304 PERIPATUS, MYRI0P0DS, AND INSECTS. 



that ants and bees, with brains smaller than pin-heads, 

 should be so " clever." The more we know about an ant, 

 " the more the wonder grows, so small a head should carry 

 all it knows," or seems to know. But these statements 

 imply forgetfulness of the relative size of brain to body, and 

 tend, moreover, to exaggerate the importance of mere size. 

 The complexity of a brain is the important fact, not its size, 

 and the cleverer insects (ants, bees, and wasps) have more 

 complex brains than the others. 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-oesophageal) are linked to the brain 

 by a ring commissure around the gullet ; and (<r) 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 degrees concentrated, and the adults are usually 

 more centralised than 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 (cf. p. 255). 



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 larva?. They have only 

 one lens (monomeniscous), whereas the compound forms 

 have many lenses (polymeniscous). Their structure varies 

 greatly, and their use is very uncertain. 



Auditory (or chordotonal) organs have been found in all orders of 

 Insects (except as yet the Thysanoptera), and occur both in the larae 



