1938] 
Lines of Descent of Insects 
179 
resembling the fossil Mecoptera. The venation of Belmontia 
is so like that of certain Rhyacophilid Trichoptera that the 
differences hardly seem to be of ordinal value. At any rate, 
the Trichoptera have departed the least of any living insects 
from the ancestors of the Lepidoptera, and the Mecoptera 
are the nearest living representatives of the forms ancestral 
to the Trichoptera. 
The origin and closest affinities of the Siphonaptera, or 
fleas, is still a subject of much dispute, and it is impossible 
to decide the question in the present state of our knowledge 
of the group. It is quite evident, however, that the Sipho- 
naptera resemble both Diptera and Trichoptera in numerous 
features of their larval and adult anatomy, and this prob- 
ably indicates that the fleas were descended from the 
common ancestors of the Diptera and Trichoptera. Since 
the Mecoptera are the nearest living representatives of the 
common ancestors of the Diptera and Trichoptera, the fleas 
were doubtless derived from Mecopteroid ancestors as yet 
unknown. 
The Hymenoptera combine in themselves so many char- 
acters occurring in both the Coleopteroid and the Mecop- 
teroid insects that it is very difficult to determine their 
closest affinities, although they have a greater number of 
characters in common with the Mecopteroid insects than 
with any other group, and their larvae (particularly those 
of the sawflies) are strikingly similar to larval Mecoptera, 
Lepidoptera, etc. These facts may be interpreted as indi- 
cating that the Hymenoptera should be grouped with the 
Mecopteroid insects in the superorder Panneuroptera 
(Neuropteria) , but since the Hymenoptera are in many 
respects intermediate between the Coleopteroid and Mecop- 
teroid insects their annectant character may be better 
indicated by placing them in a distinct superorder, the 
Panhymenoptera (or Hymenopteria) , occupying a position 
intermediate between the Mecopteroid insects (Panneurop- 
tera) and Coleopteroid insects (Pancoleoptera) , and char- 
acterized by the occurrence of an ovipositor (represented by 
a saw or a sting), forcipate male genitalia, cerci, an 
Orthopteroid head, etc., and by the absence of the meron in 
the mesothoracic coxae. 
Tillyard considers that the Hymenoptera were derived 
