ITS NERVOUS SYSTEM AND SENSE ORGANS. 



99 



filaments upon the antennae. These are taken to be the organs 

 respectively of sight, hearing, and smell. Other sense organs, 

 not as yet fully elucidated, may co-exist with these. The 

 maxillary palps of the Cockroach, for example, are continually 

 used in exploring movements, and may assist the animal to 

 select its food ; the cerci, where these are well-developed, and 

 the halteres of Diptera, have been also regarded as sense organs 

 of some undetermined kind, but this is at present wholly 

 conjecture.* 



Fig. 50. Plan of Eye of Cockroach, showing the number of facets along 

 the principal diameters, as, antennary socket. 



The compound eyes of the Cockroach occupy a large, irregu- 

 larly oval space (see fig. 50) on each side of the head. The 

 total number of facets may be estimated at about 1,800. The 

 number is very variable in Insects, and may either greatly 

 exceed that found in the Cockroach, or be reduced to a very 

 small one indeed. According to Burmeister, the Coleopterous 

 genus Mordella possesses more than 25,000 facets. Where the 

 facets are very numerous, the compound eyes may occupy 

 nearly the whole surface of the head, as in the House-fly 

 Dragon-fly, or Gad-fly. 



Together with compound eyes, many Insects are furnished 

 also with simple eyes, usually three in number, and disposed in 



* It is to be remarked that unusually large nerves supply the cerci of the Cockroach. 



