Development and Metamorphosis 43 
development the young have to develop wings and make what other change 
is necessary to reach the adult type, but the life is continually free and active 
and the change is only a simple gradual transformation of the various parts 
in which differences exist. A common locust is an excellent example of 
an insect with such incomplete metamorphosis. Fig. 72 shows the develop¬ 
ing locust at different successive ages, or stages, as these periods are called 
because of their separation from each other by the phenomenon, common 
to all insects, of moulting. As the insect grows it finds its increase of girth 
and length restrained by the firm 
inelastic external chitinized cuticle, 
or exoskeleton. So at fixed periods 
(varying with the various species 
both in number and duration) this 
cuticle is cast or moulted. From 
a median longitudinal rent along 
the dorsum of the thorax and head, 
the insect, soft and dangerously 
helpless, struggles out of the old 
skin, enclosed in a new cuticle 
which, however, requires some little 
time to harden and assume its 
proper colors (often protective). 
After each moulting the young 
locust appears markedly larger and 
with its wing-pads better developed 
(Fig. 73). But not until the final 
moulting—in the case of the locust 
this is the fifth—are the wings usable as organs of flight. So that there 
is after all likely to be a rather marked difference between the habits of 
the young and those of the adult of an insect with incomplete metamor¬ 
phosis, that difference being primarily due to structural differences. The 
young are confined to the ground, and their locomotion is limited to walking 
or hopping. The adults can live, if they like, a life in the air, and they 
have a means of locomotion of greatly extended capability. 
The insects with complete metamorphosis are the beetles, the two¬ 
winged flies, the butterflies and moths, the ichneumons, gall-flies, ants, 
bees, and wasps, the fleas, the ant-lions, and several other small groups 
of insects with less familiar names. In the case of all the thousands of 
species in these groups, the young when hatched from the egg differ very 
much in structure and appearance, and also in habits and general economy, 
from the parents. Familiar examples of such young are the caterpillars 
and “worms” of the moths and butterflies, the grubs of beetles, the mag- 
Fig. 74.—Metamorphosis, incomplete, of an 
assassin-bug (family Reduviidae, order 
Hemiptera). A, young just hatching from 
eggs; B, young after first moulting, showing 
beginning wing-pads; C, older stage with 
complex wing-pads; D, adult with fully 
developed wings. (One-half larger than 
natural size.) 
