608 
POPULAE SCIENCE EEVIEW. 
situated in the head_, another pair of little black bodies is seen 
not far from them (fig. and a third pair by the tail 
(fig. 1, o). These two pairs of black-looking bodies^ when 
viewed under the microscope^ are seen to be situated_, the one 
in the second segment, the other in the ninth. In structure 
they are found to be delicate little bags, composed of an 
elastic, finely-striated membrane, and filled with air. They 
are connected with a very fine and delicate system of air- 
tubes or tracheas, which, though not very abundant or largely 
developed, ramify through various parts of the body, being 
distributed principally to the nervous ganglia and digestive 
organs. The little black-looking sacs are in fact air-bags — 
tracheae vesicles — and, with the tubes connected with them, 
represent the extensive tracheary system, into which they 
afterwards become developed in the perfect insect. In fig*. 3, 
a, hj the anterior pair of the air sacs, with its connected air- 
tubes, is seen. In shape they are somewhat reniform ; the 
exterior point of each is slightly produced and drawn out into 
a fine and delicate tube, which becomes slightly enlarged {h), 
and then joins the main, lateral trunks of the tracheary system. 
The surface of both pairs of air sacs is more or less covered by 
dark hexagonal pigment cells of considerable size, and deep 
brown in colour. The function of these pigment cells is not 
at all obvious. It is somewhat difficult to trace the course 
of the air tubes through the body. They send out branches 
in various directions, particularly supplying the large cephalic 
ganglion and other nervous centres. Each of the terminal 
plates (fig*. 1, r) is penetrated by a branch from the tracheary 
system (fig. 6, n), and the array of lateral hairs, which are 
hollow as well as the terminal plume (fig. 1, t, u) are connected 
with it. The structure of one of the hairs of the lateral plume 
is seen in fig. 9. 
Salivary organs . — It is supposed that the organs (c, d, e) 
seen in fig. 3, in connection with the au*-sacs, are salivary 
organs. There are two elongated vesicles (c), which are com- 
posed of large nucleated cells ; each gives off* a tube, which 
unites with the other in the median line ; the tube is continued 
to near the mouth, where it becomes considerably enlarged, 
and opens apparently into the pharynx near the mouth. The 
glandular part of these bodies, as seen in the figure, is co- 
piously supplied with branches from the tracheary tubes. 
Organs of circulation . — There are no blood-vessels in the 
Corethra larva, nor should we look for them, since in insects 
the circulation is efifected by a simple dorsal vessel, or heart, 
and a series of sinuses from which it draws and to which it 
returns the nutritive fiuid. The dorsal vessel is very beauti- 
fully seen in the little creature under description. It is situ- 
