No. 395-] THE WINGS OF INSECTS. 849 
membrane of the wing. Along certain lines, seen in section in 
the figure, the two membranes remain separate, and thus are 
formed the cavities of the wing-veins. 
Fig. 84 represents a partial section of a fore wing of a 
nymph of Anax junius two thirds grown; the section was 
taken at the nodus. The general features here seen are 
common to the wings of all nymphs—two layers of very 
elongate, hypodermal cells, which meet in places and form 
the middle membrane, and remain separate in other places, 
forming the vein cavities, which usually contain tracheæ. At 
B and C in the figure is shown the character of the commoner 
hypodermal cells. 
There is in insects with a complete metamorphosis another 
type of wing development; this is so different in its external 
aspects that it may best be described, with respect to these, 
separately, after which the common fundamental features of 
wings may be considered. 
It has been abundantly shown by others that modification of 
the type of wing development has kept pace with the increas- 
ing complexity of the metamorphosis. The wing-buds are 
most erratic in the headless, appendageless larvz of the higher 
Diptera, Hymenoptera, etc., while they are simplest in larvæ 
possessing a head, legs, and mouth parts, and especially in 
those in which the structure is altered least in transforming 
to imagoes. Among coleopterous larve are some in which, 
save for the appearance of wings, the change is slight; and, 
indeed, in certain of these (notably the meal worm) specimens 
are occasionally found with the wings developing externally. 
A little coccinellid beetle (Hippodamia 13-punctata) has fur- 
nished us simpler and more instructive conditions of wing 
development with complete metamorphosis than have hereto- 
fore been fully presented. Fig. 85 shows three early stages 
in the development of the wings of this insect. Each wing 
begins as ag@isk-shaped thickening of the hypodermis (Fig. 
85, A), which was first observed when the larva was about one 
fifth grown. A prominent spine, which stands at its dorsal 
margin, is an excellent landmark to aid in finding it at the first, 
and when found it is certainly recognized by a slightly concen- 
