1894.] 
321 
[Packard. 
Fig. 6. A, dorsal view of head; eb, 
showing the mental and sular region. 
ventral view. 
In comparing the larvae of fleas with those of the Diptera^ it is 
to be observed at the outset tliat the former are less modified than 
the latter. The shape of the head in general and its parts have 
on the whole undergone less extreme modification in the fleas 
than those of any Diptera, even the most generalized of the 
Diptera eucepUala.'^ 
The Diptera, both as regards their larval and imaginal char- 
acters, have apparently undergone a greater degree of modifica- 
tion tlian any other order of insects, and this extreme degree of 
adaptation and modification is exhibited esj^eclally in the larvae. 
There seems to be no ancestral, generalized larval type of the 
order in existence. The head and its appendages have under- 
gone in response to adaptation to their unusual environment, 
most of the forms being aquatic, a greater or less reduction, not 
only in the mental and gular region, but in the appendages. So 
great is this reduction and modification, especially of the head 
and its appendages, that it is almost impossible to conjecture 
from which of the lower orders the Diptera have descended. We 
' For an excellent account and figures of the higher dipterous larvae I am indebted 
to Meinert's admirable work, Sur les larves euc6phales des dipteres. Vidensk. sels. 
skr., 188G. Also Brauer's valuable work cited below. 
^Brauer's tribe 1, Euctpliala, embraces those dipterous larvae with a completely 
differentiated head, which contains the first pair of ganglia and sometimes the eyes. 
It includes the following families: M_vcetophilidae, Bibionidae, Cliironomidae, 
Culicidae, Blepharoceridae, Simulidae, Psychodidae, Ptychopteridae, and Rhyphidae. 
Systematische studien auf grundlage der dipteren-larven nebst einer zusanimen- 
stellung von beispielen aus der litteratur iiber dieselben und beschreibung neuer 
formen. Wien, 1883. 
PROCEEDINGS B. S. X. H. 
VOL. .XXVI. 
