the metamorphoses of the respiratory system. 53 
known ; but the indisposition of their allies, the grasshoppers, to 
fly for any distance, is readily to be noticed by any one walking 
in the fields. They will jump and fly for a few yards, but it is 
very unusual to see them doing anything more. How, then, is 
it that these heavy and not over-active insects can occasionally 
maintain a prolonged and elevated flight ? Their respiratory 
system explains the apparent anomaly. When they are quiet 
their tracheae are as flat as ribbons, and contain but little air. 
On departing for their long voyage, however, the locusts, like the 
cockchafers, blow themselves full of air, and their bodies become 
very light in relation to the bulk of air they replace. More- 
over, the muscular efforts of prolonged flight produce a certain 
amount of animal heat, and this rarefies the air contained in 
the vesicles and tracheae ; so that the principles of Montgolfier’s 
balloon were foreshadowed in the insect long before they were 
elaborated by man. 
The vesicles are evidently superadded to the tracheae of the 
larva and caterpillar, and their growth during metamorphosis is 
accompanied by a prolongation of the sympathetic nerve over 
them. 
There are two kinds of transformations noticed in the insects 
or the Articulata , the complete and the incomplete, and, besides, 
there are hyper and retrograde metamorphoses. The stages or 
phases of development are usually three in number, but there are 
four in some instances, and a great many more when skin-shedding 
is accompanied by contemporaneous internal structural changes. 
Some Articidata do not undergo any metamorphosis. 
The vast majority of the articulate animals bring forth eggs 
which contain the embryo ; this becomes, after hatching, the larva, 
caterpillar, grub, or maggot; and either the larva differs consider- 
ably from the embryo, or it is so much like it as to merit the 
term of hatched embryo. But a few of the Articulata produce 
living larvae, the process of hatching going on within the body of 
the mother, which is then said to be ovoviparous ; or the young 
are developed within the parent more as offshoots of parts of her 
tissues than as the contents of eggs. The union of the males and 
females before every production of eggs is found to occur in the 
great majority of instances, but there are some insects which 
