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 well 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 trachee 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 
larve, generally speaking, and the form and arrangement of the 
trachez, within and without the body. It might be anticipated 
that the arrangement of the tracheze 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 larve 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 trachee 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 trachez 
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 effort as is possible. The 
development of the vesicles begins to take place at about the 
