BACOT : 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 entirely 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 Pupae.—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.
The 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,