AN ACCOUNT OF BRITISH FLIES. 
Trias. Lias. Oolite. | Cret. ||Tertiary. || Recent. 
Diptera: | 
Ts "EEPLOPOSCIGGa: | sess overs) saswsswnewer|esanes scanaypenveaneses<ol| essay axeas pace ic 
@;, Cyclorinpha) 9) si. 500005 |enqpeb vases sd a 
Bi DTACHYCETA | anseenennsc ES ————&x«, #42 Cl ES 
4. Nematocera — J........044: oS ee cst TE 
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 Palzeozoic 
rocks. 
At present there is only one hexapod order in the Palzeozoic rocks 
known, namely, the Paleodictyoptera, which became extinct in Triassic 
times. It may be that from these Palzodictyoptera 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. 
