their Relation to Disease. 
9 
available. We have only two tree-hole breeders in Britain, both of 
which are very vicious in their attacks. 
Under this heading may also be included those species whose 
larvae occur in early spring in temporary pools of rain-water in 
woods and on heaths, and in cold countries in the pools formed by 
the melting snows. These species, particularly in Arctic regions, 
often occur in enormous numbers, and are amongst the most 
bloodthirsty of all the mosquitoes, though at present that is the 
most serious charge that can be made against them, as none of 
them have yet been incriminated as disease carriers. They have, 
as a rule, only one brood a year, which are hatched out in the 
spring, living on all through the summer and depositing their eggs 
in the dry hollows which will, during the winter, again become filled 
with water. About half of our British species belong to this group. 
(d) Swamp Mosquitoes. —Allied in some respects to those 
last mentioned, are the species which breed in extensive and more 
or less permanent swamps, shallow inland lakes, or salt marshes 
along the coast. Anopheles is represented in this group, but they 
are not as a rule disease carriers, although in many regions they 
occur in such vast swarms as to constitute a serious pest. They 
differ from those of the preceding groups in their power of 
migration, cases having been recorded in which large clouds of 
them migrated a distance of at least fifty miles, while in other 
instances they have been known to make a nightly invasion of 
towns situated a mile or more distant from their breeding places. 
MOSQUITOES AND DISEASE. 
It has long been recognised that the most serious obstacles to 
the settlement of the tropics by the white man are the diseases 
which are rife in those regions, but it is only since the early years 
of the present century that mosquitoes have been shown to be the 
agents in the spread of some of the most devastating of these 
maladies. The blood parasites which are the actual causes of the 
ailments cannot exist continuously in one host, but require for 
their full development alternating periods in man and a mosquito; 
they are drawn up with the blood when the mosquito bites an 
infected patient, and if, after an interval of a week or so, the same 
mosquito bites a healthy individual the disease can and probably 
will be transmitted to him. 
