482 N. E. McINDOO 
pound ones. From a superficial view, a simple pit looks like a 
small circle (fig. 50, a) with a dot at its center, while a compound 
pit resembles a large circle (8) which contains radiating lines and 
two or more dots. Sections through these pits show that a single 
hair (fig. 51, Br) arises from the bottom of a simple pit and two 
or more hairs (fig. 52, Hr) from the bottom of a compound pit 
(fig. 50, C).. The mouth and sides of each pit are well protected 
by pseudohairs (Hr'!). A sense cell (SC) lies directly beneath 
each sense hair and a nerve fiber runs from each sense cell to the 
nerve (figs. 50 and 52, NV). An idea of how well the distal seg- 
ment is innervated may be had by looking at figure 50. 
Sometimes the hair in a simple pit projects out of the mouth 
of the pit (fig. 53), indicating that the primary function of such a 
hair is that of touch. All types of transitional forms of simple 
and compound pits have been found, and besides being present 
on the antennae, the compound pits are sometimes found on 
the palpi (fig. 54). Mosquitoes do not seem to have olfactory 
pits; Nagel has found two types of hairs on their antennae, and 
he calls the short, stout ones (fig. 55, Hr) olfactory organs. A 
male mosquito has only a few of these hairs, while a female has 
many. All flies seem to have olfactory pits, but some of them 
do not have the compound ones, and a few of the latter flies bear 
only one simple pit on each antennal segment. 
SUMMARY 
The disposition of the olfactory pores on the legs of Diptera is 
more similar to that of those on the legs of Hymenoptera than 
to those on the legs of Lepidoptera or Coleoptera, but those on 
the wings of Diptera are more similar to those on the wings of 
Lepidoptera than to those on the wings of the other two orders. 
The disposition of the pores on the halteres is entirely different 
from that of those on the hind wings of the other orders examined. 
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 almost one-half the total number of pores 
found. Excluding the abnormal forms, the total number of 
