pies the same position as in the front wing. Similarly there is a common stem of the radial, medial and cubital trachea. The medial trachea, however, is unbranched, and the tracheal branch preceding R, is wanting. The loss of R, is significant. This vein is one of the least stable of the Hemipterous wing veins. Its complete absence has been established for the Cicada and a Coreid.* In the front wing of the aphids R, was seen to be preceded by a wavering trachea in all the subfamilies except the Chermesina where the vein R, is altogether lacking. In the hind wing of none of the aphids does either the vein R, occur, or the corresponding trachea. In the front wing of the psyllid and Aleurodicus alone for this group is R, a strong vein and here it is evidently the response to the mechanical necessity not otherwise provided for. Subcosta is wanting and, as the only vein present in this portion of the wing, the burden of support falls upon the radius. It is exceedingly interesting to find the condition of R, as predicted for the Hemipterat on the basis of the Cicada and Coreid, fulfilled in this group of highly specialized Homoptera. It is interesting, too, to find the same veins, vein for vein, appear in the wing of the psyllid that we have in the wing of the aphid the most striking difference is a very slight one,—the cubitus branched in the psyllid and unbranched in the aphid. There is experienced no difficulty, as has been seen, in homologizing either the aphid or the psyllid veins independently of each other but the fact that in doing so the conclusions arrived at with each support and bring additional evidence for the other should in no wise be disregarded. The fact that the costal margin of the psyllid wing is strengthened in one of two distinct ways (1) by a stigma and (2) by R1, is very well brought out in the five psyllids which have been selected to represent the venation of this family. Of these Euphalerus nidifex Schwarz‡ (fig. 38) would seem to be the most generalized type so far as the radial region is concerned, R, in this wing extending to near the tip of the wing as the trachea preceding R1 does in the nymphal wing pad of Psylla floccosa (fig. 33). In Pachy *Comstock and Needham: † Comstock and Needham: Wings of Insects. Page 245. Wings of Insects. Page 245. For an opportunity to study and figure the wings of E. nidifex and A. mori, the writer is indebted to Dr. L. O. Howard, who kindly loaned specimens of these species for this purpose. psylla c. mamma Riley (fig. 39) the tip of R, has migrated slightly toward the middle of the costal margin. In both these wings the space from the base of the wing to the tip of R, is evidently too long to bear the strain of flight without further strengthening. This needed reinforcement occurs in the presence of a more or less pronounced stigma the proximal edge of which is margined by a ridge which is in some species so clearly defined as to be frequently figured as a vein. As the tip of R, approaches the middle of the costal margin as in Anomoneura mori Schwarz (fig. 40) and even more in Psylla floccosa (fig. 36) the need of the stigma is removed as R, strengthens this portion of the wing margin. The wing of Trioza (fig. 37) shows an extreme case of the migration of R1, this vein being here scarcely longer than the stigmal ridge in Pachypsylla and in about the same position. Correlated with this condition are other striking departures from the more generalized psyllid wing, perhaps the most conspicuous being the origin of the free parts of media and cubitus at approximately the same point as the origin of the free part of radius,— the common stem M+Cu present in most psyllids being lacking in Trioza. The wing of A. mori is in one respect most unusual for the family, and that is in the branching of the radial sector. This is the only instance in the four families discussed in this paper where the radial sector is branched, and it throws additional evidence on the interpretation of this vein as the sector. It should be stated in connection with the discussion of figure 40, that the venation here shown may possibly be not normal for this species. The writer had access to but a single specimen and in this the radial sector of the wing on one side bore five branches as figured while the radial sector of the other side was six branched. The purpose of this study in wing tracheation has had the vein homologies as a goal and is not intended to enter into systematic discussions. However, since the systematists of the Psyllidæ build their tables largely upon the basis of the wing veins, it is apparent that it would be more satisfactory to apply * Löw. Verh. g-b. Wien XXVIII. Maskell: Transactions of the N. Z. Institute Vol. XXII, 1889, page 158. Kuwayama: Trans. of the Sapporo Nat. Hist. Soc. 1907-08. a terminology which has the same significance for not only the closely related families of Aphididæ, Aleurodidæ, and Coccidæ, but the other insect orders as well. For instance it would certainly seem more convenient to say "M+Cu is longer than the stem of R" than to resort to the more tortuous statement of Maskell*: "the stalk of the lower branch (cubitus) of the furcation of the primary vein is longer than the stalk of the upper branch (subcosta).” At present the veins in the figures of the psyllid wings are indicated by arbitrary letters or figures with no necessary relation to symbols used by the same author for any other psyllid wing or to those of any other author for the same wing. This haphazard arrangement of lettering or numbering the figures of psyllid wings increases the confusion caused by the fact that the veins themselves are cumbered with such a system of nomenclature as the following: "Stalk of cubitus, lower branch of cubitus, lower fork of lower branch of cubitus, upper fork of lower branch of cubitus," etc. But by the use of a uniform system of wing terminology the abbreviation of the names of the veins become the natural and inevitable symbols to use for lettering the figures of the wings, and no confusion arises in associating Rs of the figure, for instance, with the radial-sector of the text. The relative simplicity, ease of abbreviation and uniformity. of such a system of terminology is recommendation enough aside from the homological significance it bears.