964 



INSECTA. 



branch from the transverse nerves posterior to 

 the third ganglion. A plexus of these trans- 

 verse nerves exists, as in the Sphinx, anterior to 

 each ganglion (o, o), to the nerves of which 

 they give a single filament near their base, and 

 another when arrived near the spiracle, while 

 their main branches, as in the latter insect, are 

 distributed separately among the tracheae and 

 muscles. Those branches of transverse nerves 

 which pass off laterally from between the 

 fourth, fifth, sixth, and seventh ganglia, become 

 approximated to the nerves from those ganglia, 

 and in the development of the insect at this 

 period afford an example of the commence- 

 ment of the interesting fact of the formation of 

 nervous trunks by the approximation of many 

 fibres. The transverse nerves anterior to the fifth 

 ganglion (5, o) are those that first become united 

 to the moto-sensitive nerves from the gangliated 

 cord, and at this period of the transformation 

 the two are beginning to become united. 



Seven hours after the insect has become 

 a pupa there is a greater enlargement of the 

 cerebral ganglia, optic nerves, and ganglia and 

 cords of the future thoracic segments. The 

 fourth and fifth have advanced closer together, 

 and the cord between them has become so 

 much shortened, enlarged in diameter, and ap- 

 proximated to its fellow, as to resemble in 

 shape a separate elongated ganglion, and 

 strongly to support the opinion formerly ad- 

 vanced by us* of the actual transmission for- 

 wards of the nervous matter within the in- 

 vesting membranes of the cord, rather han 

 that of its deposition and accumulation at 

 certain parts through the agency of the nutri- 

 tive or vascular system. At this period also all 

 the remaining ganglia have become slightly 

 enlarged, and the distance between the fifth and 

 sixth is much diminished, and the cords just 

 anterior to each ganglion are also slightly en- 

 larged and are disposed with less irregularity 

 than at a previous period. At this stage of the 

 transformation the transverse nerves (oo) also 

 are beginning to assume their temporary gan- 

 gl ionic appearance, and the terminal nerves 

 from the caudal ganglion are enlarged to sup- 

 ply the developing organs of generation. 



At twelve hours (5) the fifth ganglion has 

 almost completely coalesced with the cord and 

 fourth, and has assumed an elongated trian- 

 gular appearance, and the transverse nerves, 

 which at seven hours were beginning to be 

 united to the nerves from this ganglion, have 

 now so completely joined them as almost en- 

 tirely to have disappeared, there being in 

 some instances only a triangular elevation upon 

 the gangliated cord, with a portion of nerve 

 passing outwards to indicate their previous 

 separation, thus affording a further proof of 

 the adhesion of contiguous parts, and of the 

 manner in which nervous trunks are formed. 



At eighteen hours (6) the whole of the gan- 

 glia, cords, and nerves have become more en- 

 larged, particularly those of the wings, and the 

 transverse nerves, although continuing separate, 

 give filaments to the nerves from the gamglia, 

 and themselves exhibit at their point of di- 

 vision more the appearance of ganglia; while 

 the fourth and liiili <j;:ur>viu f the cords have 



now so completely united as to appear like an 

 irregular elongated mass. The abdominal por- 

 tion of the cord is now extended in a more 

 direct line in the body, and anterior to each 

 ganglion is still enlarged. 



Fig. 422. 

 7 8 



Thirty-six hours. 



Twenty-four hours. 



At twenty-four hours (Jig 422, 7) the fourth 

 and fifth ganglia are completely united, the 

 fifth being larger than the fourth. The cords 

 before the sixth are enlarged, as also are the 

 transverse nerves of the thorax, which seem to 

 keep pace with or rather to precede the deve- 

 lopment of the respiratory organs. 



At thirty-six hours (8) the optic nerves have 

 attained a size almost equal to that of the 

 cerebral ganglia, and after this period become 

 very little larger. During the preceding stages 

 of the transformation .the minute black patch 

 observed at their base has been gradually more 

 and more expanded, and carried forwards from 

 the posterior superior part of each lobe to its 

 lateral part, which at this period it is begin- 

 ning to cover, while the optic nerves appear as 

 if developing from within outwards, and have 

 a somewhat pear-shaped form. The first sub- 

 cesophageal ganglion, or medulla,now forms with 

 the cerebral ones a complete ring around the 

 oesophagus, the crura having almost disap- 

 peared. The fifth ganglion has decreased in 

 size and is now smaller than the fourth, while, 

 in some specimens, the nerves that arise now 

 come from the cords immediately behind it, 

 thus giving a further proof that the nervous 

 substance of the ganglion has been transmitted 

 forwards. The sixth ganglion, which at 

 twenty-four hours was much reduced in size, 

 has entirely disappeared, but the nerves that 

 belonged to it remain and are now derived 

 from the cord, very near to those which be- 

 longed to the fifth ganglion, thus further 

 proving, as formerly remarked by us,* that the 

 nervous substance has been transmitted for- 

 wards along the cord. This view of the 

 manner in which these changes in the form and 



* Phil. Trans. 1834. 



