Vinegar-fly (Drosophila funebris). 289 
defined, as there is no important difference between it and 
the rest of the dorsal vessel. 
Tracheal System.—The larva lives in semi-fluid sub- 
stances and burrows into its food. Naturally the anterior 
spiracles are not functional, while the posterior ones 
become large, and discharge the whole duty of renewing 
the air in the trachez. Two longitudinal tracheal trunks 
proceed forwards from the posterior spiracles to the anterior 
ones. ‘T'wo transverse connectives unite these main 
trunks: an anterior connective situated in the 3rd seg- 
ment, and a posterior one in the last segment. Branches 
are given off from the main tracheze 
to all parts of the body; those 
supplying the anterior segments are 
shown in fig. 2. 
In a young larva the anterior 
spiracles are solid oval bodies, which 
close the fore-ends of the main 
tracheal trunks. These are com- 
monly withdrawn into the body, but 
may be protruded by slight pressure 
(fig. 4). This simple structure of 
the anterior spiracles is only found 
in young larve. If older larve are 
examined, a very curious structure 
is seen. The simple solid spiracle 
is now replaced by a digitate struc- 
ture, the expanded extremity of the 
tracheal trunk sending out a number 
of finger-like processes, each of which | Anterior end of a young 
has its own epidermal investment anterior spiracles. (x 60.) 
(figs.2and 3). This digitate spiracle 
becomes functional in the pupa, and a further account of 
it will be given under that heading. 
The posterior spiracles of the old larva do not differ 
materially from those of the young one. They are carried 
at the distal end of a dorsal retractile process and are 
functional throughout the larval stage. They are very 
similar in general structure to those of many other 
Dipterous larvee, for instance Dicranota.* The tracheal 
trunks do not open directly to the exterior, but the cavity 
of each posterior spiracle ‘is closed by a spongy chitinous 
plug. In the centre of the plug is a flask-shaped cavity, 
* Miall (1893). 
Fic. 4. 
