 for existence on land has probably resulted in these insects taking to the 

 aquatic medium, where food in the form of planliton and aquatic vege- 

 tation is most jilentiful and though hosts of enemies exist to devour the 

 helpless eggs, larvse, pupse and even the adult insects, yet the chances 

 of escape are far greater in water than on land. The question as to 

 whether the ancestors of insects wer*^ terrestrial or aquatic crops up, 

 but a discussion of it would be quite out of place here. A point that 

 deserves mention in connection with insects that pass the earlier stages 

 of their life in water, is that the time spent in the water is comparatively 

 short, as indeed is the whole life-cycle. This is largely to be explained 

 by the abundanqe of food resulting in rapid growth and prolific breeding 

 but there are exceptions, for cases of hibernation for long periods in the 

 larval stage are quite well known in the case of large numbers of insects. 

 The direct bearing of many of the above detailed fectors is very little 

 on J;he fish-life ; still, insects, like plankton, exercise at all times a very 

 great influence on the fishes in any area. 



The relations of insects, as indeed of most influences in the sphere of 

 life, have to be considered from two different points of view, whether 

 they are of any use or they are in any way injurious ? We will consider 

 these two sides separately. 



Useful insects. In the distribution of fishes food acts as a very im- 

 portant factor, and according to the effects of this ecological factor 

 fishes are divided into various groups ; of these groups we are here con- 

 cerned with the pelagic and littoral fishes only. Both these types of 

 fishes depend largely on the plankton or insects, for their food, and in 

 accordance with it show special modifications of the mouth and the alimen- 

 tary tract. They frequent only those parts of the streams, lakes or ponds 



