ERISTALIS TENAX. 277 



migration. Many facts are on record which, seem 

 to confirm the idea that insects do occasionally 

 change their quarters in immense bodies ; and some 

 have occurred to myself, which, I have no doubt, 

 were connected with such a circumstance, not only 

 from the large numbers of the insects observed, 

 but from the steadiness of their flight, and their 

 continually persevering in one given direction. It 

 is worth noticing, with respect to the present 

 case, that King's Lodge is situate close to the 

 river Cam, which at that place runs nearly due 

 north and south ; and it is just possible that this 

 circumstance may have had some influence in di- 

 recting the movements of these insects. I find 

 also, by referring to a journal of the weather, 

 kept in the neighbourhood of Cambridge, that, 

 about the time when they were first observed, the 

 wind was N. N. W, and that it had been blowing 

 steadily from that quarter for four successive 

 days.* 



ERISTALIS TENAX.f 



FROM the middle of September onwards during 

 the autumn, these flies are much in the habit of 

 entering houses, where they prove tiresome in rooms 

 from the incessant humming they keep up. This 

 is so like that of a bee, and the fly itself so much 



* The above account was published at the time in London's 

 Mag. of Nat. Hist. (vol. v. p. 302), and is reprinted here. 

 + Musca tenax y Linn. 



