1921] Crampton: Sclerites of the Head 99 
in turn lead to the Mecoptera, Diptera, and Siphonaptera, 
on the one hand, and to the Trichoptera and Lepidoptera on 
the other, the neuropterous head which most resembles the 
head of the Mecoptera and Diptera being that of Nemoptera, 
while the neuropterous head which most resembles that of the 
Trichoptera (such as Philopotamus) and Lepidoptera (such as 
the micropterygids) is the head of Oliarces and other Ithoniidz.* 
The head of a Zorapteron is intermediate between that of 
certain Plecoptera such as Capnia, Leuctra, etc., and the 
Psocid type. Certain of the Psocide, in turn, have elongate 
heads suggestive of affinities with the Thysanoptera, while 
others exhibit undoubted affinities with the Homoptera and 
Hemiptera. The head of a mallophagan was undoubtedly 
derived from the psocid type, and the head structures of the 
Pediculide exhibit affinities with the Mallophaga, etc., as well. 
The head structures of Hymenoptera are rather puzzling. 
They exhibit features suggestive of the Mecoptera on the one 
side, and with the Neuroptera, as well as the Zoraptera and 
Psocide, on the other. The Psocide likewise exhibit certain 
features very suggestive of a relationship to the Neuroptera, 
and there is apparently a rather complicated interrelationship 
between these groups, as is shown in the head structures as 
well as in other features of the body, such as the ovipositor, 
etc. In certain respects, the head structures of Hymenoptera 
_ (even of the sawflies) are rather highly modified, but the sawfly 
type is a far more ancient one than it is commonly supposed 
to be, and probably arose from ancestors intermediate between 
the Zoraptera (with the Isoptera) on the one side, and the 
Coleoptera (with the Dermaptera) on the other. 
The study of the head and its appendages has shown that 
these structures are of prime importance for a study of the 
interrelationships and phylogeny of insects, and before we can 
arrive at the correct conclusion concerning the evolution of the 
various orders, and the origin of the Hexapoda, it will be neces- 
*The Hemiptera (and Homoptera) are related not only to the Psocidx, but 
to the Lepidoptera, Mecoptera, and Neuroptera, as well and exhibit many features 
suggestive of affinities with the Lepidoptera and Mecoptera. The lines of develop- 
ment of these forms apparently arose at the point where the Neuroptera and 
Psocide began to diverge, and therefore took over in their development, features 
common to both. Under these conditions it is readily seen that the Lepidoptera 
may resemble the Psocide and Hemiptera (Homoptera) on the one side, and the 
Neuroptera, etc., on the other, and this fact has not been sufficiently emphasized 
by students of insectan phylogeny. 
