WHIRLIGIG BEETLES. S7 



of them have not this difference ; or if they have, it is scarcely 

 perceptible. 



Students have of late derived an immense advantage from 

 tabulating and stating only those characters which are necessary to 

 determine and identify the insect under examination from all 

 others. Let me try to explain. Suppose there are only two 

 species in a genus, one of which is red and the other black. It is 

 sufficient to state that fact only, and the species is determined as 

 clearly as it could be by a whole page of description. Or, should 

 there be many species, the red ones may be separated into one 

 group and the black ones into another, and then some other 

 characteristic selected for their further determination. In this way 

 the genus Gyriniis is tabulated. By referring to the table at the 

 end of this paper, it will be noticed that only two species have the 

 under side of the body red — G. mimitus and G. urinator. The 

 first is a Scotch insect, so that when I noticed a Gyrinus red 

 underneath, I knew at once I had G. urinator, as I could not 

 expect to find G. ininutus in Bath. At first I only found one 

 specimen, but knowing it to be a rare insect I was determined to 

 find more if possible, and for this purpose I made up my mind to 

 try the canal* on both sides until I succeeded. I certainly felt very 

 disappointed, after working for nearly two hours without any 

 success. However, presently I came to a corner with a bank of 

 Anacharis weed, and there they were in abundance, together with 

 a still larger number of G. inarinus. There were a few G. natator 

 and G. opacus, and one G. bicolor only. I obtained about eighty 

 specimens of G. urinator, nearly all of which have since been 

 distributed to coleopterists in various parts of the country. 



It is a very general idea that the commonest species is G. 

 natator, but this certainly is not the case in Bath, G. rnarinus 

 being far more common. I think G. natator occurs more freely in 

 ponds than any of the other species. In the London district G. 

 natator does not occur at all ; at least, I am informed so by a 

 friend who has collected beetles round London for ten years. 

 From the name Marinus, one would suppose this species to be 

 peculiar to localities near the sea ; but such is not the case. It 



* The Kennet and Avon Canal. 



