AN ACCOUNT OF BRITISH FLIES. 



It is here seen that the first traces of flies are to be met with in 

 the Lias. No doubt future discoveries will place the date of appear- 

 ance at least in the Trias, or possibly even in the later Palceozoic 

 rocks. 



At present there is only one hexapod order in the Palaeozoic rocks 

 known, namely, the PalcEodictyoptera, which became extinct in Triassic 

 times. It may be that from these Palasodictyoptera all the existing 

 orders became differentiated. They die out, as I said before, in the 

 Trias, and are there replaced by Orthoptera and Neuroptera, to 

 which they present close affinities. So far no Diptera, Lepidoptera, 

 Hemiptera or Hymenoptera have been discovered prior to Liassic 

 times. From this fact some weight may be given to the assumption 

 that the existing orders were differentiated in the Triassic period. 



The Orthoptera and Neuroptera were the most abundant Mesozoic 

 forms, and are now replaced by the more highly developed orders, 

 such as Lepidoptera, Coleoptera, and Diptera. 



As to the point where the Diptera deviated from the ancestral 

 order, we are at present in the dark ; but further researches in the 

 rocks of new localities may in time throw light on this interesting 

 point. 



In another chapter we will see if the Embryology of the flies 

 throws any light on their phylogeny. 



