18 THE AMERICAN NATURALIST. [VoL. XXXVI. 
of four segments was demonstrated, and the presence of a fifth 
em ocular) suggested. From this point the work has been 
carried on chiefly by the embryologists. 
The existence in the embryo of distinct 
segments, each corresponding to a pair of 
mouth parts, was early shown. Among the 
better of the older figures for this purpose 
are those of Brandt (69, Fig. 12) and of 
Packard (71). Fig. 6, which is copied from 
Brandt, represents an early stage in the devel- 
opment of the embryo of a damsel fly (Calop- 
teryx) In this stage the labial and maxillary 
| segments are quite distinct, appearing to be 
body segments rather than cephalic. This 
Mag poco doubtless represents a phylogenetic stage, in 
Mors el fy (Calopter, which the head proper consisted of fewer seg- 
ments than it does in existing insects. 
It was also found that the subcesophageal ganglion, which 
innervates the mandibles, maxilla, and labium, is formed by 
the union of at least three pairs of primitive ganglia. Fig. 7, 
from Heider (89), represents a stage in the development of 
Hydrophilus, in which these ganglia are still distinct, each pair 
of ganglia corresponding to a pair of mouth parts. 
So far the results of embryology merely con- 
firm the conclusions of comparative anatomy. 
But the embryologists have also demonstrated 
the existence of vestiges of segments, which had 
not been recognized as such by the early writers. 
In his work on the embryology of the honey- 
bee, Bütschli ('70) described a pair of evanescent 
appendages situated between the antennz and 
the mandibles. Later these were observed by 
= others, and writers began to refer to a “pre- 
mandibular," or * intercalary,' ' segment in the 
head of insects. More recently the appendages ! 
of this vestigial segment, which is properly ^fe" Heider. 
. termed the second antennal segment, were observed in the 
oe rhon M n 







Fic. 7. — Embryo 
of H 1 
» 