† As is stated in the discussion of Redtenbacher's homologies (see page 125) except for his interpretation of the alternate concave and convex veins his terms for the psyllid veins are in the main those applied in this present paper upon the basis of the tracheation. ALEURODIDÆ. Four fine but distinct trachea are present in the freshly emerged wing of Aleurodes sp.,—the costal, subcostal, radial and cubital trachea (fig. 44). All of these are uncoalesced to the *Maskell: Trans of the N. Z. Institute, Vol. XXII, 1899, page 158. The Comstock-Needham system of terminology has been adopted by Handlirsch in all his recent papers dealing with venation of fossil insects and in his extended monograph of the fossil insects of the world. (Die Fossillen Insekten und die Phylogenie der Rezenten Formen 1906-1908). It has been with interest that the writer has noticed the application of this system of nomenclature to the group of insects with which this present paper deals, especially as the basis for his conclusion was a study of the treacheation of the wings. base of the wing. The medial trachea is suggested merely by a very faint and delicate but constantly appearing tracing in the wing. As in the wings of Aphididæ and Psyllidæ the radial trachea in Aleurodidæ is branched, being represented by the branches corresponding to R, and Rs. And as in the hind wing of all aphids and the front wing of the Chermesinæ, and the hind wing of psyllids, the vein radius in Aleurodes is unbranched, R, being lacking. The formation of the vein radius in Aleurodes is of exceeding interest. It follows the course of the radial trachea to the branching of the trachea and then proceeds along the radial sector. R, in the mature wing is lost and its position, if indicated at all, is suggested by the most delicate "shadowing" in the wing tissue. As has been previously stated,* the "complete absence of vein R," was predicted as characteristic of the Hemiptera on the basis of the two widely separated insects,-the Cicada and a Coreid, and the phylogenetic significance of the added testimony of the weakness of this vein and the trachea that precedes it, in the remote and highly specialized group of Homoptera here under consideration seems to the highest degree interesting. The tracheation of Aleurodicus I have not had an opportunity to study. On the basis of figures of the mature wing (not a safe basis for this group by any means, as has been shown), however, the additional vein found there can only be interpreted as R1. Its prominence is to be accounted for here doubtless, as in the case of its prominence in the forewing of the psyllid, by the rounded shape of the wing which results in a large wing expanse not otherwise strengthened. In response to mechanical necessity this vein, weak or absent when not needed, becomes more strongly developed. The second vein of the wing of Aleurodes is cubitus as will be seen by comparing figures 44 and 45 where the second vein is shown to follow the course of the cubital trachea. The hind wing of Aleurodes (fig. 46) has but one trachea and one vein, the radial sector it seems inevitable to conclude. COCCIDE. Before examining wings of freshly emerged male coccids it seemed possible only to echo the sentiment of Redtenbacher,— "Cocciden konnte ich nicht untersuchen."† Imagine the surprise, *See Page 119. † Vergleichende Studien uber das Flugelgeader der Insekten. Page 188. then, which was caused when mount after mount of Dactylopius sp. on cactus showed five very delicate but perfectly distinct tracheæ. See Figure 42. Four of these trachea are simple and uncoalesced to the base of the wing. They are arranged in two groups, the base of the subcostal, radial and medial tracheæ lying close together and the cubital and first anal trachea forming the other group at a little distance from the first. The subcostal and radial trachea are both very wavy and as they lie close together, they cross and recross, often for the greater part of their length. The medial trachea takes a median course. This consists of two interwoven branches, pursuing a common course. The trachea in the wing of this coccid remain distinct until after the veins begin to form so that the relation of the two is at once discerned. One vein follows the general trail of the subcostal and radial trachea. This vein very evidently represents radius. The second vein follows the base of the first tracheal group to about the point where the medial tracheal separates from the subcostal and radial. The vein here takes a direct line for the middle of the caudal margin of the wing. For slightly less than one-third the length of this vein it frequently joins the path of the cubital trachea. This corresponds most closely with media. Besides these two main veins a short spur representing the subcosta is present. In a wing so highly specialized as the coccid wing it is not improbable that the tracheation has lost its value as a basis for the venation. Certainly in the species studied there seems no necessary connection between the trachea and the veins which are found later. In Pseudococcus citri the tracheation was exceedingly difficult to trace. The same veins occur in this species (fig. 41) as in the preceding (fig 43). In many species of Coccids there are shadowed portions of the wings. It is due to this fact that we find the coccids sometimes figured with apparently four long veins, alternately dark and light, as for instance, in Westwood.* *Westwood, J. O. Arcana Entomologica. Vol. I, Plate 6. |