1938] 
Lines of Descent of Insects 
175 
investigators insist is the case) as the result of mutational 
changes in the genetic or hereditary material of these 
insects. Thus in larval Aleurodidse, for example, the wings 
arise internally from wing buds, and become external in a 
quiescent pupal stage , 1 as they do in the Holometabola ; 
and the development of certain Thysanoptera and other 
Hemipteroid insects clearly suggests the beginning of 
Holometabolism. 
In this connection, it may be noted that the internal de- 
velopment of the wings in the Aleurodidse, and the external 
development of the wings in the closely related Psyllidse and 
other Hemiptera-Homoptera, clearly indicates that the usual 
division of winged insects into Exopterygota and Endoptery- 
gota, on the basis of the external or internal development of 
the wings, is utterly meaningless from the standpoint of 
phylogeny. Such a division would group together the 
remotely related Palseodictyopteroid, Orthopteroid and 
Hemipteroid insects, and would separate the Holometabola 
from their Orthopteroid relatives, etc. ; and this division of 
winged insects should be abandoned in favor of the more 
natural and fundamental grouping of winged insects into 
Palseopterygota and Neopterygota (on the basis of the 
method of folding the wings in repose), since this grouping 
does not separate the Holometabola from their Hemipteroid 
and Orthopteroid relatives. 
The section Holometabola (or Neuropteradelphia) in- 
cludes all insects with complete metamorphosis, and the 
group is evidently a monophyletic one since their larvse 
intergrade so intimately that all of the Holometabola must 
have had a common ancestry. The fact that some Holo- 
metabola are somewhat Psocid-like, and the fact that some 
Psocoid (or Hemipteroid) insects, such as the Aleurodidse, 
etc., clearly foreshadow Holometabolism, would seem to in- 
dicate that both Parametabola (Hemipteroids) and Holo- 
metabola were descended from closely allied Protorthoptera 
— which may have resembled the Protoblattids venationally, 
although their bodies probably exhibited features occurring 
in the ancestral Isoptera, Embiids and Grylloblattids. 
The most primitive representatives of the Holometabola 
are the Neuroptera, Hymenoptera and Coleoptera, and their 
1 A quiescent pupal stage is also foreshadowed in the Isoptera. 
