138 Zoological Society. 



longed ; at any rate it would have been putting to the test the geo- 

 graphical characters of the species, and this arrangement can only be 

 made by an American acquainted with the branches and creeks of 

 the different rivers. 



Mr. Lea uses this test for the European species, and reduces all the 

 Anodons to a single species, but believes that a very little stream in 

 America affords at least one, and often many, distinct species of these 

 animals ! — J. E. G. 



PROCEEDINGS OF LEARNED SOCIETIES. 



ZOOLOGICAL SOCIETY. 



December 10, 1850.— Prof. Owen, V.P., F.R.S., in the Chair. 



Observations on the destructive species of Dipterous 

 Insects known in Africa under the names of the 

 Tsetse, Zimb, and Tsaltsalya, and on their supposed 

 connexion with the Fourth Plague of Egypt. By 

 J. O. Westwood, F.L.S., Pres. Ent. Soc. etc. 



The species of insects which attack the larger of our domestic qua- 

 drupeds may be divided into two chief classes ; first, those which do 

 so in order to obtain a supply of food for their own support ; and 

 second, those which do so with the object of depositing their eggs in 

 such a position, that the larvae, when hatched from them, will be cer- 

 tain of finding a proper supply of food derived from some part of the 

 animal, either external or internal. 



The insects composing the first of these two classes require for the 

 performance of their dreaded functions an organization of the parts 

 of the mouth especially fitting them to pierce the skins and hides of 

 the quadrupeds upon the blood of which they subsist, and we accord- 

 ingly find that it is precisely these insects which have the mouth- 

 organs most fully developed in the different families to which they 

 respectively belong. The Stomoxys calcitrans, and especially the 

 different species of Tabanus, are pre-eminent in this respect ; and 

 the formidable array of lancets in the mouth of one of the latter 

 insects is not to be met with elsewhere among the whole of the flies 

 composing the order Diptera, to which they belong. The effects of 

 the attacks of these insects upon the horse are perceived by the 

 drops of blood which flow from the orifices caused by their bites, 

 and sometimes these wounds are so numerous, that the beasts "are 

 all in a gore of blood." A still smaller species, named by Linnaeus 

 the Culex equinus, also infests the horse in infinite numbers, running 

 under the mane and amongst the hair, and piercing the skin to suck 

 their blood. This insect, although given by Linnaeus as a Culex, 

 appears from his description to belong to the genus Simulium, to 

 wliich genus also belongs an insect of fearful note, which attacks the 

 horned cattle in Servia and the Bannat, penetrating the generative 



