THE PUPA OR CHRYSALIS. 115 



chrysalis state during irregular periods afterwards, 

 a few not expanding till the following season. La- 

 cordaire mentions a particular case of this kind in 

 Cayenne, where, out of a brood of the fine Moth 

 Baturnia Paphia, which he had reared, the greater 

 part only remained in the chrysalis state fourteen 

 days, while a few continued for several months in 

 that state, afterwards producing the matured insect 

 in a perfectly healthy state. This would appear to 

 be a providential arrangement to prevent a species 

 from becoming extinct by any sweeping calamity, 

 such as a storm, change of temperature, or insect 

 epidemic, which might sweep off a whole species if 

 all came out precisely at the same time. In such a 

 case, the reserve, safely secured from the calamity 

 in the pupa, would come forth at a more propitious 

 season and restore an otherwise lost species. 



While speaking of the coming out of a particu- 

 lar kind at an exact season, I ought to mention 

 some of the Ephemera, one species of which, accord- 

 ing to Eeaumur, invariably appears between the 

 10th and 18th of August, and at no other time. 

 Fishermen are well acquainted with this fact, and 

 are never disappointed in the appearance of these 

 aquatic flies, which they sometimes use as attractive 

 baits. Swammerdam had previously observed simi- 



