NERVOUS SYSTEM OF INSECTS. 



291 



the latter to run to the limbs and to the muscles and other organs of the abdomen, are then 

 given off in a radiating fashion from this elongated nervous mass, those of the abdomen forming 

 a brush-like tuft at the extremity of the consolidated chain. 



With a few words upon these nerves we may quit this part of the subject. The 

 upper ganglion of the head (supracesophageal or cerebral ganglion) gives off in front a pair of 

 nerves, which run to the antennae, 

 and above a nerve which goes to 

 the simple eyes or ocelli when these 

 are present, whilst on each side it 

 is directly continued into the thick 

 optic nerves that run to the large 

 compound eyes. The lower, or 

 subcesop/tageal ganglion, furnishes 

 paired nerves which go to the upper 

 lip and the paired organs of the 

 mouth. The thoracic ganglia espe- 

 cially provide the nerves for the 

 wings and legs ; and the nerves 

 of the abdomen govern the move- 

 ments of that part, and especially 

 the function of respiration. 



The only organs of sense, to 

 which we can with certainty assign 

 a definite function are the eyes and 

 ocelli. The latter are found as the 

 sole organs of sight in many larvae, 

 when they are placed in groups on 

 the sides of the head in the position 

 afterwards occupied by the com- 

 pound eyes. In perfect insects 

 the ocelli are situated on the vertex 

 or crown of the head, and they 

 are then either two or three in 

 number. They are small round 

 organs, showing externally a con- 

 vex transparent cornea, beneath 

 which is the termination of a ner- 

 vous branch, specially modified for 

 the purpose of vision. The com- 

 pound eyes are much larger, and 

 usually very prominent organs 

 situated on the sides of the head, 

 the greater pavt of the surface of 

 which they occupy in some insects. 



They may be regarded to a certain 



* * 

 extent as composed of a multi- 



tude of ocelli, which, in consequence of their being brought as close as possible together, assume 

 a hexagonal form, and thus divide the surface of the composite organ into a number of facets 

 of that shape. Their number is often exceedingly great, but it varies much in different insects. 

 The smallest number recorded is 15 in the eye of a little Bee-parasite; the common House-fly 

 has 4,000, and a species of Dragon-fly as many as 20,000 facets in its eye. Each facet is 

 a small horny lens, usually flat on the outer surface and convex within. The centre of each 

 lens is in contact with the base of a cone, which is frequently regarded as a crystalline body, 



Fig. 10. NERVOVS SYSTEM OF PERFECT BEE. 



