GENERAL NOTES ON INSECTS. 343 
hairs, and spines. Chitin is not favourable to the develop- 
ment of skin glands. Most insects have “salivary glands” 
opening in or near the mouth. Bees have wax-making 
glands opening on the abdomen; aphides have glandular 
tubes ; not a few have poison bags; and many larvz besides 
silkworms have organs from which are exuded the threads 
of which a cocoon is made. 
Muscular system.—In very active animals like Insects, 
we of course find a highly developed set of rapidly contract- 
ing striped muscles. These work the wings, the legs, and 
the jaws. The resulting movements have this further 
significance, that they help in the respiratory interchange of 
gases, and in the circulation of the blood. 
Nervous system.—As in other Arthropods, the nervous 
system consists—(a) of a dorsal brain or supra-cesophageal 
ganglionic mass, and (4) of a double ventral nerve-cord with 
a number of paired ganglia, of which the most anterior (the 
sub-cesophageal) are linked to the brain by a ring com- 
missure around the gullet; and (c) of nerves given off from 
the various ganglia to the sense organs, the alimentary canal, 
and the other organs. In many of the higher insects the 
ganglia of the ventral nerve-card are in some degree con- 
centrated, and in the adults are usually more centralised 
than in the larvee. 
Sensory structures.—Animals so much alive as Insects, 
and in surroundings so stimulating as many of them enjoy, 
have naturally highly-developed sense organs. 
Two compound eyes are present on the head of all adults 
except the primitive Collembola, the degenerate lice, the 
likewise parasitic fleas, and blind insects which live in caves 
or other dark places. Each eye contains a large number of 
similar elements, in each of which we can distinguish—(1) a 
cuticular or corneal facet; (2) a glassy lens-like portion ; (3) 
a retinal portion in association with which are fibres from 
the optic nerve; and there are also pigmented cells between 
the elements. 
In addition to the compound eyes, simple eyes or ocelli 
are present in the adults of many insects, e.g. ants, bees, 
and wasps; they occur without the accompaniment of com- 
pound eyes in Collembola, lice, and fleas, and they are 
usually the only eyes possessed by larve. They have only 
