their Relation to Disease. 
T 3 
and it is probable that it is too small to be seen even with the 
highest powers of the microscope. We know, however, that it 
is a blood-parasite with a definite life-history, requiring to live 
first in the mosquito and then in man in order to complete its 
development. 
The Yellow Fever mosquito is a beautiful insect, strikingly 
marked with black and white; its most characteristic feature, by 
means of which it may be distinguished from every other mosquito, 
is the lyre-shaped white mark on the thorax. Apart from its 
Fig. 6 .—Culex fatigans, Wied. ?. x 4. 
The chief carrier of Elephantiasis. 
conspicuous coloration, it differs from the malaria mosquitoes in 
the points already indicated. It is a purely domestic mosquito, 
rarely found far away from towns or villages, and like other 
brightly coloured mosquitoes it bites chiefly during the day-time, 
only occasionally at night. It is practically universally distributed 
in tropical and sub-tropical regions, but fortunately the disease 
which it conveys is much more restricted in its range. 
Mosquitoes and Elephantiasis.— This is another tropical 
disease which frequently, though not necessarily, results from the 
presence of the worms known as Filariae in the blood. Manson 
