262 BENJAMIN P. YOUNG 
in the group that the use of another more descriptive term would add to the 
confusion already existing in the nomenclatures of different orders of insects. 
Full lines representing sutures in drawings are not limited to any one 
kind of suture, but to sutures in general, whether these are spaces between 
approximated sclerites or plates of the integument, impressed lines, or 
lines formed by the approximated lips of an infolding of the body wall. 
GENERALIZATIONS 
The scope of this investigation makes it necessary to limit the discus- 
sion of homologies to the metathorax and the first few abdominal segments; 
consequently many interesting modifications and details of structure in 
the remaining thoracic segments which have been figured may prove of 
advantage to those interested in the order, but must be disregarded at 
present by the writer. However, sclerites have been named in the meso- 
thorax according to the dictates of this study, and a short discussion of 
the terms employed will of necessity be given. 
If the Apterygote are to be considered the most generalized insects 
(forms in which wings were never present and therefore forms in which 
the muscular tension and mechanical stimulus due to the proper func- 
tioning of these locomotor appendages has never been experienced), 
and a thoracic segment of such an insect in which there is practically no 
chitin laid down in the membrane is to be considered as rather primitive, 
then our conclusion must be that a membranous condition must have 
been that of the thoracic segments of primitive insects, and only when 
the development of appendages gave rise to muscular stresses and strains 
and friction of parts was it necessary that their walls should be strengthened 
by the deposition of chitin in their integument. All theories to the origin 
of insects from annelid-like ancestors tend to strengthen this hypothesis. 
Panorpa venosa (Plate XXXII, 76), therefore, has been figured simply to 
show the appearance of a more primitive winged form. The development, 
of wings has meant the chitinization of terga and pleura, together with the 
invagination of the body wall to form the pleural ridges, while the increase 
in size of the legs has meant their chitinization and their division into the 
true coxae and mera. The straightness of the pleural and coxal ridges 
is a mark of primitiveness. The large amount of membrane in the abdo- 
men is another, while the distinctiveness of the basalares and the subalares 
might be considered a third. The two divisions of each tergum of the 
