THE NEMOCERA NOT A NATURAL GROUP OF DIPTERA. 
By FREDERICK KNAB, 
Bureau of Entomology. 
The classification of the Diptera is based upon recognized 
evolutional principles. The forms in which the larva has a well 
developed head and the imago multi-articulate antennze are 
placed lowest and from them are derived the higher flies, with 
the system of nerve-ganglia concentrated, headless larvee and 
in the imago the number of antennal joints reduced and the 
venation specialized. Brauer, in his earlier attempts to formu- 
late a natural classification of the Diptera on larval characters, 
gave primary importance to the reduction of the larval head. 
He considered phylogenetically lowest those forms in which 
there is a well developed head with the mouth-parts in normal 
position, and arranged the other families in sequence in accord- 
ance with the degree of reduction of the head, the relative con- 
centration of the nerve centers and the modification of the 
mouth-organs.!. Later, when further study had revealed the 
existence of many refractory elements, he laid greater stress 
on the progressive concentration of the nervous system, and 
readjusted his classification with the help of this factor.? 
Brauer’s ideas have been generally accepted and our present- 
day classifications practically follow him. However, while the 
transition from the lower to the higher flies along the lines 
indicated by him may be readily followed in a general way, 
detailed study reveals many contradictions, more particularly 
among the lower forms, within the so-called Nemocera. Very 
naturally these contradictions have led to various attempts 
toward a more natural grouping, none of which can be pro- 
nounced successful; the only ones worthy of mention in this 
connection are those of Osten Sacken* and of Lameere?. 
As already stated, the principles applied to the classification 
of the Diptera by Brauer are fundamental and must be accepted. 
But the presence of contradictory elements indicates that some 
fundamental factor has been overlooked, and such proves to be 
the case. This factor, more fundamental, as we shall see, than 
the reduction of the larval head, is the organization of the larval 
respiratory system. We have in dipterous larve several dis- 
93 
