540 
MR. NEWPORT ON THE RESPIRATION OF INSECTS. 
six rather broad and thick fibres, which form two sets, and originate at some distance 
from the median line, posteriorly to, and on the outer side of the tendons of the great 
oblique, and passing outwards are inserted into the integument about half way across 
the segment. They contract the under surface very powerfully, and bring the sides 
towards the median line. Anteriorly to these muscles, but further from the median 
line, is another set, the abdominales anteriores [fig. 18.], consisting of six short muscles, 
which are inserted into the inferior margin of the lateral surface of the body. The 
abdominales laterales arise in the posterior half of the segment by three great fasciculi 
of narrow tendons [fig. 19.], eight in the first, four in the second, and seven in the 
third. These tendons are formed from very powerful muscles, which interlace with 
each other, and are inserted at different distances of attachment posteriorly to the 
spiracle [fig. 20.]. These are the great muscles of the false feet, and are connected at 
their origin with other transverse muscles near the median line. When these muscles 
are removed there are two layers of fibres which arise from the anterior line of attach- 
ment in the posterior part of the segment. The inner layer, or obliquus posterior [fig. 21.], 
consists of nine small muscular fasciculi of fibres, which pass diagonally outwards, and 
are inserted into the skin at different distances beneath the lateral abdominal muscles. 
The other set consists also of nine distinct fasciculi [fig. 22.], which arise from the same 
margin in the lateral part of the segment, and after crossing over the last lateral abdo- 
minal, pass between it and the one immediately before, and are inserted into the inte- 
gument : they may be called the postero-laterales obliqui. Besides these layers of mus- 
cular fibres there are four other sets which seem particularly concerned in the function 
of respiration. The first of these, the transversus lateralis [fig. 23.], arises tendinous 
from beneath the outer margin of the great ventral rectus, and passing upwards and 
outwards internal to the great longitudinal trachea, which it crosses, is gradually en- 
larged, and is inserted a little beneath the external margin of the dorsal rectus. The 
second transversus lateralis [fig. 24.] arises lower down than the first, and is inserted 
beneath the dorsal rectus at about half way across the segment. These muscular fas- 
ciculi appear to be directly concerned in contracting the segments in expiration. The 
other two muscles of respiration have been noticed when speaking of the spiracles. 
The first, retractor spiraculi [fig. 25.], as before noticed, originates from the third 
ridge of muscular insertion, at the anterior part of the segment, on the ventral surface, 
by a small tendon before the transverse abdominal muscles [fig. 26.]. It gradually 
increases in size, and passes upwards and obliquely outwards and backwards, and 
terminates in a tendon which is inserted, as before described, into the circular fibres 
which surround the spiracle. This muscle appears to be directly concerned in forcible 
expiration, during which it draws the spiracle inwards and downwards, and when 
the fibres which surround the spiracle and form the orbicular muscle act in conjunc- 
tion with it, it assists in closing the spiracle. The other muscle directly concerned 
in the function of respiration, retractor valvula?, is the immediate antagonist of the last, 
and has been described as one of the proper muscles of the spiracle [fig. 2 7 -]. 
