HARDWICKE'S SCIENCE-GOSSIP. 



2C3 



of the three last segments consolidated into this 

 terminal one ? and is this an approach to the state 

 of things which we find in the larva of the Blow-fly, 

 where the whole of the nervous system is, as Mr. 

 Lowne states, collected in the anterior segments? 



I have now to call attention to that portion of 

 the nervous system referred to in the closing sen- 

 tence of my quotation from Burmeister. In man 

 and in the higher animals the viscera are supplied 

 by a separate system of nerves and ganglia, con- 

 nected with, but distinct from, the cerebro-spinal 

 axis. It would appear that this is also the case 

 in insects ; Burmeister refers to it in the following 

 terms :— "The sympathetic system is peculiar to all 

 insects; but in the several orders it takes a different 

 form : we may distinguish in it two main divisions, 

 — a single cord, which runs upon the surface of the 

 oesophagus and stomach, giving off delicate branches 

 on all sides, and where the oesophagus passes 

 through the brain running with the oesophagus 

 beneath the cerebrum ; and a double nervous web, 

 consisting of ganglia, which originates on each side, 

 by one branch from the posterior portion of the 

 cerebrum, running down the oesophagus, and giving 

 off here and there fine auxiliary branches to the 

 single nervous cord." As this is a general descrip- 

 tion, we must not expect it to tally too closely with 

 particular cases, nevertheless, the single cord is 

 well marked, and is seen on the upper surface of 

 the oesophagus, between the muscles which guard 

 the orifice of the dorsal vessel. It commences with 

 a small heart-shaped ganglion, called by Burmeister 

 the frontal ganglion, situated immediately behind 

 the three pairs of muscles already described as 

 joining the oesophagus at this point with the upper 

 internal surface of the head, and which is connected 



J i s 



as before stated, by two pairs of arching nerves 

 with the cerebrum. It ends also in a rather smaller 

 ganglion immediately beneath the cerebrum, from 

 which a few fine filaments appear to connect it 

 with what Burmeister, as above quoted, calls the 

 second division of the sympathetic system, viz. 

 "the double nervous web," though in the present 

 case 1 scarcely recognize in it the character of a 

 web, but that of two broad, flat, nervous expan- 

 sions, originating, as he states, from "the posterior 

 portion of the cerebrum," closely attached to the 

 dorsal vessel, and occupied by very large nerve-cells. 

 These are followed by two smaller- nerves, which 

 first bend a little forward and then again backward, 

 along the course of the oesophagus, till they are lost 

 on the muscular coat of the proventriculus. 



I wish lastly to notice those portions of the 

 organization of the larva from which the head and 

 thorax of the future fly are developed. Mr. Lowne, 

 in speaking of the formation of the pupa of the 

 Blow-fly, says* that "all the tissues of the larva 



* Lowne's "Anatomy of the Blow-fly," p. 116. 



undergo degeneration, and the imaginal tissues are 

 redeveloped from the disintegrated parts of the 

 larva, under conditions similar to those appertaining 

 to the formation of the embryonic tissues from the 

 yolk." This development takes place within a 

 delicate membrane called 'the pupa-skin, which en- 

 velops the whole of the future insect, and in the 

 Blow-fly is inclosed within the last larval integu- 

 ment, which dies and forms around it a hard horny 

 outer case. In the insect before us, however, the 

 corresponding larval integument is shed when the 

 larva reaches maturity, and the fly is formed within 

 the pupa-skin alone, which becomes hardened to 

 withstand the exposure. 



This pupa-skin, Mr. Lowne says, is partly formed 

 from seven pairs of delicate cellular expansions, 

 called by Dr. Weismann imaginal discs, and partly 

 from cells formed upon the inner surface of the 

 larval integument, the head and thorax being formed 

 in the former manner, and the proboscis and ab- 

 domen in the latter. Although Mr. Lowne men- 

 tions only seven pairs of discs, I think I have very 

 distinctly made out eight in the Crane-fly, the addi- 

 tional pair being, I believe, the upper prothoracic 

 discs, about whose existence he does not speak 

 positively. Following his nomenclature, as adopted, 

 from Dr. Weismann, they will be as follows. The 

 two pairs, viz. the antennal and optic, which enter 

 into the formation of the head, and the six pairs 

 which are concerned in forming the thorax and its 

 appendages, viz., the upper and lower prothoracic, 

 mesothoracic, and metathoracic discs. Of these the 

 antennal and optic, and the three lower pairs of 

 thoracic discs, ate certainly connected, as before 

 stated, with the cephalic and thoracic ganglia by 

 means of thick nervous cords. I think that this is 

 the case also intermediately with the upper thoracic 

 discs, through one or more small nervous filaments 

 connecting them with the corresponding lower ones. 

 They are all iflclosed within membranous capsules, 

 which Mr. Lowne seems to regard as a continuation 

 of the neurilemma of the nerves which support 

 them. The antennal and optic discs are situated on 

 each side of the oesophagus, in front of the cere- 

 brum, and appear as flat cellular expansions lying 

 in a vertical plane between the oesophagus and the 

 great muscles of the mandibles. The inferior one 

 is the antennal disc, and, when in an advanced state, 

 shows clearly the indications of the joints as they 

 appear in the pupa ; the optic discs lie above these, 

 and show the commencing areolation of, and 

 deposition of pigment in, the future facets of the 

 eye. The three thoracic discs are situated imme- 

 diately beneath the ventral integument of the tho- 

 racic segments, and, when somewhat advanced, may 

 be distinguished as whitish spots through the skin : 

 they are concerned in the formation of the legs, 

 which may be seen coiled round inside them, the 

 several parts, viz. the coxse, femora, tibia?, and 



