i88i.] On Diptera as Spreaders of Disease. 533 
The sections into which these segments have been divided 
are each composed of an oblong and a triangle, of a large 
wave crossed by a small one ; but circles represent on 
Globe III. conveniently the mean surfaces and character- 
istics. 
Meridian circle Af passes through L and S. The meridian 
circle which equally divides segment Af in the equator, 
equally divides the land belonging to segment Af, and also 
all land and sea generally. The configuration of the earth 
seen in the profile of this circle shows the symmetry of the 
halves. 
Segment Pr is divided into four sections ; the extremes, 
the arCtic and antarCtic, are equal, so are the middle ones. 
Each extreme is to each middle 1 : ( ^2*83) 3 = 1 : 4*78. 
The middle sections are connected by an isthmus ; the 
extremes are divided into archipelagos by the ocean rings 
in their tendency to maintain the circulation of the water 
complete in the amalgamated senses of longitude and lati- 
tude, with separation of retrogressing and progressing 
streams, of arteries and veins. 
Segment Pr gets not only inclined and divided : it is, to- 
gether with Ap and Af, pushed north by the 1-29*78 of 
surplus water by depth round Ps and its zone, the region of 
absolutely greatest gravity. 
The parallel 34 0 12' from Ps divides half the prototypic 
segment Pr in proportion 1 : ( ^2*83) 3 = 1 : 4*78. This is 
io° 44' outside the antarCtic circle. The southern point of 
America is io° 44' from that circle. 
(To be continued.) 
V. ON DIPTERA AS SPREADERS OF DISEASE. 
By J. W. Slater. 
S ^HE two-winged flies, in their behaviour to man, stand 
^ in a marked contrast to all the other orders of inseCts. 
The Lepidoptera, the Coleoptera, the Neuroptera, the 
Hymenoptera no doubt occasion, in some of their forms at 
least, much damage to our crops. But none of them are 
parasitic in or upon our bodies ; none of them persistently 
