1909] Homologies of Wing Veins 107 



of these tracheas unite before they reach the body tracheae, 

 approachmg the condition of the mature wing. 



This change in the condition of the corresponding tracheae 

 at ditlerent stages has been designated in this paper as the 

 "coalescence" of tracheae, as this term seems to express the rela- 

 tion of the tracheae of the mature wing to those of the nymphal 

 wing pad. This sort of coalescence of tracheae is a constant fac- 

 tor in the aphids, not alone as to the main tracheal stems but a 

 similar tendency is shown in the branches of the tracheae. For 

 instance notice that the medial trachea in figure 21 branches 

 desidedly nearer the base of the wing than the medial trachea 

 in figure 22, these branches being coalesced to much nearer the 

 margin of the wing than in the earlier stage. 



How or when this coalescence takes place the writer has as 

 yet made no attempt to ascertain. No pains has been spared, 

 however, in accumulating evidence that it does occur, or in mak- 

 ing sure that it is an actual union of tracheae and not an approxi- 

 mation. A 1-6 objective was usually sufficient for the exami- 

 nation of the cases in question. Where it was not, an oil emersion 

 was used. The manner and time of the coalescence is a m^^stery 

 well worth solving but it is only the fact of this coalescence and 

 not its method which has any bearing on the present problem- — 

 the homologies of the veins. However, it is a problem which the 

 writer hopes to undertake in the near future. 



Besides the normal coalescence of the basal portions of medial, 

 cubital, and the first anal tracheae just described, very rare 

 instances of abnormal coalescence occur. Figure 2 shows a 

 wing of Melanoxanthiis in which the radial, medial and cubital 

 tracheae are anastomosed for a considerable distance. 



Attention is also called to Plate XIX, which shows three unusual 

 examples of tracheation. Normally the forewing of Chermes 

 has two basal tracheae as in the more generalized genera, and as is 

 shown in figures 24 and 28. But in this highly specialized genus 

 the tracheation seems to be unstable. Figure 26 shows an in- 

 stance of anastomosis similar to that in figure 2, while figure 27 

 represents a single tracheal stem at the base of the wing, a condi- 

 tion which approaches the normal conciition of the hind wing of 

 Chermes (fig. 30.) On the other hand figure 25 records an 

 instance where the first anal trachea is separate from the common 

 stem of the medial and cubital tracheae to the base of the wing, 

 giving in this case three basal tracheae. 



