MR. NEWPORT ON THE RESPIRATION OF INSECTS. 
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the smaller rectus. The origin of this set is exceedingly narrow, and resembles the 
origin of the oblique abdominal muscles from the crest of the pubis in Man. It is 
distinctly tendinous, and does not extend quite so far as the outer margin of the 
smaller rectus. This layer of muscular fibres begins to expand immediately above 
its origin, and is inserted beneath the greater rectus throughout nearly the whole 
extent of that muscle across the segment [fig. 10.]. When these oblique fasciculi are 
employed alone, they are evidently opposed to the latter, and draw the posterior part 
of each segment backwards and to the median line of the body ; consequently they 
bend the anterior part of the body into the segment of an arch, and when both are in 
action they bend the anterior part of the body backwards. These muscular layers, 
from their form and direction, may be called the first [fig. 6.], second [figg. 7, 8.], 
and third [figg. 9, 10.] oblique. Beneath these there is another set of oblique fibres, 
which is formed of only two broad fasciculi. It originates from the anterior of the 
three ridges of attachment a little towards the inner edge of the smaller rectus [fig. 2.], 
and passing a little diagonally forwards and outwards beneath the great oblique, is 
attached to the third ridge of insertion, and maybe called the fourth oblique [fig. 11.]. 
Beneath the posterior extremity of this set lies the third rectus, which is formed by 
three muscular fasciculi [fig. 12.], rather broader than those which constitute the 
second or smaller rectus, but running in the same direction longitudinally, and having 
the same origin and insertion. On removing the third rectus we arrive at the eighth 
layer of muscular fibres. This arises from the anterior ridge of attachment, and is 
formed by three rather broad fasciculi, which are partially crossed at their origin by 
the third rectus. This layer passes diagonally outwards, and is inserted into the third 
ridge as far as the outer margin of the great rectus and third oblique, which cover it, 
and may be called the fifth oblique [fig. 13.]. When this layer is removed we have 
exposed the triangular and transverse muscles. The triangularis [fig. 14.] consists 
of nine distinct muscular fasciculi, originating in a longitudinal series very near the 
median line of the body, alongside the nervous columns, and extending through the 
posterior half of the segment. The posterior of these fasciculi originates by three, 
and the one immediately preceding it by two, distinct heads or tendons, which, with 
the tendons of the other fasciculi of this set, indigitate with a set of short transverse 
muscular fibres [fig. 15.], ten in number, which occupy the median line beneath the 
nervous columns, and may be called the transversus medius. The fibres which form 
the triangular set pass backwards and outwards, with varying degrees of obliquity, 
and are inserted by strong tendons into the anterior transverse margin, or ridge of 
insertion [fig. 16.]. When this layer acts with its fellow of the opposite side it 
shortens the posterior half of the under surface of the segment ; but when it acts 
singly, or in conjunction with the great or third oblique, it shortens that side of the 
segment, and bends that part of the body to the opposite side. It is a very powerful 
muscle in locomotion, and is probably also of great use in contracting the segment 
during transformation. The transversi abdominales [fig. 17.] are shorthand consist of 
3 z 2 
