INTRODUCTION 5 



too. But this is not very frequent, and the natur- 

 alist, exercised in cases of affinity, who studies a 

 table of aquatic Insects, is likely to conclude that 

 different families, and different genera within one 

 family, and different species within one genus have 

 betaken themselves independently to an aquatic life. 

 I think we can say with a considerable degree of 

 probability that this change of habitat from terres- 

 trial to aquatic has taken place in the class of Insects 

 at least a hundred times quite independently, and 

 the number may be very much higher than a 

 hundred. 



I have spoken with confidence of the original 

 terrestrial habitat of Insects, and this requires some 

 further discussion. Writers have been found to main- 

 tain the very high antiquity of the aquatic habitat, 

 and to suppose that many, if not all, of existing 

 Insects had aquatic ancestors. What can be found 

 out on this point ? It is of no use to appeal to the 

 geological history of the earth, for the fossil remains 

 of Insects are too few and scattered to justify any 

 conclusion as to the mode of life of the first Insects 

 which appeared on the globe. But we are not alto- 

 gether without the means of forming an opinion. One 

 of the most universal features of Insects in all stages 

 of growth is the aeration of the tissues by air-tubes 

 or tracheae, which ordinarily open on the outside of 

 the body by special valvular inlets, the spiracles. Even 

 in aquatic Insects the spiracles are commonly present 

 and functional, and we can hardly name an Insect 

 which has not tracheal tubes, though they may in rare 

 instances be greatly reduced. This is pretty good 



