224 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 
fibers proceed to the facets of the eye. The optic ganglion connects 
with the brain by the large optic nerve. There are, then, two optic 
nerves, besides three slender nerves (ocellar nerves) sent to each of the 
three ocelli or simple eyes ; moreover, a nerve is sent to each of the an- 
tennae and are hence called the antennal nerves. The relations of the 
brain to the head, and to the succeeding ganglion, and the origins of 
the nerves distributed to the eyes, antennas and ocelli, as well as of the 
nerves sent to the jaws, etc., are clearly seen in the figures on Plate 
IX. 
On the other hand the mouth parts, i. e., the jaws (mandibles) and ac- 
cessory jaws (first and second maxilla:, the latter called the labium or 
under lip) are each supplied by a pair of nerves, called, respectively, the 
mandibular, maxillary, and labial nerves. These three pairs of nerves 
arise from the subcesophageal ganglion. (See PI. IX, Fig. 2.) 
The brain of insects as distinguished from the brain of vertebrates. — The 
"brain" or supraoesophageal ganglion is, as we shall see, a much more 
complicated organ than any of the succeeding ganglia, having impor- 
tant parts which are wanting in all the others, hence it is jpar excellence 
nearer to our idea of a brain than any of the other nervous centers. It 
should be remembered, however, that the word "brain" is applied to 
this compound ganglion simply by courtesy and as a matter of conve- 
nience, as it does not correspond to the brain of a vertebrate animal, the 
brain of the horse or man being composed of several distinct pairs of 
ganglia. Moreover, the brain and nervous cord of the fish or man is 
fundamentally different, or not homologous with that of the lower or 
invertebrate animals, though the nervous system of the insects and 
Crustacea present greater analogies to that of the vertebrates than any 
other of the lower animals, with the exception, perhaps, of the cuttle- 
fish. The nervous cord of the insect consists of a chain of ganglia con- 
nected by nerves or commissures, while the spinal cord of the fish or 
man is essentially "a double and fused series of nerve-centers." More- 
over, if this cord is cut through, a section shows that it consists of two 
kinds of substances or tissues, called the "gray" and "white" substance. 
The gray matter is situated in the center, and consists largely of nerve 
or so-called "ganglion cells," while the external white matter of the 
brain or cord is composed of a mass of nerve fibers. Xow, in the nervous 
system of insects there is nothing to compare with these substances, but 
the ganglia, on the contrary, as we shall see farther on, consist primarily 
of an external layer of ganglion cells, whose fibers pass in to form a 
central fibrous mass or net-work, the meshes of which are filled with a 
fine granulated nerve substance, the nature of which is not clearly 
understood. Moreover, the entire brain of an insect is white, as are all 
the ganglia. 
A ganglion in its simplest form is a little rounded mass, or nodule, of 
ganglion cells, with fibers leading from them ; such cells are represented 
by Fig. 3a, on Plate XL Xow when the fibers lead in from the sensitive 
