50 TRANSFORMATIONS OF INSECTS. 



back, SO that it can breathe by coming up to the surface, and 

 allowing the wing-cases to appear above water. Other insects in 

 the larva state support themselves at the surface of the water by 

 a coronet of hairs, arranged in a star-shaped manner, which 

 includes a bubble of air ; they descend with it for the purposes of 

 respiration, and return for a fresh supply. During their meta- 

 morphosis, the ordinary spiracles of flying insects are added to 

 their organisation, and that which was so Avell adapted for the 

 larval condition can no longer be distinguished. The larva of 

 the gnat breathes through a tube in its tail, and the nymph by 

 means of two tubes on its back, but the perfect insect has the 

 usual spiracles. 



A most interesting alteration takes place in the tracheae of 

 those insects which fly, or are great jumpers in the adult state. 

 The tubular nature of the respiratory apparatus in the caterpillar 

 has already been noticed, and it is evident from what has been 

 stated that there is a definite relation between the habits of the 

 larvae, generally speaking, and the form and arrangement of the 

 tracheae, within and without the body. It might be anticipated 

 that the arrangement of the tracheae of the slow-moving cater- 

 pillar, or of the sluggish grub, would hardly suit the purpose of 

 the restless moth or the vigorous beetle ; and that Nature, which 

 has specially adapted the varieties of the respiratory structures 

 in the larvre of the aquatic insects to meet the exigencies of 

 their position, would do as much for the flying kinds. During the 

 metamorphosis of the larva into the chrysalis, and just previously, 

 the future requirements of the adult insect are recognised, for 

 the trachea:^ begin to suffer some curious modifications before 

 the pupa state commences. 



When the metamorphosis of the pupa or chrysalis into the 

 beetle or butterfly, as the case may be, is complete, the tracheae 

 are no longer perfectly tubular structures, for many of them are 

 enlarged and dilated into sacs or vesicles, which can be filled with 

 air at the will of the insect. Their use is to enable the insect to 

 alter its specific gravity by enlarging its bulk, the weight remain- 

 ing the same, and thus rendering it better able to support itself 

 on the wing with as little muscular eftbrt as is possible. The 

 development of the vesicles begins to take place at about the 



