W. WESCHE OX SOME NEW SENSE-ORGANS IN DIPTERA. 101 



same objection prevents A being considered as an auditory 

 organ, but does not apply to B, which is on situations where 

 it would well receive impressions, though the very small surface 

 of the membranes might not be a favouring character. (-4) That 

 leaves smell as the remaining sense. As regards both A and 

 B, though A has analogies in detail, yet the whole organ is 

 so much unlike the structures which so far we have been able 

 to connect with that sense ; and as B has no analogies, we 

 must reject the hypothesis that they are olfactory organs. 

 (5) They are not secondary sexual organs, as I find them in 

 both sexes. (6) We then see that Structure B is possibly 

 an auditory organ, but in the case of A we are driven back 

 on Lord Avebury's idea, as to a sense of which we have no 

 conception, and there the inquiry necessarily comes to an end. 



In certain minute flies in the Empidae, apparently but 

 distantly connected, as they are each in different sub-families, 

 I have found sense-organs on the fore-legs which are obviously 

 homologous. They are situated in much the same place as 

 the auditory organs of Jleconema, already alluded to as illustrated 

 by Packard. 



In Hyhos femoratus, Mull. (PI. 6, Figs. 14, 17, 22), I find a 

 ciliated process t \q of an inch long, and underneath the hair 

 a central pit ; this is situated in that part of the tibia which 

 is directly opposite the femur, and close to the knee-joint. In 

 Ocidromia glabricula, Fin., the organ is situated lower down 

 the tibia, is -g^ of an inch long, and is bounded on three sides 

 by a chitinous ridge. In Chersodromia cursitans, Z., it is in 

 much the same place as in Hi/bos, and is —-$ of an inch 

 long. In this species I can trace a long tube which opens 

 into the circular pit. This can readily be distinguished from 

 the trachea, as its structure is without annulations (PI. 6, 

 Figs. 13, 16, 20). 



Now whether these are auditory organs is difficult to determine. 

 The question at once arises, What particular advantage would 

 the sense of hearing be to these very minute insects ? Certainly 

 H. femoratus and C. cursitans are uncompromisingly raptorial 

 in their habits ; they frequent damp herbage, and it may be 

 of service as a means of informing them of the approach of 

 prey or danger. That being so, why are the nearly related 

 Empinae without the process, and why is not the possession 



