1S68.] 387 [Packard. 



and deprive those of the others of such a prominence ? Why give 

 one long and the other short anal cerei, or clothe the hind tarsal nails 

 of one with short hairs and leave the other naked '? What have 

 these features to do with the differences of structure we have men- 

 tioned in the palm-shaped fore leg, or in the length of the hind leg? 

 These and similar difficulties, arising on every hand, seem to attend 

 every derivative theory of the origin of sjiecies. 



Dr. A. S. Piickard, Ji-., read the following account of two 

 species of salt fly, by Mr. Cox : — 



"I send you the larva and pupa of a dipterous insect (Ephydrd) 

 found in the salt brine at the salt works near Equality, Gallatin 

 County, Illinois, in such prodigious quantities as to fill up the Avooden 

 conduit pipes. These pupae are gregarious, collecting in masses, and 

 form great rope-like bunches, by clinging around small fibrous roots 

 on the sides of the little ditch that conveys the brine from the first 

 'Graduation or Thorn house,' to the pump at the furnace. 



"The brine, as it comes fi'om the well, has a strength equal to 7^ 

 Baume, and is graduated after the German plan, by showering it suc- 

 cessively over thorn bushes arranged on beams from top to bottom of 

 three separate frames, from forty to forty-five feet high, called 'Grad- 

 uation or Thorn houses.' 



"What is remarkable in this is, that the above larva3 can nowhere 

 be found except in the brine after first graduation, that is, passed 

 over the first house, when they are found in such quantities as to 

 prove a great nuisance. Neither in the fresh water, weak brine, or 

 brine of second and third graduation can they be found at all. 



"The people at the works believe that they are generated by some 

 peculiar property in the water acquired after first graduation. I 

 send them in their favorite brine. Professor Leo Lesquereux found a 

 new species of 25lant in the brine pools. The short time at our dispo- 

 sal was so much taken up with geology, the object of our visit, that I 

 forgot to collect specimens of the jjlant, which was abundant. I hope 

 soon to revisit the locality, when I will try to collect some. 



"The pupa of a species of Eristalis was found in the same place. 

 The fly of the first worm sent you (identified by Baron Osten-Sacken 

 as a S2iecies of Ephi/dra') was seen in great abundance on tiie pool 

 at the bottom of the Graduation house. When alarmed they will fly 

 up a few inches from tlie water, then alight upon it again in another 

 place, and will glide about upon its surface with rapidity and the 

 greatest ease. 1 think the worms come trom a small egg, deposited 

 by the fly, which sinks to the bottom of the water, where it is 

 hatched, and the first visible stage of life is a very small white 



