BA COT : MOSQUITOES AND THE DANGER OF MALARIA. 245 
three times before they attain their full size. They then cast 
their larval skin for the fourth and last time and become active 
free-swimming pupae. 
During both larval and pupal existence, the insects are 
mainly, and in the case of pupae entirely, dependent upon a 
supply of oxygen from the air ; but, in addition to the specialized 
spiracular openings on the 8th abdominal segment, which admit 
air into the tracheae, the larvae are provided with anal processes 
which serve as gills for utilizing the oxygen in the water. There 
is, however, very considerable variation in the extent to which 
these are developed. Most species have to rise frequently to 
the surface for air, but a few can remain submerged for long 
periods, and some of these seem able, under certain conditions, 
to dispense entirety with oxygen from the air. With some 
species of the cannibal group Megarhinus, the anal gills are very 
much reduced in size. In Mansonia, the air-tube is modified 
so that it can be inserted into the roots of water plants, from 
which the larvae obtain a supply of oxygen. 
The PuftcB .—The change from the more or less vermiform 
larva, with its lashing method of swimming and abdominal 
spiracles, to the comma-shaped pupa, which breathes through 
air-tubes, situated on the head, and usually termed “ trumpets,” 
is most striking. In this stage, swimming is due to the dorso- 
ventral movements of the flexible abdomen, which is terminated 
by two relatively-large paddle-shaped appendages. As no 
food is required during the pupal period, there is neither mouth 
nor vent. Owing to this, it is possible for pupae to survive 
when larvae would be killed. For instance, the pupa of the 
yellow fever mosquito can complete its development in salt 
water, in which the larvae die. Pupae are also less easily de¬ 
stroyed than larvae by oiling the surface of the water. 
Fhe pupal period is generally short, usually not more than 
two or three days, but development may be prolonged by cold. 
There is no evidence, so far as my knowledge goes, that the 
pupal stage is ever used by any species for hibernation. 
The Adults and their Methods of Feeding .— When the adult 
mosquito is developed, the pupa rises to the surface of the 
water and its skin splits along the suture in the middle line 
of the thorax. The edges of the ruptured portion of the pupal 
envelope curl outwards above the surface-film of the water, 
