The sense organs of Insects are very variable, both in position and structure. Three special senses are indicated by transparent and refractive parts of the cuticle, by tense membranes with modified nerve-endings, and by peculiar sensory rods or filaments upon the antennæ. 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.[110] [110] It is to be remarked that unusually large nerves supply the cerci of the Cockroach. [Illustration: 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, irregularly 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 a triangle on the forehead. The white fenestræ, which in the Cockroach lie internal to the antennary sockets, may represent two simple eyes which have lost their dioptric apparatus. In many larvæ only simple eyes are found, and the compound eye is restricted to the adult form; in larval Cockroaches, however, the compound eye is large and functional. [Illustration: Fig. 51.--One element of the Compound Eye of the Cockroach, × 700. _Co. F_, corneal facets; _Cr_, crystalline cones; _Rm_, nerve-rod (rhabdom); _Rl_, retinula of protoplasmic fibrils. To the right are transverse sections at various levels. Copied from Grenacher.] [Illustration: Fig. 52.--Diagram of Insect Integument, in section. _bm_, basement-membrane; _hyp_, hypodermis, or chitinogenous layer; _ct_, _ct_′, chitinous cuticle; _s_, a seta.]