2 Journal New York Entomological Society. tVoi. xxi. 



primitive and primitive animals are aquatic. Among the inverte- 

 brates, with the exception of the dubious Mycetozoa, all the Protozoa 

 spend the active part of their lives in fluid surroundings ; sponges, 

 jelly-fishes and their allies, star-fishes and their allies, and most worms 

 are aquatic; while only the higher arthropods are at all terrrestrial. 

 Among the vertebrates, on the other hand, only the most primitive, 

 the fishes, are typically aquatic, the vast majority being terrestrial. 

 Returning to the invertebrates and considering the insects, we find 

 that low in the scale whole orders — Plecoptera, Trichoptera, Odonata, 

 and so on — have exclusively aquatic larvae. Here again the distinc- 

 tions are not clear cut. The Thysanura are not aquatic and the 

 Isoptera are also both terrestrial and primitive, but, in general, there 

 seems to be at least the shadow of a rule. 



Even when we consider the higher orders, we find that the aquatic 

 Diptera, Coleoptera and Hemiptera are the more primitive members 

 of their respective orders while aquatic Hymenoptera are exceedingly 

 rare. The Orthoptera come in to disturb us by being both primitive 

 and terrestrial but here also the more nearly aquatic, the now dis- 

 tinct order of earwigs and certain of the cockroaches, are the more 

 primitive. 



All this may seem foreign to the purposes of our symposia. Still 

 it seemed to me interesting to note that in a very general way the 

 division into aquatic and terrestrial insects is also a division into 

 primitive and modern. The division is not distinct but it is probably 

 as distinct as most of those we will be able to draw. Admitting it, 

 the question of cause now looms large as it always should. The 

 answer that would be given by most theorizers is that life originated 

 in the water and that our present day aquatic fauna is a survival .of 

 the early origins. Nevertheless, the very most primitive insects are 

 very far removed indeed from the origin of life. I suspect that a 

 better reason may be that in early times certain insects took to the 

 water and that aquatic environments, although variable, are less vari- 

 able than terrestrial and have therefore not given rise to a series of 

 modifications resulting in great changes from the primitive types. 



Consider the factors of the aquatic environments as contrasted 

 with terrestrial. There is no variation in absolute humidity since 

 water is always as wet as it can be. There is, however, a variation 

 in physiological humidity. That is, when the water contains certain 



