116 THE PERTODTCAL ( UADA. 
without thoir being reported, unless some oVjserver chances on a pupal 
shell or notes the exit holes in the ground about the trees; hence 
the slight value of a negative report as opposed to a positive one. 
HISTORY OF THE LARVAL AND PUPAL STAGES. 
A careful study of the material collected in the course of the experi- 
ments described in the last section demonstrates the interesting fact 
that this species, in spite of its very long period of growth, presents 
the same number of adolescent stages as is found in insects which go 
tlu'ough their entire development within a single year or even of the 
more rapidly multiplexing species, wliich have many annual genera- 
tions. But six distinct stages are found, four of which belong to the 
larval condition and two to the pupal. In other words, the larval 
and pupal changes in the periodical Cicada are normal and are not 
increased by its long preparatory existence. 
It has been inferred hitherto, and notably by Professor Riley, that 
owing to the continual use of the claws in burrowing, this species found 
it necessary to shed its skin and undergo a molting once or twice a 
3^ear, and instead of the normal number of changes or molts there 
were probably from twenty-five to thirty. An examination of types 
of the different larval stages which Professor Riley had provisionally 
separated demonstrates that the differences on which these supposed 
stages were based are either individual and exceptional or due to the 
difference of age within the same stage, and that as far as structure 
and size of the hard parts of the larva and pupa are concerned the 
normal number of stages only is represented in this species. 
For the separation of these different stages, of growth useful char- 
acters are found in the size and structure of the legs, and especially of 
the anterior pair, in the antennae, and in the development of the wing 
sheaths. It is the rule with insects that wdth each molt there is a 
decided increase in the size of the head and hard parts generally, and 
with the periodical Cicada especially it is also very doubtful if there 
is ever a molt without a decided change of the sort indicated. Its life 
beneath the ground in its moist cell over a rootlet is a vers^ quiet one and 
free from an}^ of the wearing action of rain -or the drying of the outer 
air, so that the need of a molting or change of skin would apparently be 
much less than that in an exposed or much more active insect. It prob- 
ably also very rarely has occasion to burrow^ to an}^ considerable extent 
and probably often remains for ^^ears in the sanie,£ell, which it enlarges 
from time to time without change of location. For these reasons the 
^^Titer is inclined to believe that mol tings onl}^ occur when change of 
form becomes necessary b}^ the increased size of the insect, and this 
seems to be borne out by definite structural peculiarities, wliich easih" 
permit us to recognize the different stages or determine the age of any 
larva within a year or two. The larva of a particular molt or stage 
