118 
A nopheles Maculipennis 
( pp.m .) becomes a good deal stretched and is reduced to a thin layer of 
tissue. At this stage the first three basal joints are already developed 
but the remainder of the organ is still unsegmented. In a fully grown 
larva the second joint is greatly enlarged, especially in the male, and its 
cellular wall is very much thickened so as to almost obliterate the cavity 
of the joint. Tins condition is a stage in the development of the 
prominent antennal sense organ known as Johnston’s organ. Although 
the antennal buds are situated the deepest below the hypodermis 
of all the imaginal buds, they retain their connection with the surface 
of the head by means of a short canal which is the persistent mouth of 
the early invagination. 
The buds of the mouth-parts. The earliest indications of the forma¬ 
tion of the future mouth-parts are noticeable as special thickenings of the 
hypodermis, situated at localised points beneath the cuticle of the larval 
mouth-organs. Of these buds, the rudiments of the future labium, 
labrum, and maxillary palpi alone attain the condition of imaginal folds 
prior to pupation. Those of the mandibles and the maxillae remain in 
the condition of simple, hypodermal thickenings, but are easily recog¬ 
nisable on account of their great affinity for staining reagents. The 
invaginations of the first mentioned series of buds are superficial in 
position, and they maintain their communication with the sui’face by 
means of wide mouths or openings. The bud of the labrum is unpaired 
and situated in the middle line. It commences as a hypodermal 
thickening on the roof of the head and gradually elongates and comes to 
lie some distance backwards. Those of the maxillary palpi are small 
structures, and are situated at the bases of the same organs in the 
larva. The labial bud is extremely prominent (Fig. 31) and takes the 
form of a paired structure projecting from the bottom of a wide pocket. 
It is the earliest to appear of the imaginal buds that go to form the 
mouth-parts. Its first indication in the young larva is in the shape of 
a thickening of the hypodermis underlying the anterior border of the 
labial plate. In longitudinal sections through the head of a fully grown 
larva, the labial buds form a prominent pair of hollow pointed projections 
which are confluent only at their bases. 
II. The thoracic buds. In the thoracic region three successive 
pairs of buds are present on either side belonging to the pro-, meso-, and 
metathorax respectively. They are disposed in a dorsal and ventral 
series, the latter eventually giving rise to the three pairs of legs, while 
of the former series the first pair form the pupal l’espiratory siphons, 
the second pair the future wings, and the third pair the halteres. All 
