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 larvae 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 (b] 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-cord are in some degree con- 

 centrated, and in the adults are usually more centralised 

 than in the larvae. 



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 (i) 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 larvae. They have only 



