92 Mr. A. S. Packard on the Development 
an attempt has been made to trace these parts im the rings of 
the head by those who have proposed theories of the number 
of arthromeres in the head of insects. 
As we can understand the structure of the thorax better after 
studying the abdomen, so we can only homologize the different 
head-pieces after a careful study of the thorax of Insects and 
the cephalothorax of Crustacea, which thus afford us a standard 
of comparison. 
Since the arthropleural is the limb-bearing region in the 
thorax, it must follow that this region is largely developed in 
the head, to the bulk of which the sensory and appended diges- 
tive organs bear so large a proportion; and as all the parts of 
the head are subordinated in their development. to that of the 
appendages of which they form the support, it must follow logi- 
cally that the larger portion of the body of the head is pleural, 
and that the tergal and, especially, the sternal parts are either 
very slightly developed or wholly obsolescent. Such we find to 
be the fact. As to the number of rings composing the head, it 
is evident that it is correlated with the number of appendages 
they are to support. Hence, as in the thorax there are three 
rings bearing three pairs of appendages or legs, it follows that 
in the head, where there are seven pairs of appendages, there 
must be seven rings. That there are seven such appendages, 
among which we would include the eyes, which, if not homolo- 
gous with the limbs, or, more properly speaking, repetitions of 
the ideal appendage, are at least their equivalents, in that they 
are situated on a distinct ring, as are the ocelli, which are exact 
equivalents or repetitions of the eye, is evident. 
The larvee of Ephemera and Libellula, in the head of which 
these parts of the cephalic rings, by reason of the degradational 
character of the insects, appear in their simplest forms, afford 
us the best material for study. In the head of the larva of 
Libellula we have observed that the greatly elongated labium, 
masking, when at rest, the mandibles, is in reality composed of 
three sternites, immersed in and surrounded by three pleurites, 
all bearing appendages, the basal pair being the mandibles, the 
middle pair maxille, and thirdly, the pair of labial palpi, all of 
which are placed behind the mouth-opening. Beyond and in 
front of the mouth are successively placed the sensory organs, 
the antenne, the pair of eyes, and what we must consider 
two pairs of ocelli, since the early forms of Ephemera and the 
early stages of Bombus show the three ocelli resting on three 
separate pieces, the two posterior pieces (pleurites) forming a 
pair, while the single ocellus in advance is placed on a trian- 
gular piece which we regard as two pleurites united on the 
median line of the body, as the ocellus has a double form, being 
