108 Anopheles Maculipennis 
to. The two series cross one another (Fig. 5) and are inserted into the 
median plate (m.pl.) of the spiracular lobe along its middle line. 
Thirdly, a pair of stout muscles arise one on either side close to the 
imaginal bud of the gonapophysis of its side and slightly dorsal to it 
( o.mus .). These muscles pass backwards and upwards in an oblique 
direction, and just external to the longitudinal muscles, and are inserted 
on either side of the median area ( m.pl.). 
When the animal dives suddenly down into the water, the upper 
surface of the spiracular lobe is converted into a kind of cup which 
retains, as a rule, a bubble of air. The contractions of the two sets of 
muscles (i.mus. and o.mus.) depress the floor of the lobe, with the result that 
the sides of the organ are curved inwards, and brought much closer to¬ 
gether, and, furthermore,by the action of the longitudinal muscles ( l.musc .), 
the anterior and posterior portions of the organ similarly become drawn 
closer together, and in this way a temporary cup is formed. When the 
muscles are relaxed, the organ spreads itself out flat, and forms a plate 
which helps to support the larva at the surface of the water. 
In the ninth abdominal segment, a pair of stout muscles take their 
origin one on either side of middle line of the tergum, and pass obliquely 
downwards and backwards to become attached, one on either side, to 
the base of the skeleton which supports the ventral fan of hairs 
(Fig. 6). By means of the contractions of this pair of muscles the 
ventral fan, or “ tail-fin,” can be moved to one side or the other as 
occasion may demand and, in this way, it appears that the fan of hairs 
functions as a kind of rudder which steers the animal as it is swimming. 
If this analogy be a correct one, the two muscles may be regarded 
as being comparable with the tiller cords attached to the rudder of 
a boat. 
The Fat-body. 
The fat-body is almost entirely confined to the thorax and the first 
seven abdominal segments. It consists of a parietal layer situated just 
beneath the integument, a visceral layer lining the body-cavity, and 
lying between the longitudinal muscles and the gut-wall, and a peri¬ 
tracheal layer which invests some of the principal tracheal trunks. The 
limits of distribution of each of these divisions varv according 1 to the 
age of the larvae, and a certain amount of individual variation is also 
noticeable among specimens of as near as possible the same age. 
The visceral layer of the fat-body (Figs. 3 and 9although but 
a thin sheet of tissue, is very conspicuous, as its cells are loaded with a 
