No. 395-] THE WINGS OF INSECTS. 853 
till after a molt, their mouths being closed at first by the pre- 
existing intima of this trachea. Fig. 86, 4, will show clearly 
that in beetles we have the same conditions, though here the 
temporary system is much less highly developed. This is from 
a well-grown larva; ¢/ is a tracheole; only such are present in 
the younger wing shown at A inthis figure; ¢ is a developing 
trachea; and Z/, ¢/ are developing tracheoles attached to single 
cells of the wall of the trachea. 
We believe that this peculiar temporary system of tracheoles 
is due to and dependent upon the retention of the wing within 
the narrow limits of its hypodermal pouch; for its small size 
alone renders its aération by simple tracheoles possible. We 
believe that this also explains the retarded development of the 
trachee. In an externally developing wing it is necessary that 
the trachez should grow with the wing, in order to carry the 
tracheoles out within reach of the tissues; but when a wing 
develops internally its length for a long time does not exceed 
the length of normal tracheoles. In such a wing the tracheze 
develop only when needed —at the approach of the time when 
rapid extension is to take place. 
III. THE BEHAVIOR OF THE HYPODERMIS. 
The cells of the hypodermis are remarkable, not only for 
their secretory and excretory activity, but also for their capa- 
city for rapid shifting and readjustment. Their life history is 
one of alternating conditions: first, growth beneath a limiting 
layer of chitine ; then, sudden lateral extension when the chitine 
is thrown off at molting. 
The typical hypodermis of prismatic hexagonal cells is found 
only where the body wall is smooth ; in short curves and angles, 
and in folds of the integument, and in the wings where close 
crowding is followed by enormous expansion their change of 
form is very great. At their ends, however, these cells main- 
tain fairly constant relations. Externally they must needs cover 
the surface to provide its integument, internally they join the 
basement membrane; between these two planes, however, they 
may assume almost any shape, according to the conditions of 
