272 
Psyche 
[September 
mally and has not yet changed into a typically inserted 
sector. Nevertheless, here this vein takes a definite con- 
vex position and forms at its base a projection towards the 
concave vein R. This is sufficient for the secondary trache- 
ation to be adopted. The main convex veins, R and M, 
had orginally and have still in many species their own 
tracheal trunks, which penetrate into them; but in the 
other weaker trachese and in the more specialized groups, 
as in Bsetoidea, for example, where the membrane is very 
fine, all superfluous veins are discarded and in the re- 
maining ones the alternaton of convexities and concavities 
is sharply expressed up to the very base, where the main 
convex veins, R and M, lose their own trachese. 
The indicated relation becomes clear to us, if we look 
at it from an historical point of view. There was certainly 
once a time when the alternation of convex and concave 
veins was only suggested. There was still 15 no inserted 
sectors, and the present convex RS 4 and RS 2 of recent 
types had a normal origin from R. Because of the absence 
of plaiting of the wings and the diversity in the methods 
of origin of the different veins receiving the trachea nor- 
mally (that is, through the base from larger trachea of 
other veins of which they were branches), this position 
of the tracheation is approximately preserved at the pres- 
ent time, as mentioned above, in a great many primitive 
forms, and also in the earliest stages of development of 
the wing arilage in nymphs. As the wings “mechanized” 
more and more, that is, as the membrane became finer, 
the veins adopted an alternation of convex and concave 
positions, the basal part of the veins at the time of the 
change from the concave position (e. g., of R to RS 4 ) weak- 
ened and finally was obliterated. The normal tracheation 
through the base into the convex vein became more and 
more difficult, and then became impossible, when the vein 
15 By this I do not mean to say that all, even the smallest irregular 
distal veins, were originally normal branches which arose from other 
veins. Small branches were formed at their places from the primi- 
tive network, were unstable, irregular, diverse and as a rule gave 
rise to no (secondary) alternation of convexities and concavities. 
Now, as before, they are tracheated by small branches probably in 
very diverse ways. These supplementary, secondary veins I do not 
consider here. 
