420 Mr. Newport on the Formation and Use of the Air-sacs 



considerably enlarged throughout the greater part of their course, their ex- 

 treme ramifications only retaining their original setiform structure and distri- 

 bution. In the truly apterous insects the tracheae are invariably arborescent, 

 and diminish in size from their origin to their extremest point in their perfect 

 as in their larva condition ; and they are invariably smaller in diameter, and 

 have fewer ramifications, in the most inactive species. 



The respiratory organs are always simply tracheal in the larva state of all 

 insects, and it is not until the period of change to the pupa is fast approach- 

 ing that they begin to be enlarged, even in those in which vesicles afterwards 

 are most numerous. The enlargement, as I have elsewhere shown*, com- 

 mences in Lepidopterous insects at about the time when the larva ceases to 

 feed. It is perceptible first in the longitudinal tracheae of the thoracic seg- 

 ments of the Sphinx, immediately before the insect enters the earth ; and by 

 the time that the cell in which it is to undergo its transformation is com- 

 pleted, the tracheae from the second to the fifth spiracles are distinctly en- 

 larged. In the Diurnal species, which do not enter the earth, but undergo 

 their changes in the open air, the dilatation of these tracheae commences while 

 the insects are spinning their silken threads. When this labour is finished, 

 and they have remained for a few hours at rest, the skin is fissured along the 

 dorsal surface of the thoracic segments and thrown off, the change to the pupa 

 is effected, and the longitudinal tracheae in the fifth and sixth segments are 

 dilated into vesicles, which continue to be enlarged during the first few days 

 after the change. The tracheae of the third and fourth segments each give off 

 a small trunk on their external surface, which is divided into two branches, 

 and is involved in a fold of the new tegument that is formed beneath the old 

 skin of the larva some days before its change. The fold of tegument on each 

 side of the third and fourth segments is supplied with ramifications of tracheae 

 from these minute trunks, and very closely resembles in appearance the ex- 

 ternal abdominal branchiae of the aquatic larvae of Nenroptera. It is these 

 folds which become the most important organs in the perfect state of the in- 

 sect, its wings. When the old skin of the larva is fissured, and the thoracic 

 segments become shortened, as the skin is thrown off, previous to their form- 

 ing one region, the thorax, the tracheae in these folds are rapidly enlarged 



* Phil. Trans. 1836, part ii. p. 535. 



