162 INVERTEBRATE ANIMALS. 
which collectively constitute what is called the “ metamor- 
phosis.” In some insects, however, there is no proper meta- 
morphosis, and in some the changes which take place are not 
so complete and striking as in others. By the absence of meta- 
morphosis, or by its completeness when present, insects are 
divided into three convenient, though perhaps not strictly 
natural, sections, as follows :— 
Section I. Ametabolic Insects.—The insects belonging to this 
section are said to be “ Ametabolic” (Gr. a, without ; me¢adole, 
change), because they pass through no metamorphosis. The 
young, on their escape from the egg, resemble the adult in every 
respect except in size, and they undergo no alteration in reach- 
ing maturity, except that they grow larger. All the insects of 
this section are destitute of wings in the adult state, and they 
are therefore often called “ Aptera” (Gr. a, without; Azeron, a 
wing). 
Section LI. Hemimetabolic Insects.—In the insects belonging 
to this section (Gr, #emz, half ; and metadole, change), there is a 
metamorphosis consisting of three stages, but these stages do 
not differ much from one another in appearance. The young, 
on escaping from the egg, is known as the “ larva,” and it is not 
only much smaller than the adult, but is destitute of wings, 
After several changes of skin, the larva enters into the second 
stage, when it is termed the “pupa.” The pupa is active and 
locomotive, and rarely differs much from the larva, except that 
it is bigger, and rudimentary wings have now appeared on the 
back of the thorax. After a certain period, and after some 
changes of skin, the wings burst from their sheaths, and the 
pupa is now converted into the third and final stage, when it 
is known as the “imago” or perfect insect. In all the insects 
belonging to this section—such as grasshoppers, dragon-flies, 
&c.—the second stage or pupa is active and locomotive ; and 
for this reason the metamorphosis is said to be “ incomplete.” 
Section ITI, Holometabolic Insects (Gr. holos, entire; and 
metabole, change).—The insects belonging to this section—such 
as butterflies, moths, and beetles—pass through three stages, 
just as do the preceding, but these stages differ from one another 
very much in appearance, and the metamorphosis is therefore 
