IN' THE ANTENNA A\D PALPI OF DIPTERA. 67 



readily fall over. I think the structure of these organs shows 

 that the mere cutting of these large nerves must lead to ver}^ 

 great nervous disturbance, and there does not seem to be any 

 very good reason to believe that they function in control of the 

 legs. 



An enormous number of observations with flies shows that they 

 are extremely sensitive to the presence of certain compounds. 

 If sugar is put out in a room in which there are manj^ flies, the 

 sugar is presently covered with flies ; if with the sugar is mixed 

 a chemical substance, it is possible to determine if the flies avoid 

 it, or if it is attractive to the flies. The only compounds to which 

 House-flies ai'e sensitive are the volatile constituents of essential 

 oils, the volatile organic compounds which are soluble in fats, 

 and those compounds which give off ammonia. The compounds 

 are generally those which are fat-solvents, and, apart from the case 

 of Musca, the most active are the constituents of essential oils. 



The antenna contains (1) a large nerve network, (2) secreting 

 cells and frequently, (3) fat cells. Apparently the hairs over the 

 main surface are filled with secretion, probably of a fatty nature, 

 which picks up essential oils readily. A fly that is a flower- 

 feeder is attracted b}' the traces of essential oils secreted by the 

 flowers, and directs its course to the flower : arriving there, its 

 large outer surface is probably saturated with the absorption of 

 the perfume ; it has still to locate the honey-secreting glands and 

 it is then that the pits come into play. The pits are genei-ally 

 guarded, often on the inner unexposed surface of the antenna, 

 and only very great concentration of the perfume afliects them ; 

 but they are extra sensitive and aid the fly in locating the source 

 of the perfume in the flower. Flies not only have to find food in 

 flowers. l)nt also to find breeding-places : for some of these, other 

 kinds of odours are certainly the attraction ; and some flies have 

 two kinds of pits, one kind of which, I suggest, function for the 

 food, the other function for the breeding-place. Musca, for 

 instance, is definitely attracted by ammonia, whether from the 

 manure heap, or from chemical compounds that slowly disengage 

 ammonia : it has two kinds of pits. It is a pui'e supposition, 

 that one of these pits locates one class of compound, another 

 locates another class ; but it is borne out by the occuirence of the 

 two kinds in Eristalis, which is a flower-feeder and which breeds 

 in deca3'ing organic matter. The explanation that is suggested 

 of these organs is that they are purely olfactory, that the general 

 surface of the anteniiEe acts for delicate perceptions, that the pits 

 come into play when the concentration of the absorbed liquid 

 has dulled the simple organs on the outer surface, and that the 

 final location of the source of scent is due to the protected pits. 

 Further it is suggested that the presence of two kinds of pits in 

 some species is correlated with the dual perception in the 

 female of food and of breeding-place, in the male, of food and of 

 the female. 



