110 DR. HOOD — NOTES OJf THE MAY FLY. 



out, some most interesting facts might be obtained. Making allow- 

 ance for the difference of temperature between the water in the 

 aquarium and in a river, an approximation might be arrived at 

 as to the length of time that occurred before the egg was hatched. 

 When hatched, the changes that would take place in the larva, 

 if any, — whether it in any manner resembled that of the silk-worm, 

 with which we are familiar, — also the period that would elapse 

 before the larva was of a size to construct for itself a home, — and 

 how long it would remain a tenant of such home, anterior to its 

 wonderful metamorphosis, — should be observed. Watching these 

 various stages could not fail to be most interesting to the 

 naturalist. 



In connexion with the history of the birth and progress of the 

 May fly to maturity, we are able to draw most important conclu- 

 sions as to the condition of the water — its healthfulness or other- 

 wise — on which she is found. No May flies will be discovered on 

 streams that are polluted by sewage and other noxious elements. 

 Many streams that have even been celebrated for the abundance of 

 these flies are now no longer tenanted by them. This fact has 

 excited a good deal of remark, and various opinions have been 

 assigned for their disappearance. I cannot think there is any 

 mystery or difficulty in explaining the true cause of their absence. 

 If the bottom of a river is contaminated by materials that should 

 never have been permitted to be passed into the stream, we cannot 

 feel surprised at the tender egg of a May fly being poisoned and 

 rendered rotten when it comes in contact with it — for it follows as 

 a matter of course, that if the eggs deposited by the flies are not 

 hatched there can be no May flies. That this is the true cause of 

 the disappearance of this fly fi-om many rivers, it is unfortunately 

 too easy to prove. One of the most striking amongst them is to 

 be found in the Colne at Rickmansworth. This river formerly 

 abounded with May flies as well as trout ; both have disappeared 

 in consequence of the deleterious materials employed by Mr. 

 McMurray, at Loudwater and Scott's Bridge Mills, which flnd 

 their way into this portion of the river. The laws of nature are 

 thus subverted in order to further the interests of man. 



EXPLAXATIOX OF PLATE I. 



On the left-hand blade of ijrass is the female green drake, which changes into 

 the grey drake on the right-hand blade, her cast-off pellicle being on the grass 

 stem below. The flying insect is the perfect male, or black drake. 



