1921] Crampton: Sclerites of the Head OT 
The tentorium is apparently formed by three pairs of invaginations, 
two of which are anterior (a dorsal and ventral anterior pair of invagina- 
tions) forming the dorsal and ventral anterior arms of the tentorium, 
while the posterior ventral pair of invaginations form the posterior 
ventral arms of the tentorium. These tentorial arms, whose position 
is marked by the frontal pits, gular piis, etc., extend internally to the 
body of the tentorium. The ventral anterior arms of the tentorium 
may meet and unite to form a plate called the frontal plate of the ten- 
torium by Comstock and Kochi, 1902. 
Infoldings of the integument called implexes or endoplice are fre- 
quently formed in the different regions of an insect’s body, for the 
attachment of muscles, or for the strengthening of the body wall 
somewhat after the manner of corrugations in sheets of metal. In the 
head capsule, however, these internal folds for the most part form 
ridges for muscle attachment. These have been described under the 
discussion of the regions of the head capsule in which they occur. 
The foregoing description of the parts in insects in general, is offered. 
merely as the basis for a further more detailed discussion of those 
external features of insect morphology which appear to be of value in 
determining the phylogeny and interrelationships of the various groups 
of insects (and their arthropodan relatives), and on this account the 
modifications occurring in many of the orders have not been treated of 
in the present paper, since these can be taken up more profitably in a 
detailed discussion of each group taken separately. The evidence 
bearing upon the question of the phylogeny or of the interrelationships 
of the different groups of insects which have been described in the 
present paper, may be briefly summarized as follows, leaving the more 
detailed comparison of the parts in the different orders to be discussed 
later. 
RELATIONSHIPS INDICATED BY THE HEAD STRUCTURES. 
A comparison of the head structures of the Diptera with those of 
the Mecoptera would indicate a very close relationship between these 
two orders. The fact that the labial palpi, ‘“‘lp,’’ assume the form of 
fleshy lobes at the end of the labium in the Mecopteron shown in Fig. 
84, the fact that the lacinia of the maxilla is lost, and the galea, ‘‘ga,”’ is 
reduced to a short slender structure, and the presence of the sense organ 
labeled ‘“‘so”’ in the maxillary palpus of the scorpion fly shown in Fig. 
84, all point to a close relationship to the Dipteron shown in Fig. 83, in 
which the same tendencies occur, and the sense organ, ‘‘so,’’ occurs on 
exactly the same segment of the maxillary palpus as in the Mecopteron 
shown in Fig. 84. These facts are in full accord with the evidence of 
relationship between the Diptera and Mecoptera drawn from the study 
of the other regions of the body, such as the thoracic terga and wing 
bases, the genitalia and terminal abdominal structures, etc. 
The Mecoptera may, therefore, be regarded as the nearest living 
representatives of the types ancestral to the Diptera, although it is by 
no means certain that both Diptera and Mecoptera were not derived 
from Neuroptera-like ancestors. In fact the latter is even more probable, 
