INSECTA. 



Fig. 400. 



937 



Diagram of the muscles and nerves of the ventral surface of the segments in the larva 

 of Sphin* ligustri. ( Newport, Phil. Trans.) 



beneath the upper one. The second layer con- 

 sists of seven distinct fasciculi, which extend 

 from the anterior margin of the segment, close 

 to the smaller rectus, and beneath the larger, 

 about half their breadth across the segment. 

 They run backwards and outwards in a dia- 

 gonal direction, and are attached to the middle 

 ridge below the rectus as far across the seg- 

 ment as the outer margin of that muscle (8). 

 These layers of muscles when in action draw 

 the outer part of the anterior margin of the 

 following segment diagonally forwards in the 

 direction of the median line of the body, 

 and, consequently, when these muscles in se- 

 veral segments on one side of the body act 

 ogether, they bring forward the posterior por- 

 ion of the body of the same side, and bend 



it in a semicircular direction. When these 

 layers on both sides of a segment act together, 

 they draw forwards the posterior part of the 

 body in a straight line. The most internal 

 of these layers (6) which lies close to the rectus 

 may be called the first oblique, and the other 

 the second oblique (7). Beneath this there is 

 another diagonal layer of fibres which originates 

 close to the median line of the body (9), be- 

 neath the nervous cord, almost in a line with 

 the insertion of the smaller rectus. The origin 

 of this set is exceedingly narrow and distinctly 

 tendinous, and the fibres diverging pass di- 

 agonally upwards and outwards, forming a flat 

 triangular muscle, the third oblique (9, 10), 

 which is attached to the posterior ridge, and 

 extends outwards to the margin of the greater 



