THE CANADIAN ENTOMOLOGIST 281 



bifurcates to innervate various setae on the laterodorsal wall of the head. The 

 second, the lateral Nerve, arises from the outer end of the first ganglion and 

 passes laterad and then cephalad , in close connection with a tracheal branch, 

 bending ventrad and innervating certain muscles at theirattachment to the ventral 

 wall. In argenteomacidatiis larv?e, PI. XI, Fig. 3, the arrangement of this group is 

 closely similar to that just described, but there are, as in many other parts 

 of the nervous system, constant differences. An instance of this occurs in con- 

 nection with the third ganglion, which has there a direct connection with the 

 caudal part of the brain. 



A tracheole crosses the inner face of the 3rd lateral ganglion in thule and 

 extends into minute tracheoles which mainly supply the lateral fan-like muscles 

 of the oesophagus, innervated by the 2nd ganglion. 



Crossing this tracheole, and extending from the caudal part of the inner 

 face of the 3rd ganglion, is a minute rather elongate ganglion giving threads 

 to the tracheoles entering the brain, PI. X, Fig. 2. 



The Paired Median Section. 



This consists of the so-called Median Nerves of the central system, with 



their branches, and there seems to be little reason for treating it as separate from 



the ventral chaiui The Median Nerve appears in front of each ganglion of the 



ventral chain as a v^ery short, subtriangular nerve arising from the connective 

 in the median split immediately in front of the ganglion. In the abdomen 

 each median nerve divides into two transverse nerves which proceed outward 

 at right angles to the vetral chain. Their distribution is shown in PI. XI, Figs. 2, 4, 

 5 and 6. A cephalic branch connects with a branch of the posterior nerve of the 

 preceding segment ; a basal thread goes to the integument, and the distal portion 

 of the nerve innervates the spiracular muscles in addition to giving one or more 

 connections to the anterior nerve of its segment. The median nerve of the 

 eighth ganglion, which should appear in front of the ninth if that ganglion were 

 separate, has apparently disappeared. 



The median nerves in front of the second and third thoracic ganglia give 

 ofif each two pairs of transverse nerves. The second or posterior pair are really 

 oblique in position and pass latero-caudad, eventually uniting with the median 

 nerve of the succeeding segment. It has already been suggested that the poster- 

 ior nerves of the abdominal ganglia represent these posterior branches of the 

 thoracic median nerves, which have become fused with the ganglia and lost 

 their original connections. The median nerve in front of the first thoracic 

 ganglion is apparently not always present, but has been demonstrated on some 

 specimens of both thule and argenteomaculatus. It is simple with very few fine 

 branches and passes cephalad and ventrad to muscles at the base of the mandi- 

 bles. It is possible that the minute branch, F"ig. 2, "sy", represents the caudal 

 branch of the mediau nerxc which has degenerated and become fused with the 

 base of the superficial nerve. 



