D. Keilin 441 
the skin shows also supplementary folds which mask the real segmentation of 
the body (Text-fig. 4). 
The chitinous hooks, which are small and transparent, vary in shape and 
there are all the transitions between a real hook-like process on the one hand 
and wide flattened scale on the other; these hooks or scales are disposed in 
series of sinuous rows. 
The sensory organs of the skin are either in the 
form of small pits surrounded by chitinous rings, or 
in the form of hairs; the latter being in six groups, 
each composed of three hairs, which represent the 
remains of the thoracic legs, and are connected with 
the imaginal discs of these legs (cf. Keilin, 1915 a , 
pp. 166-173). 
The larva is amphipneustic, the anterior, pro- 
thoracic spiracles are composed of five papillae (PI. 
XXV, fig. 26) similar to those of the second stage. 
Each of the postabdominal spiracles (PL XXV, fig. 
20) bears three oblong clefts surrounded by oval peri- 
tremes having the internal border dentate; the peri- 
tremes lie so that their axes converge towards the 
scar (sc.). The spiracular tubercle shows also four 
openings (o.) of perispiracular glands, which, as in 
the previous stage, are surrounded by divergent chiti¬ 
nous hairs. In a few cases these spiracles were ab¬ 
normal, one peritreme being smaller than the other 
two (PI. XXV, fig. 21). A similar abnormality was Fig. 4. Third stage larva 
observed by Nielsen (1909, PI. II, fig. 18 and PI. Ill, of Melind " c0 ° n " t ' 1 secn 
fig. 49 6) in a third stage larva of Ptychomyia selecta atera y- 
Meig. 
The Buccopharyngeal armature is of the same general structure as in the 
previous stage (PL XXIV, fig. 9) but it is much larger and the basal sclerite 
( b.scl.) shows, near the pharynx, a supplementary emargination, which is 
narrow and deep, thus marking out a small triangular plate (t.). The two 
lateral hooks ( l.h .) are narrower and more slender than those of the second 
stage. 
For the details of the structure of this organ I need only refer to PL XXIV, 
Fig. 9 which shows also the reduced antennae (an.), maxillary palpe (m.p.), 
and radiating furrows of the cheeks. 
The alimentary canal (PL XXV, fig. 14). The pharynx (ph.) is followed by 
a short oesophagus ( oe .), which, after passing through the nerve ring, pene¬ 
trates the proventriculus ( pr.) forming the oesophageal valve. Four short 
coeca (c.) arise from the anterior portion of the midgut, just behind the pro¬ 
ventriculus. The midgut (m.g.) is very long and sinuous so that it is very 
difficult to trace its course. The two pairs of Malpighian tubes arise at the 
