No. 386. ] THE WINGS OF INSECTS. r21 
difference, so far as this point is concerned, whether we believe 
that the wing is a modified tracheal gill or a transformed para- 
chute-like expansion of the body wall. In either case it is 
highly improbable that it was fanlike at first. It was not until 
the wing became an organ of flight that a corrugation of it was 
beneficial; and even then this corrugation did not spring into 
existence suddenly, only to be lost in most of the orders of 
insects ; as must be inferred, if we accept the theory of Adolph, 
that the wing of a May-fly represents the primitive type of this 
organ. 
The stiffening of the costal margin of the wing by the for- 
mation of a subcostal furrow has been attained in most of the 
orders of insects; and in several of them the formation of folds 
has extended, to a greater or less degree, to other parts of the 
wing. But, as a rule, this method of specialization has not 
been the most important one in perfecting the wing. In the 
Odonata it has been carried farther than elsewhere, among liv- 
ing insects, except in the Ephemerida. But in the Odonata 
it has been supplemented by other methods of specialization, 
already discussed, with the result that an exceedingly efficient * 
organ of flight has been developed in that order; while in the 
Ephemerids the cephalization of the flight function and the 
corrugating of the wings have been the chief lines along which 
specialization has extended. The former has doubtless added 
much to the efficiency of the wings ; but a too close adherence 
to the latter method of specialization has resulted in the forma- 
tion of a rather indifferent organ; although it is the most per- 
fect development of its peculiar type. 
We have studied the tracheation of many nymphs of May- 
flies, but with results much less satisfactory than those we have 
reached in the study of other orders of insects with many- 
veined wings. In all nymphs of May-flies that we have ex- 
amined, a greater or less reduction of the trachez appears to 
have taken place; and in many of them a large proportion of 
the longitudinal veins contain no trachez. And, too, the pres- 
ence or absence of a trachea in a vein appears to have little sig- 
nificance. As an example of this the wings of two nymphs are 
before the writer, in which the venation is so similar that there 
