938 



INSECTA. 



rectus. These oblique muscles are the anta- 

 gonists of the recti, and when acting alone 

 draw the posterior part of each segment back- 

 wards and to the median line ; consequently, 

 when the layers of only one side are in action, 

 the anterior part of the body is flexed laterally 

 in the form of a curve, but when those on both 

 sides are in action the anterior part of the body 

 is carried directly backwards. Beneath these 

 oblique muscles there is another, which is 

 formed of only two broad fibres. It arises 

 from the anterior of the three ridges of attach- 

 ment in the median line, and passing diago- 

 nally forwards and outwards parallel to the third 

 oblique, beneath its inner margin, is attached 

 to the third ridge of insertion. This may be 

 called the fourth oblique (11). Beneath the 

 posterior extremity of this muscle lies the 

 third rectus (12), which is formed of three 

 fibres, somewhat broader than those of the 

 second or smaller rectus, but running longitu- 

 dinally in exactly the same direction, and hav- 

 ing the same origin and insertion. On re- 

 moving the third rectus we expose the eighth 

 layer of muscular fibres. This arises from the 

 anterior ridge, and is formed of three broad 

 fibres which are partially crossed at their origin 

 by the third rectus. It passes diagonally out- 

 wards, and is attached to the third ridge, extend- 

 ing as far as the margin of the rectus and third 

 oblique, by which it is concealed. This is the 

 fifth oblique ( 1 3). When this layer is removed, 

 the triangular and transverse muscles are exposed. 

 The triangularis (14) is composed of nine dis- 

 tinct fibres, which originate in a longitudinal 

 series that extends about half-way across the 

 segment. The fibres pass from their origin 

 diagonally backwards and outwards, with vary- 

 ing degrees of obliquity, and are inserted by 

 strong tendons into the anterior of the three 

 transverse ridges (16). They arise also by 

 distinct tendons, which indigitate with a set 

 of short transverse fibres, ten in number, and 

 which occupy the median line beneath the ner- 

 vous cord, and form the transversus medius(\5). 

 This muscle contracts the diameter of the mid- 

 dle of the under surface of a segment. The 

 triangularis, when acting with its fellow of the 

 opposite side, shortens the posterior half of 

 the ventral surface of the segment ; but when 

 acting singly, or in conjunction with the third 

 oblique, shortens that side of the segment, and 

 assists to bend the body laterally. It is a very 

 powerful muscle in locomotion, and probably 

 is of great use in shortening and contracting 

 the segments during the transformations. The 

 transversi abdominales (17) are six short broad 

 and thick fibres, that form two sets, and 

 originate at some distance from the median 

 line, posteriorly to and on the outer side of 

 the tendons of the third oblique, and passing 

 transversely outwards are inserted directly into 

 the tegument, about half-way across the segment. 

 Like the transversus medius they contract the 

 diameter of the ventral surface of the segment, 

 and bring the sides towards the median line. 

 Anteriorly to these muscles, but further from 

 the median line, is another set of six short 

 fibres, the abdominales anteriores (18), which 



arise at some distance from the median line, 

 and passing transversely outwards are inserted 

 into the lateral part of the segment. The 

 abdominales laterales (19) are situated in the 

 posterior half of the segment. They are in- 

 serted by three great fasciculi of narrow ten- 

 dons into the inner and inferior part of the 

 segment, and consist of eight muscular fibres 

 connected in the first tendon, four in the se- 

 cond, and seven in the third. They form very 

 powerful muscles, which interlace with each 

 other, and originate directly from the tegu- 

 ment of the sides of the segment, at different 

 distances posteriorly to the spiracle. Some of 

 them (20) are much longer than others, and 

 the whole of them are connected with the false 

 feet of the abdomen. On removing these 

 muscles we expose the attachment of the ob- 

 liquus posterior (21), which is composed of 

 nine small fibres that pass diagonally outwards 

 from their origin, the anterior ridge, to their 

 insertion in the tegument at different distances 

 beneath the lateral abdominal muscles. Ano- 

 ther set of nine distinct fibres, the postero- 

 laterales obliqui (22), which originate from the 

 same ridge at the lateral part of the segment, 

 cross over the last lateral abdominal muscle, 

 and are inserted between it and the one im- 

 mediately before it. Besides these layers of 

 fibres there are four other sets which seem to 

 be particularly concerned in the function of 

 respiration. Of these the transversus lateralis 

 (28) arises tendinous from beneath the lateral 

 part of the great rectus, and passing upwards, 

 internal to the great longitudinal trachea (), 

 which it crosses, is inserted beneath the ex- 

 ternal margin of the dorsal rectus (A). The 

 second transversus lateralis (24) arises pos- 

 teriorly to the first, crosses the trachea, and 

 continuing its course upwards is inserted into 

 the tegument of the back, about half-way 

 across the dorsal rectus. These muscles ap- 

 pear to be directly concerned in contracting 

 the segments during expiration. Besides these 

 muscles there are also the retractor spiraculi 

 and the retractor valvulte, which belong also to 

 the ventral and lateral surface of each segment. 

 The retractor spiraculi (25) is attached by a 

 long tendon (26) to the third ridge of insertion. 

 It is a long, fleshy, and somewhat conical mus- 

 cle, which passes upwards and obliquely back- 

 wards to the spiracle, unto the lower margin 

 of which it is attached, and is blended with 

 the circular fibres that constitute an orbicular 

 muscle to that orifice. It appears to be di- 

 rectly concerned in forcible expiration, and 

 draws the spiracle inwards and downwards, 

 and when the orbicular muscle acts in con- 

 junction with it assists in closing the spiracle. 

 The remaining muscle, the retractor valvul<s 

 (27), is the direct antagonist of the last. It 

 is composed of five distinct fibres (jig. 401, c), 

 which arise from the posterior margin of the 

 spiracle, and partly also from the attachment 

 of the retractor spiraculi (e\ and then, con- 

 verging, end in a tendon that passes diagonally 

 upwards and backwards, and is inserted into 

 a little elevation in the tegument. It is the 

 proper retractor or levator muscle of the spira- 



