ANATOMY AND PHYSIOLOGY 45 
number of segments entering into the composition of the insect 
head has been a difficult problem. As no segment bears more 
than one pair of primary appendages, there are at least as 
many segments in the head as there are pairs of primary 
appendages. On this basis, then, the antennze, mandibles, 
maxillz and labium may be taken to indicate so many seg- 
ments; but in order to decide whether the eyes, labrum and 
hypopharynx represent segments, other than purely anatom- 
ical evidence is necessary. The key to the subject is furnished 
by embryology. At an early stage of development the future 
segments are marked off by transverse grooves on the ventral 
surface of the embryo, and the pairs of segmental appendages 
are all alike (Fig. 194), or equivalent, though later they dif- 
ferentiate into antenne, mouth parts, legs, etc. Moreover, the 
nervous system exhibits a segmentation which corresponds to 
that of the entire insect; in other words, each pair of primitive 
ganglia, constituting a neuromere, indicates a segment. Now 
in front of the cesophagus three primitive segments appear, each 
with its neuromere (Fig. 55): first in position, an ocular seg- 
ment, destined to bear the compound eyes; second, an antennal 
segment; third, an imtercalary (premandibular) segment, 
which in the generalized orders Thysanura and Collembola 
bears a transient pair of appendages that are probably homol-. 
ogous with the second antenne of Crustacea. In the adult, 
the ganglia of these three segments have united to form the 
brain, and the original simplicity and distinctness have been 
lost. The labrum, by the way, does not represent a pair of 
appendages, but arises as a single median lobe. Behind the 
cesophagus, three embryonic segments are clearly distinguish- 
able,each with its pair of appendages, namely, mandibular, max- 
lary and labial. Finally, the hypopharynx, or rather a part of 
it, claims a place in the series of segmental appendages, as the 
author has maintained; for in Collembola its two dorsal con- 
stituents, or superlingue, develop essentially as do the other 
paired appendages and, moreover, a superlingual neuromere 
(Fig. 55) exists. .The four primitive ganglia immediately 
