1938] 
Lines of Descent of Insects 
169 
as adults) and are sometimes called Anamerentoma, in con- 
tradistinction to the rest of insects (called Holomerentoma) 
which exhibit no such postembryonic increase in segmenta- 
tion. Their line of development evidently branched off at 
the base of the Apterygotan stem, and ends blindly, unless 
it leads to the Collembola. 
The line of development of the Collembola is a rather 
isolated one, but the Collembola resemble the Protura in that 
they have a ventral head-groove, a postantennal organ, and 
similarly modified endognathous mouthparts, etc. The 
Collembolan line of development may have branched off 
from that of the Protura, although it is also possible that 
the Collembola represent degenerate offshoots of the primi- 
tive Machiloid ancestors of the Apterygota, since the lacinial 
fringes of such Collembola as Tetrodontophora are very 
like those of certain Machilids. In any case, the line of 
development of the Collembola is a very isolated one, and 
has no significance for tracing the lines of descent of other 
insects. 
As was mentioned before, the “larvae” (naiads) of the 
Ephemerida, with their Lepisma-\ike terminal filaments (a 
pair of cerci and an unpaired median terminal filament), 
and their large paragnaths (superlinguae), and primitive 
type of mandibles, provided with a lacinia mobilis like the 
mandibles of higher Crustacea, etc., are the most archaic 
representatives of the Pterygota, and suggest that winged 
insects arose from Lepisma- like forebears, which lead back 
to the Crustaceoid Apterygota such as Machilis. “Larval” 
Ephemerida and Odonata agree in having the lacinia and 
galea united to form a single lobe in the maxilla, and “larval” 
Ephemerida and Zygoptera agree in having an unpaired 
median terminal structure, represented by a gill plate in 
the Zygopteran naiad, and by a terminal filament in the 
Ephemerid naiad. 
The Ephemerida, Odonata, Megasecoptera and Palseo- 
dictyoptera, etc., comprise the division of Pterygota called 
the Palseopterygota, characterized by their inability to lay 
the wings back along the body in repose. They consequently 
do not develop a basal fold of the wing, and do not have 
more than two or three axillary sclerites. Their wings are 
primitively homonomous, and a neala is not developed in 
