CTA. 



967 



itself vertically to mndergo Us change, the di- 

 latation commences while the insect is spinning 

 the silkeu hangings from which it suspends 

 itself; so that the changes commence at a 

 corresponding period in both insects. It is in 

 the butterfly that we hare most closely watched 

 the development of the vesicles. During the 

 period that the insect remains suspended it 

 makes several powerful respiratory efforts, 

 accompanied by much muscular exertion, and 

 these efforts are continued at intervals until the 

 old skin is fissured and thrown off. It is at 

 this period that the tracheae become much en- 

 larged, as we have found at about two hours 

 after the insect has suspended itself. Meckel 

 observed the sacs soon after the insect has 

 entered the pupa state, but it will thus be seen 

 that the expansion of the tracheae in the for- 

 mation of these sacs commences very much 

 earlier. At about half an haxr before the in- 

 sect becomes a pupa we have found the whole 

 of the tracheae 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 tracheae 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 tracheae, to the 

 entire closing of the spiracles, and expansion 

 of the air contained within them, which he 

 thinks is increased in quantity during the deve- 

 lopment of the insect. But from the circum- 

 stance 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 trachea?, 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 trachea! tubes, which then become 

 distended by the natural elasticity of the air 

 contained within them ; and further, that the 

 subsequent enlargement of these tracheae into 

 distinct bags is occasioned, not by an in- 

 creased quantity of air in the vesicles, as 

 Cams imagines, but simply by a continuance 

 of the same cause that effects the first dila- 



* Introduction to Comparative Anatomy, 

 lated by Gore, 1827, vol. u. p. 167. 



tation of the trachea?, the elasticity of the 

 contained air, since the dilatation appears to 

 keep pace with the gradually decreasing site 

 of the digestive organs, and the spiracles are 

 not permanently closed during the pupa state, 

 respiration being continued at intervals, ex- 

 cepting perhaps in the most complete state of 

 hyberuation. In accordance with this opinion 

 we find that, at about halj am hour after the 

 change, the pro-thoracic tracheae that ramified 

 over the oesophagus are enlarged to double 

 their original diameter, and have begun to be 

 detached from that organ. At seven boms 

 these changes have been carried much farther. 

 At twelve hours they are still further enlarged, 

 and the principal 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 

 tracheae of the head and thorax are still further 

 enlarged, and those from the third spiracle are 

 detached from the cardiac extremity of the 

 stomach, and are more enlarged than the others, 

 and those from the ninth spiracle, in the twelfth 

 segment, which supply the colon, are begin- 

 ning to be distinctly vesicular. At thirty-sir 

 hours not only have the longitudinal tracheae 

 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 papa state, during 

 a great part of which time die changes are 

 almost or entirely suspended. The butterfly 

 remains but a few 

 all the chj 



days, and in consequence 

 eed with rapidity, which is 



either greater or less in proportion to the 

 of the year and temperature of the atmosphere. 

 function of 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 tne^aatr- 

 rum, which properly belong to a distinct sub- 

 ject. (See RESPIRATION.) 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 consist of 

 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 

 tebrata. The number and frequency of these 



