102 Rymer Jones, on Corethra plumicorms. 
five in number, suspended in the cellulosity surrounding the 
heart. 
But perhaps the most characteristic feature in the economy 
of these larvae is the existence of four remarkable organs, 
situated — two of them in the thoracic region and two near 
the centre of the posterior half of the body. These strange- 
looking masses, conspicuous from their jet-black colour, are 
placed in pairs, and uniformly occupy the same situations 
corresponding with the centre of gravity, or rather of flotation 
of the two halves of the animal to which they respectively 
belong. In form they are more or less kidney-shaped/ and 
are to all appearance completely isolated and unattached to 
the surrounding structures. On crushing these kidney-shaped 
bodies beneath the compressorium, they are found to be filled 
with air, by the compression or rarefaction of which the 
creature is enabled to rise or sink in the surrounding water, 
without apparent effort, just as a Gold-fish rises or descends 
by means of its swimming- oladder. There is no visible 
outlet for the air thus confined in their interior, and the most 
rigid scrutiny only shows a few delicate air-vessels of extreme 
tenuity, in their immediate vicinity. Each of these air-sacs 
consists of several coats ; of these the outermost, when feebly 
magnified, seems of a uniform hue of jet-black ; but appears 
under higher powers to be made up of numerous distinct 
spots of black pigment, separated by considerable interspaces, 
so as to give the organ a reticulated appearance ; and it is 
only when this black pigment has been removed, together 
with a dull opaque membrane, whereon the black patches 
rest, that the real air-sac is displayed. When thus denuded, 
the true wall of the air-sac appears to be composed of a dense 
membrane, possessing great refractive power ; the effect of 
which upon transmitted light is extraordinary. When highly 
magnified, it is found to be entirely composed of numerous 
coils of a delicate fibre, similar to that which maintains the 
permeability of the tracheae of ordinary insects, arranged in 
several superimposed layers, and having the appearance of 
being closed on all sides. It is not until the larva thus con- 
stituted has arrived at its full size that the appearances 
described become complicated, by intermixture with organs 
belonging to the pupa condition of the insect. At this period, 
however, the rudiments of future limbs begin to show them- 
selves under the form of transparent vesicles, which, as they 
enlarge, crowd the thoracic region of the body. 
The change from the larva to the pupa condition involves 
several remarkable phenomena. The air-sacs, situated both in 
the thoracic region and in the hinder portion, burst and un- 
