66 MR. K. M. SMITH OX CERTAIN SEXSE-ORGAXS 



Appendix. 



This papex" is the outcome of inquiry into tlie habits of the 

 House-fly which was initiated at the Zoological Gardeus in 1915. 

 Miss Lodge's paper on the behaviour of flies has appeared in the 

 ' Bulletin of Entomological Eesearch," November 1918 ; this 

 paper is complementary to it, and in trying to ariive at an inter- 

 pretation of these sense-organs, we have also further unpublished 

 work on the behaviour of flies towards cheujicals which aids us 

 in understanding- the function of the antennte. The strnctni-e of 

 the sense-organs described above shows a general similarity in 

 fundamental design but marked variation in detail. Speaking 

 broadly the' third joint of the antennpe is alone concerned, it is 

 set over its surface with simple sensitive hairs, it contains pits or 

 cavities which have a more complex structure; these pits ai-e as 

 a rule protected or situated on the inner surface of the antenna 

 and the whole joint is set on to the second in a curiously complex 

 fashion. Further, there is in some species an inner chitinous 

 skeleton, which has probably to be considei-ed in conjunction with 

 the elaborate conical structui'e of the base of the sensoiy joint. 



lu' considering the function of this antenna, one point sug- 

 gested itself which we may first dispose of : some flies have the 

 curious habit of hovering, i. e. of resting poised in the air, 

 stationary, with a wing velocity adjusted to maintain them so ; 

 it seemed possible that the antennas functioned as a speed- 

 perception organ, I'clative not to space but to air: that is, the 

 fly was able to feel if it were progressing against the air and 

 when sensation was nil, was stationary. As these organs are less 

 well developed in the Hover-fly section of the Syrphid?e than in 

 the •' Bee-fly " section, this is not supported. Another possible 

 view is that they are auditory in function, and some of the more 

 complex pits suggest the auditory organs of Locnstidfe ; but there 

 is little reason to believe that any flies respond to sound. I think 

 that a great deal of furthei' observation will be I'equired to 

 ascertain if some of the Syrphidas especially do not have auditoiy 

 pits, but I think it is not the explanation of the function of the 

 sense-organ generally. 



A third point that seems to be clear is that the antennae and 

 pits are not modified in the sexes : it has long been known that 

 the females of some Fruit-flies secrete essential oils or complex 

 ai'omatic ketones or aldehydes which are limited within the 

 species and which attract the males in a very wonderful manner: 

 in such cases males must have extreme delicacy of sense percep- 

 tion ; but females have equally perfect sense-organs (if these 

 organs are olfactory in function) and they are not secondary 

 sexual characters, as they occur equally well developed in both 

 sexes of each species. Miss Lodge's observations show that flies 

 with the antenna? cut oft' are unable to retain theii' balance and 



