INSECTA. 



Ml 



itself vertically to undergo lU change, the di- 

 latation colllllielK I's while tin- Ill-ret is -[.Hilling 



the silken hangings t'niin wliirli it sus|>t:iids 

 itself; so tliut tin 1 changes commence at .1 

 OOmfpOnding period ill both insects, ll is in 

 the butterfly that we have most closely watched 

 the development of the vesicles. During tin 1 

 period that the insect remains suspended it 

 nukrs several powerful respiratory efforts, 

 accompanied !>y much muscular exertion, and 

 these elliirts are continued at intervals until the 

 old skin is fissured arid thrown off. It is at 

 this period that the trachea? become much en- 

 larged, as we have found at about two knurs 

 alter the insect has suspended itself. Meckel 

 dl>-erved the sacs soon aAer the insect has 

 entered the pupa state, but it will thus be seen 

 that the exclusion of the trachea in the for- 

 mation of these sacs commences very much 

 earlier. At about half an hour before the in- 

 sect becomes a pupa we have found the whole 

 of the trachea; more distended, particularly 

 those on the under surface of the thorax, from 

 which branches are given to the legs, so that 

 the elongation of these trachea: is probably 

 connected with the subsequent rapid develop- 

 ment and extension of those organs. At this 

 period the trachea: of the abdomen have ex- 

 perienced but little alteration. It is at the 

 actual period of transformation that all the 

 changes take place most rapidly. At that time 

 the laborious respiratory efforts made by the 

 insect appear greatly to affect the condition of 

 all the organs. When the skin is thrown off, 

 these efforts cease for a few minutes, after 

 which the abdominal segments become short- 

 ened, and the circulatory fluid is propelled 

 forwards, and the wings, then scarcely so large 

 as hemp-seeds, are gradually distended at their 

 base, and at each respiration are perceptibly 

 enlarged, and carried downwards over the under 

 surface of the thorax and first abdominal seg- 

 ments. Carus* attributes the development of 

 the sacs, and dilatation of the trachrse, to the 

 entire closing of the spiracles, and expansion 

 of the air contained within them, which he 

 thinks is incn nscil in ijiianliti/ during the deve- 

 lopment of the insect. But from the circum- 

 stanee that all the tracheae are enlarged imme- 

 diately after the insect has entered the pupa 

 state, it seems probable that this enlargement 

 is occasioned simply by the closing of the 

 spiracles, and the expansion of the air within 

 the tradie.r, during the powerful respiratory 

 efforts, aided by the receding of the circulatory 

 fluid from the abdomen into the partially de- 

 veloped wings, suddenly removing pressure 

 from the tracheal tubes, which then become 

 distended by the natural elasticity of the air 

 contained within them; and further, that the 

 subsequent enlargement of these trach< 

 distinct hags is occasioned, not by an in- 

 I quantity of air in the vesicles, as 

 Cams imagines, but simply by a contimianee 

 of the same cause that effects the first dila- 



Introduction to Comparative Anatomy, trans- 

 lated by (..in , 1827, vol. ii. p. 167. 



tation of the trachea:, the elasticity of the 

 contuiiK.il air, since the dilatation appears to 



. e with tlie gradually decreasm 

 of the digestive organs, and the spiracl. 



iiiianently closed during the pupa state, 

 respiration being continued at intervals, ex- 

 cepting perhaps in the most complete M 

 hybernation. In accordance with this opinion 

 we find that, at about halt 'i // .if 

 change, the pro-thoracic trachea: that lannlied 

 over the oesophagus are enlarged to double 

 their original diameter, and have begun to be 



d from that organ. At seven hours 

 these changes have been carried much farther. 

 At twelve hours they are still further enlarged, 

 and the princi|xd alteration observed is the 

 diagonal direction of those from the seventh 

 spiracles, which supply the posterior extremity 

 of the digestive stomach, owing to that organ 

 having now become shorter, previously to its 

 subsequent change. At eighteen hours all the 

 trachea: of the head and thorax are still further 



d, and those from the third spiracle are 

 detached from the cardiac extremity of the 

 stomach, and are mote enlarged than the otheis, 

 and those from the ninth spiracle, in the twelfth 

 segment, which supply the colon, are begin- 

 ning to be distinctly vesicular. At thirty-tit 

 hours not only have the longitudinal trachea: 

 and their many branches become dilated, but 

 those distributed to the different viscera have 

 also become vesicular. At forty-eight hours 

 the development of these parts is so far ad- 

 vanced that the whole have assumed the vesi- 

 cular form, and those at the anterior part of the 

 abdomen occupy a great proportion of that 

 region, and the dilatation of others proceeds 

 until within a few days before the perfect insect 

 is developed, before it is completed. The 

 only difference we have observed between the 

 development of these organs in the Sphinx 

 and the butterfly is in the rapidity with which 

 the changes are effected. The Sphinx re- 

 mains many months in the pupa state, dining 

 a great part of which time the chan-ji 

 almost or entirely suspended. The butterfly 

 remains but a few days, and in consequence 

 all the changes proceed with rapidity, which is 

 either greater or less in proportion to the season 

 of the year and temperature of the atmosphere. 

 function iif respiration. Having dwelt so 

 long upon the structure of the parts concerned 

 in respiration we cannot venture at any length 

 upon the phenomena connected with the /'unc- 

 tion, which properly belong to a distinct sub- 

 ject. (See KESPI RATION.) We would remark, 

 however, that the circumstances connected 

 with it are in many respects particularly in- 

 teresting, while the results are similar to those 

 of the respiration in other air-breathing ani- 

 mals. Thus the acts of respiration con 

 alternate dilatations and contractions of the ab- 

 dominal segments, the air entering the body 

 chiefly at the thoracic spiracles, and partly also 

 at the abdominal, during which the dorsal and 

 ventral arches of the abdomen are alternately 

 elevated and depressed, like the ribs of V er- 

 tebrata. The number aud frequency of these 



