538 
MR. NEWPORT ON THE RESPIRATION OF INSECTS. 
sixth, which is in reality the commencement of the true abdomen, that they can pro- 
perly be considered as recti muscles, since it is at this part where they begin to be 
most developed. While passing through the thoracic region they are thinner, narrower, 
and somewhat differently arranged. They are the most powerful of all the muscles of 
the abdomen, and are those which are most concerned in contracting and effecting the 
duplicature of the external teguments during the changes of the insect, and mainly 
assist in locomotion during the larva state. There are four sets of these muscles in the 
abdomen [Plate XXXVII. figg. 1. 1.], two on the dorsal and two on the ventral surface 
on each side of the nervous column and dorsal vessel. These muscles on the ventral 
surface are again divided into four sets, two on each side of the nervous column 
[figg. 1, 2.], and between which there is a slight interspace. The set which is situated 
on each side nearest the nervous column consists only of three narrow fasciculi of 
fibres, and may thence be called recti minor es [fig. 2.] ; while the other set, situated 
more externally in the segment, is broad and powerful, and consists of from twenty 
to twenty-five distinct fasciculi, and may thence be named recti majores [fig. 1.]. The 
attachments of these are different from those of the smaller recti. The larger recti 
are inserted into the middle ridge of attachment, which forms the margin of each 
segment [figg. 3, 3.], close to the insertion of the muscles of the preceding segment; 
while the smaller are inserted at about one fifth of a segment further back [fig. 4.] 
towards the tail, or extremity of the body, and pass over the ridge of attachment for 
the larger recti to their insertion in a corresponding part of the next segment. There 
is a small muscle which originates from the same line of attachment as the greater 
rectus, between it and the smaller, which goes to the stomach and attaches that 
viscus to the exterior tegument of the body, and may thence be called retractor ven- 
triculi [fig. 5.]. There is one of these on each side the nervous column, from the 
fourth to the eleventh segment. On removing the recti we expose two layers of very 
thin fine muscles. The upper layer consists of nine distinct fasciculi of fibres, which 
pass backwards and outwards in a slightly diagonal direction [fig. 6.], but less so 
than those of the second layer [fig. 7-], which lies immediately beneath it. This con- 
sists of seven distinct fasciculi, which originate from the anterior margin of the seg- 
ment close to the smaller recti and beneath the larger, and extend about half the 
breadth of the latter across the segment. They run backwards and outwards in a 
diagonal direction, and are attached below the recti as far as their outer margin 
[fig. 8.]. These layers of fibres, when inaction, draw the outer anterior margin 
of the next segment towards the middle line of the body, and consequently when 
those of several segments on one side of the body act together they bring forwards 
the posterior portion of the body of the same side, and bend it in a semicircular di- 
rection. If these muscular layers on both sides of the body act together they draw 
forwards the posterior part of the body in a straight line. Immediately beneath these 
there is another diagonal layer of fibres, which originates close to the median line of 
the body [fig. 9.] beneath the nervous column, and a little anterior to the insertion of 
