THE OLFACTORY ORGANS OF DIPTERA 465 
much difficulty and doubtless many of them were overlooked. 
The writer is the only observer who has seen a group of them on 
either side of the halter. Since the number of pores on the hal- 
teres has never been tabulated, the following table is presented. 
A reference to this table will show the minor variations in these 
pores better than a description of them, therefore only the more 
important variations need be pointed out. The Hicks’ groups 
were found on 75 per cent of the halteres; a basal group on each 
halter, except in one species (no. 2); one or two scalpel groups 
on each halter; and the undetermined pores on 45 per cent of the 
halteres examined. One basal group (excepting no. 2) was 
invariably present on each halter, while two scalpel groups were 
observed on each halter examined, except in the three mosquitoes 
(nos. 3 to 5) and two of the wingless forms (nos. 13 and 48); 
only one scalpel group was seen on each halter of these five 
species. 
d. Pores on abnormal species. To determine what effect en- 
vironmental conditions has had upon the disposition of the ol- 
factory pores, seven species were selected for this purpose. 
Table 3 (p. 470) shows to what families they belong and the 
number of their olfactory pores in comparison with the pores of 
the normal species. In table 2 they are arranged according to 
the degree of degeneracy of the wings and halteres and they shall 
be described accordingly. 
The sheep tick (45 Melophagus ovinus) is much compressed; 
has no signs of wings and halteres; its legs are short and the 
segments are wide; the entire integument is thick and tough. 
Olfactory pores were found only on the trochanters and femora; 
their distribution is normal, but their number is reduced. The 
bat tick (48 Nycteribia bellardii) is also much compressed and 
has no compound eyes; its wings are totally wanting and its 
halteres are unusually small. The disposition of the pores on 
its legs is normal, but on the halteres the pores are comparatively 
few; the scalpel type being reduced to only one group per halter 
(table 1). The so-called wingless female of the snow-fly (2 
Chionea valga) copulates on the surface of the snow and it 
seems to be abnormal in four ways; 1) The number of pores on 
