No. 421.) SKELETON OF THE HEAD OF INSECTS. I9 
They were also observed in Campodea by Uzel (97) The 
last writer states that these appendages persist in the adult 
Campodea, and Folsom ('99b) says, * I may add that rudimen- 
tary chitinized intercalary appendages persist in adults of 
Tomocerus, Orchesella, and other Collembola.” 
Equally important evidence as to the existence of a second 
antennal segment in insects has been furnished by studies of 
the nervous system. It was shown by Viallanes (87a) in his 
study of the structure of the brain (supra-cesophageal ganglion) 
of Vespa that there are three principal divisions in the brain 
of insects. These he named the fprotocerebrum, the deutocere- 
brum, and the ¢ritocerebrum. Almost immediately after Patten 
(88) demonstrated that the brain is formed from three pairs of 
primary ganglia; and the same fact was shown by Wheeler ('89). 
Viallanes also showed that the protocerebrum innervates the 
compound eyes and ocelli; the deutocerebrum, the antennae ; 
and the tritocerebrum, the labrum. This demonstrates the 
existence of three premandibular segments: first, an ocular 
or protocerebral segment, without appendages, unless the com- 
pound eyes represent them (the supposed discovery of other 
appendages on the ocular segment by Carriere ('90) has not 
been confirmed); second, an antennal or deutocerebral seg- 
ment, bearing antennz ; and third, a second. antennal, or trito- 
cerebral segment, of which the labrum is a part, and to which 
the so-called intercalary appendages doubtless belong. As 
Viallanes has shown that the tritocerebrum of Crustacea inner- 
vates the second antennz, we are warranted in considering the 
tritocerebral segment of insects to be the second antennal 
segment. 
The evidence thus far brought forward demonstrates the exist- 
ence of six cephalic segments, — three innervated by the brain 
and three by the subcesophageal ganglion. We have now to 
refer to the evidence indicating the existence of a seventh 
cephalic segment. . 
The hypopharynx of insects is usually, in the Pterygota, a 
tongue-like organ lying below and projecting in front of the 
beginning of the alimentary canal In the Apterygota it 
consists of three distinct parts, — a median organ termed th: 
