THE OLFACTORY ORGANS OF DIPTERA 475 
and then never distinctly; it would never have been regarded as 
an opening had not the writer seen many good examples of it in 
the olfactory pores of other insects. These pores are the smallest 
ones ever examined by the writer, and this fact easily explains 
why other observers have never seen the pore apertures. The 
sections were studied under a magnification of 1900 diameters, 
and with the aid of a camera lucida the drawings made represent 
a magnification of 3000 diameters before reduction; and they 
were reduced to 1000 diameters. 
As already stated, the pores on the legs, wings and groups 
nos. 16 and 17 on the halteres belong to the Hicks’ type, while 
group no. 15 belongs to the basal type and nos. 14 and 18 to the 
scalpel type. A glance at figures 19 to 25 shows that the struc- 
ture of the basal type (figs. 24 and 25) is like that of the Hicks’ 
type (figs. 20 and 23), and the only difference (not shown in these 
figures) between these two types is that a row of pseudohairs 
(fig. 16) lies between each two rows of pores in the basal group. 
Pseudohairs (figs. 19 and 20, Hr!) also protect some of the pores 
on the wings, but they are never arranged in rows as they are on 
the halteres. 
The size of the pores in any type varies considerably. This is 
shown by comparing the smallest and largest basal pores (figs. 
24 and 25) and the largest and smallest scalpel pores (figs. 26 and 
27). The scalpel type differs from the other types in the follow- 
ing three particulars: 1) The domes lie totally above the sur- 
rounding chitin; 2) the bottoms of the domes are considerably 
constricted, while in the basal and Hicks’ types on the halteres 
the domes are projected about one-half their height above the 
surrounding integument and their bases are constricted little 
or not at all, and 3) the tops of the domes are beautifully sculp- 
tured and assume a more or less flat surface. One pore (fig. 21) 
on the wing, resembling a scalpel pore, was found, while several 
(fig. 20) on the wing are identical to those in the Hicks’ and basal 
groups on the halteres. 
Sections passing longitudinally through the rows of basal pores 
show the pores as drawn in figures 25 and 29 (6Por,), while sec- 
tions passing transversely through the rows show the pores as 
