536 
MR. NEWPORT ON THE RESPIRATION OF INSECTS. 
thing approaching to sacs, but the respiratory organs are purely tracheal. The 
common Humble-bee, which has the largest pulmonary vesicles of any insect in the 
perfect state, has its respiratory organs in the larva state exceedingly small and pen- 
cillated. But further proof of the vesicles being for the sole purpose of lightening 
the body is to be found in the male Lucanus cervus, Linn. In that insect the large and 
apparently heavy mandibles and head, instead of being filled with solid muscles, are 
filled almost entirely with a string of vesicles, which are developed from the sides of 
some large tracheae that extend through the mandibles. By this beautiful provision 
of nature these apparently heavy and unwieldy structures are rendered extremely 
light, while their solid exterior fits them for all the purposes of strength and defence 
required by the insect. 
2. The Spiracles. 
The structure of the spiracles, the orifices of respiration, is somewhat complicated. 
In the larva of Sphinx ligustri , L., the spiracles are oval apertures closed externally 
by valves, which open perpendicularly in their long axis, like the iris in feline Mam- 
malia. They are placed on a level with the external or cuticular surface of the body, 
and are formed by a series of converging fibres, edged, as in the iris, by circular ones 
[Plate XXXVI. fig. 6.], and guard the entrance of the spiracles. At a little distance 
within this valve the spiracle is considerably enlarged, and there is situated a second 
valve, of a more complicated structure. The anterior half of this second valve 
[Plate XXXVI. fig. 7- «•] is of a darker colour and firmer substance than the outer 
valve. Its inner surface, or that which looks towards the viscera, is concave, and its 
margin crescent-shaped, and it is not acted on by any muscles. The posterior portion 
of the valve [fig. 7- b~] is thick, moveable, and of a dark colour, and closes on the an- 
terior half like a cushion or pad. This portion is acted on by a remarkable muscle, 
the retractor valvulae [fig. 7- c], composed of five distinct fasciculi of fibres uniting 
in a common tendon, and by their conjoined action opening the valve, just as the 
levator palpebrce elevates the eyelid in Man and other animals. The tendon into 
which the converging fasciculi of this muscle are inserted, passes diagonally" upwards 
and backwards, and is inserted into a little elevation in the common tegument of the 
body. A few circular fibres surround the edges of the inner valve [fig. 7- d.] of the 
spiracle, and constitute the sphincter muscle which closes the valve. It is among 
these fibres that the retractor muscle originates. The sphincter muscle and valve 
are still further acted upon by another muscle, which may be considered the great 
constrictor muscle of the spiracle, retractor spiraculi [fig. 7- c.]. This muscle ori- 
ginates from the posterior margin of insertion at the anterior ventral surface of each 
segment, at a little distance from the median line [Plate XXXVII. fig. 26.], and 
passing diagonally upwards and outwards terminates in a tendon, with which some 
of the fibres of the orbicular muscle are blended. The internal or proper valve 
of the spiracle appears to be continuous with the mucous lining of the tracheae 
