582 THE SENSES AND SENSORY ORGANS. 
*Probable acuity of vision in the Blow-fly from = to 2. 
(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 retinulz - - ‘008 to ‘ol mm. 
*Diameter of retinal image corre- 
sponding tor” - ‘0089 mm. = ('000357 X 25). 
9. ON THE STRUCTURE AND FUNCTIONS OF THE ANTENNZ. 
a, General Considerations (Plate XLI., Figs. 1 and 2). 
The antennz 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 antennz 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 Krapelin, 
in a critical monograph [273], came to the same conclusion. 
Bibliography : 
259. LEHMANN, M. C. G., ‘ De sensibus externis animalium exsanguium.’ 
4lo., Goettingee, 1798. 
260. LEHMANN, M. C. G., ‘De antennis insectorum dissertatio prior, 
fabricam antennarum describens.’ 12mo., Hamburgi, 1799. 
261. LEHMANN, M. C. G., ‘De antennis insectorum dissertatio posterior, 
usum antennarum recensens,’ 12mo., Hamburgi, 1800, 
These three works are the most important in the older literature, 
especially from an experimental point of view. 
262. LEFEBVRE, A., ‘ Note sur le sentiment olfactif des Antennes.’ Ann. 
Soc. Entom. France, tom. vii., 1838. 
2622. DUPONCHEL, ‘ Réflexions sur usage des Antennes dans les Insectes.’ 
Revue Zoologique per la Société Cuvierienne, tom. iii., 1840. 
263. ERICHSON, W. F., ‘ De fabrica et usu Antennarum in Insectis.’ 4to. 
Berolinii, 1847. 
