262 ARTHROPODS. 



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 ex- 

 aggerate the importance of mere size. The complexity of a 

 brain is the important fact, not its size, and there is no 

 doubt that 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-oesophageal 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 (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 concentrated, and the adults are usually more 

 centralised than the larvse. 



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. Compound eyes vary considerably in 

 structure, and it is not certain to what extent they form 

 definite images of things. 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. 



Simple eyes or ocelli are present in addition to the 

 compound eyes in the adults of many insects, e.g., ants, 

 bees, and wasps ; they occur without the accompaniment of 

 compound eyes in Collembola, lice, and fleas, and they are 

 usually the only eyes possessed by larvae. They have only 

 one lens (monomeniscous), whereas the compound forms 

 have many lenses (polymeniscous). Their structure varies 

 greatly, and their use is very uncertain. 



