440 Mr. NEw»ronr on the Anatomy and 
perfect and larva states of insects of that Order. Perla, with its enlarged 
tracheze, its sacculated stomach and colon, and its more concentrated nervous 
system, is much in advance of Pteronarcys, which retains the larva type of 
organization in all its structures, branchial organs of respiration, a capacious 
esophagus and elongated alimentary canal, and more numerous and sepa- 
rated ganglia in its nervous cord. On passing from the general conformation 
to the details of its nervous system, Pteronarcys still preserves the same infe- 
riority. The cephalic ganglia, which constitute the brain, have not completely 
coalesced, as in some of the more perfect insects, but have only partially united 
in the middle line above the cesophagus; while the antennal ganglia in front 
of them are also distinct and separate. The ocelli on the front and vertex of 
the head in the perfect insect (fig. 10) are supplied by short nervous trunks, 
which proceed directly from the cephalie ganglia (fig. 11). "The anterior or 
median ocellus is primarily a double organ. It derives its nerve from two 
trunks, which originate one from the front of each of the cephalic ganglia, 
and which immediately are united laterally to form the nerve to the ocellus. 
This is the mode of origin of the anterior ocellar nerve in Pteronarcys, as I. . 
have found it to be also in Hymenoptera, and other insects. The posterior 
ocelli are supplied each by single trunks from the two cephalic ganglia. The 
true optic nerves, which supply the compound lateral eyes of the insect, have 
distinct gangliform enlargements at their base, and are expanded at their 
termination into a broad retina. 
The nervous cord exhibits distinct indications of its compound structure. 
The aganglionic portion, which I formerly described * in Lepidoptera, is very 
distinctly seen on its superior or visceral surface, while passing over the 
ganglia in the thoracic segments (u,v, w). It gives off a branch on each side 
in its course between the pro- and meso-thoracie ganglia in company with 
Some organic or transverse fibres. This branch passes diagonally back- 
wards, distributes some ramifications to the respiratory organs connected 
with the prothoracic spiracle, and then joins the first nerve from the meso- 
thoracie ganglion, and with it forms the anterior alar nerve that supplies 
the muscles of the first pair of wings, thus directly associating the function 
of respiration with that of flight. The origination of the wings, during 
* Phil. Trans, 1834. Eu 
