Ohfcrvations on the Hejfian Fly, 99 



degrees of heat may produce this ,e5e6l, yet this miifl; be under- 

 ftood to be within certain liraitsi. fqr the chrylahs can endure 

 but a very fmall degree of. heat beyond that >yhich is ufual in 

 its natural fituation in the field. Befides thefe effefts proceeding 

 from heat, I am inchned, for feveral reafQns, to fuppofe tliat 

 different degrees of moiiiure will ;produce efiefts ofafnnilar 

 nature. 1. Becaufe the chryfalis: will perifh, when expofed 

 to the rays of the fun, in fuch manner as to become very dry, 

 and have all its natural moifture exhaled. 2. Becaufe the fly 

 when hatched in the houfe, where the chryfalis has been kept 

 .more dry than in the open air, will always come forth very feeble 

 . and will never acquire that life and a6livity which it mufl be 

 fuppofed to have in the field. 3. . Becaufe there is rcafon to 

 believe, that too great a degree of moifture, will in this cafe, 

 .produce effects hmilar to thofe which muft arife from too fmall 

 a degree of heat. Although it will prove fatal to the chryfalis, 

 [ to be expofed to any great degree of heat, beyond that which is 

 ufual in the open air in the fummer feafon, yet cold and wet, 

 even in the greateft extremes ever known in our climate in the 

 winter feafon, appear to affecl it no otherwife, than to continue 

 it in the fame ftate, and prevent its transformation into a fly, 

 until it can again enjoy that degree of heat and moifture which 

 is moft agreeable to its conftitution. Thefe circumftances 

 render it difticult to afcertain with precifion, the length of time 

 that the infect will continuea chr\falisduring the fummer feafon. 

 But from the beft obfenations I have been able to make, it is 



