THE OLFACTORY ORGANS OF DIPTERA 479 
b. Internal structure. A reference to figures 35 to 49 shows 
that the size of the pores usually varies according to the size 
of the insect studied; thus the average size of the pores in the 
robber fly (figs. 39 to 43) is greater than that of the pores in the 
flesh fly (figs. 44 to 49), although the pores on the halters of 
both flies are about equal in size. While the pores on the legs 
are always dome-shaped, many of those on the wings (figs. 42 
and 46) and a few on the halteres (fig. 47) are not dome-shaped. 
Many of those on the wings (fig. 45) of Sarcophaga project far 
above the surrounding chitin and their tops slightly resemble 
those of the scalpel pores on the halteres. A study of these pores 
shows a complete series of variations, ranging from the Hicks’ 
type to the scalpel type, and perhaps each type is still in the trans- 
Figs. 33 and 34 External views of olfactory pores of other flies. Fig. 33, a 
group from trochanter of Tipula, showing slitlike pore apertures, and fig. 34, a 
group from tibia of Sarcophaga plinthopyga, showing 3 areas of chitin around pore 
wall. X 500. 
itional stage. . Morphologically the scalpel type is the most highly 
developed, but physiologically it is probably little or no better 
developed than any other type of pores. - 
All of these results indicate that while the hind wings of 
Diptera have been gradually reduced in size, consequently 
gradually diminishing their flying ability, their sensory function 
has been greatly increased, and now they bear the highest type 
of olfactory pore yet found. The latter statement is supported 
by the fact that in Hymenoptera, the hind wings bear about one- 
half as many pores as do the front wings; in Lepidoptera’ the 
hind wings do not bear quite as many pores as do the front 
wings, while in Diptera the halteres bear about as many pores as 
do the wings and legs combined, or close to one-half the total 
number of pores found. 
