582 THE SENSES AND SENSORY ORGANS. 



*Probable acuity of vision in the Blow-fly from - to - 



60 60 



(See p. 569.) 

 Size of retinal elements : 



Length of bacilli - '075 mm. 



Length of outer segments '03 mm. 



Diameter of outer segments "002 mm. 



Diameter of retinulae - '008 to "oi mm. 



*Diameter of retinal image corre- 

 sponding to i 3 "0089 mm. = ('000357 x 2 5)- 



9. ON THE STRUCTURE AND FUNCTIONS OF THE ANTENNA. 



a. General Considerations (Plate XLL, Figs, i and 2). 



The antennae in the Insecta are always a single pair of 

 jointed organs, rarely reduced to a single joint, situated in 

 front of or above the great compound eyes. The function of 

 the antenme will be discussed hereafter, but I will premise 

 here that I regard them as olfactory organs an opinion which 

 is now, I think, very generally held. C. Claus (' Grundziige 

 der Zoologie/ 1876) regarded this as established, and Kriipelin, 

 in a critical monograph [273], came to the same conclusion. 



Bibliography : 



259. LKII.MANN, M. C. C,., ' Ue sensibus externis animalium exsanguium.' 

 410., Goettinga;, 1798. 



260. LKHMANN, M. C. G., ' De antennis insectorum dissertatio prior, 

 fabricam antennarum describens. : 121110., Hamburg!, 1799. 



261. LKHMANN, M. C. G., ' De antennis insectorum dissertatio posterior, 

 usurn antennarum recensens.' icmo., Hamburgi, 1800. 



These three works are the most important in the older literature, 

 especially from an experimental point of view. 



262. Li i i.i;\ KK, A., ' Note sur le sentiment olfactif des Antennes.' Ann. 

 Soc. Kntoin. France, torn, vii., 1838. 



262^. Uri'ONCHKL, ' Reflexions sur 1'usage des Antennes dans les Insectes.' 

 Revue Zoologique per la Societe Cuvierienne, torn, iii., 1840. 



263. KRICHSO.V, \V. F., ' Ue fabrica et usu Antennarum in Insectis.' 410. 



ii, 1847. 



