1915] ~- Nemocera Not a Natural Group’ of Diptera 95 
phylogenetically the relatively secondary, closed form existing 
among larve of the present time’ (1. c., p. 72). 
Strangely enough none of the attempts at the classification 
of the Diptera have taken this fundamental factor into con- 
sideration. While the condition of the larval respiratory system 
in the various families is stated in a perfunctory way in some 
classifications, its significance seems to have remained wholly 
unappreciated.* 
Let us now survey the result of grouping the families of the 
so-called Nemocera according to the character of the larval 
tracheal system, first placing on the one side the forms with 
the primitive series of lateral spiracles and on the other the 
specialized forms with partially closed spiracles. | Under the 
primitive peripneustic type we have the families Bibionide, 
Scatopside, Mycetophilide, Sciaride, Chironomide (part) 
and Cecidomyide. The forms with partly closed tracheal 
system include the Psychodide, Culicide (part), Ptycho- 
pteride, Tipulide and Rhyphide. The apneustic forms do 
not, as one might expect, form a concrete natural group in 
culimination of the metapneustic type, but are of diverse origin. 
Thus, in the Culicide the derivation of the apneustic 
Chaoborus larva is clearly indicated by the existence of a closely 
related, more primitive form (Eucorethra) with open posterior 
spiracles. On the other hand, we have in the larve of most of 
the Chironomidz apneustic forms derived from peripneustic 
stock. This is demonstrated by the existence of peripneustic 
and apneustic larve in the closely related genera Ceratopogon 
and Culicoides. Where entire families have apneustic larve, 
as is the case with two within the family-complex here consider- 
ed, their proper position must be decided by other evidences. 
of relationship. With the two families in question, the Simu- 
iide and the Blepharoceride, this evidence is so strong that 
they can be unhesitatingly associated with the peripneustic 
forms. In fact, in the larve of the Simuliidz the series of 
lateral spiracles, although minute and not functional, may 
be still detected. 
* Only Osten Sacken has felt, at least to a degree, the fundamental significance 
of differences in the respiratory system. He excluded Mycetobia pallipes from the 
Mycetophilide, on the ground that according to several good observers its larva 
is amphipneustic, while all other known forms are peripneustic (Proc. Ent. Soc. 
Phila., vol. 1, p. 151; 1862). I have had no opportunity to examine this larva, but 
nevertheless believe that the supposed difference rests upon an error of observation. 
