ORGANS OF SENSE. 



21 



again alluded to under classification ; they have been particularly 

 worked out by Bordas (Ann. Sci. Nat. (8) xi, 1900, pp. 283-448) ; 

 Leon Dufour ("Recherches anatomiques sur les Carabiques, etc.," 

 Ann. Sci. Nat. (1) vi, 1825, p. 152) ; and Escherich (" Anatomische 

 studien iiber das mannliche Grenital system der Coleopteren," 

 Zeitschr. fur wissensch. Zoologie, lvii, 1894, pp. 620-641, Taf. 

 xxvi). 



There are many secondary characters belonging to the male. 

 Some of these, which might be called direct characters, are adap- 

 tations for holding the female, e. g. the dilated front tarsi of many 

 Carabid^e, the suckers of the front tarsi of the Dytiscim;, the 

 enlarged and toothed femora and curved tibiae which occur in 

 various genera ; while others, which might be termed indirect 

 characters, consist in considerable differences in length and breadth 

 (the male being often much smaller than the female), longer and 

 more serrate or plumose antennae, a greater development of the 

 head and its appendages (especially in the Lamellicornia), etc. 

 These will be noticed in the course of the work. 



Dimorphism within the limits of a single sex is of rare occurrence, 

 but we have a good instance of it in the elytra of the females of 

 certain Dytisoid^e, which may be either smooth or deeply 

 canaliculate in the same species. 



The Organs of Sense. 



The organs of sight. — These, in the Coleoptera, are of two kinds, 

 the compound facetted eye, and the simple eye or ocellus, which 



Fig. 15. — Diagrammatic section of the eye of a beetle, au, facetted eye ; 

 c, transparent cornea made up of numerous lenses (cl) ; Jc, layer of 

 crystalline cones concealed by pigment ; rh, rt, rhabdoms and retinulse, 

 partly concealed by pigment ; nbs, nervous structures ; go, globular 

 apex of the optic nerve; no, optic nerve; tr, two tracheae belonging 

 to the optic nerve ; or, part of the chitinous orbit of the eye. (After 

 Kolbe.) 



is only found in the imago of a few species, and then in conjunction 

 with the compound eye (as in Omalium, etc.). Some of the cave- 



