Disholcaspis bassetti

Macrodiplosis majalis

Acalitus ferruginea

Aceria querci

Aceria negundi

Phyllocoptes didelphis

Eriophyes emarginatae

Phytoptus abnormis

Eriophyes cerasicrumena (on-p-serotina)

Pachypappa pseudobyrsa

Hormaphis hamamelidis

Hamamelistes spinosus (on-hamamelis)

Hamamelistes spinosus (on-betula)

Mordwilkoja vagabunda

Melaphis rhois

Adelges abietis

Pachypsylla celtidismamma

Rabdophaga salicisbatatus

Rabdophaga salicistriticoides

Rabdophaga strobiloides

Euura salicispisum

Euura orbitalis

Euura salicisovum

Euura salicisovulum

Euura serissimae

Euura salicispomum

Euura salicisdesmodioides

Euura proxima

Euura lucidae

Euura cosensii

Euura crassicornis

Holcaspis bassetti

Host Quercus macrocarpa

A monothalamous gall occurring singly or in clusters around the stems of the host. When grouped the galls often cover completely 4 to 5 inches of the stem.

When the gall is not deformed by crowding, it is irregularly circular in outline at the base, gradually tapering to a distinct point that is recurved in most cases. The gall is attached to the host by a small stalk at the centre of the base. Colour green, often tinged with pink when young; becoming brown when more mature. The larval chamber resembles closely that found in the former species [Disholcaspis quercusglobulus] in being oval and free at maturity, but it differs in being placed nearer the base of the gall and in tapering to a point at the end nearer the twig.

Diameter at base, average, 16 mm.

Cecidomyia majalis

Hosts Quercus rubra, coccinea

A flat pouch-like gall on the under side of the leaf. The opening which extends the entire length of the gall is on the upper side. It is produced by a folding of the blade of the leaf; this fold is parallel with and very close to the midrib or a main vein.

Along the main line of its attachment to the leaf, diameter 4-7 mm.

[Photo caption]
Fig 27. Cecidomyia majalis on the leaf of Quercus coccinea. Section at right angles to the midrib. The folding of the leaf is shown and the uniform character of the mesophyll of the gall. 

Eriophyes sp.
Chadwick's No. 65 

Host Fagus grandifolia Ehrh. 

This gall occurs on the under side of the leaves of the host plant. It is an erineum, white and frost-like at first, turning to a brown colour at a later stage. The anatomical structure of the interior of the leaf remains perfectly normal, the effect of the stimulation being confined to the epidermis. This produces a large number of unicellular, capitate hairs, resembling miniature mushrooms, the resemblance to which is the more striking since the stalks of the hairs are bulbous at the base, as shown in Fig. 3. The hairs on the normal leaf are long, acicular and unicellular. As this gall is produced with no increase in the number of cells in the leaf, anatomically it stands as a simple type of "hypertrophy", the remaining forms all exhibit the more advanced phenomenon of cell proliferation.

Eriophyes querci (Garman). 
Chadwick's No. 112 

Host Quercus macrocarpa Michx. 

In this gall, which is of the "dimple" type, the depression is on the under and the elevation on the upper side of the leaf. The hollow is filled with a dense mass of brown pubescence. In rare cases the elevation is on the under side of the leaf, when the pubescence then covers it. 

The leaf blade in this case has become of nearly twice the normal thickness. This has resulted from cell division which has occurred in the tissues of both the palisade and the spongy parenchyma, producing a compact mass of undifferentiated cells, entirely without intervening air spaces. This tissue is shown in Fig. 6.

The epidermis has responded to the stimulation by producing many long, acicular, unicellular hairs, which are narrowed at the base. The hairs are stellate on the normal leaf of the host, but are simple, uni- cellular and acicular on its reproductive axis. The same feature is true of Quercus robur var. pedunculate, and in all probability of the other oaks. This appears to be a clear case, not of the production of a new type of trichome, but of a reversion.

Eriophyes Sp. 
Chadwick's No. 2

Host Acer negundo L. 

A shallow dimple on the under side of the leaf, filled with a white pubescence. In this gall as in the preceding species, the leaf blade has been very much thickened by proliferation in the mesophyll. The cells produced are circular in outline and of about the same size as those of the normal spongy parenchyma. The hairs produced in this case not infrequently consist of from 2 to 3 cells which are very much convoluted. They are well shown in Fig. 4. The hairs on the normal leaf of the host are straight or only very slightly curved, but the glandular, convoluted type of hair is found on the inflorescence. Both the normal and abnormal hairs are composed of the same number of cells.

Eriophyes Sp. 
Chadwick's No. 84 

Host Populus grandidentata Michx. 

A deep dimple gall with the convex part on the upper surface of the leaf. The gall is light green above, with the contents of the depression dark red. 

The cells produced by the palisade parenchyma can be distinguished in this case from those arising from the spongy parenchyma. The former are almost square in outline and placed in two very regular rows immediately below the upper epidermis, and parallel with it. This tissue can be seen in Fig. 2. The latter constitute a tissue made up of cells which are circular to elliptical in outline. These cells are much smaller than those produced by the palisade layer. The outgrowths from the lower epidermis described in various ways as trichomes, granules, etc., are in reality produced by a complicated folding, in which only the lower epidermis and the spongy parenchyma participate, as shown in Fig. 2. 

A cross section of the gall shows a number of vascular strands, but in my opinion the gall-producing stimulus only enlarges the veins that are already present in the normal leaf; it does not originate a special vascular system for the gall. 

 Eriophyes Sp. 
Chadwick's No. 88. 

Chadwick considers this form, first described by Jarvis, the same as his No. 87. But the latter is characterized by a depression on the under side of the leaf, the former by an elevation, so that the two are constantly distinct. 

Host Populus tremuloides Michx. 

A dimple gall with the elevation on the under surface of the leaf. The elevation is a lighter green than the surrounding normal leaf and the folds that occupy the concavity are greenish-yellow or reddish in colour. 

In dealing with the anatomical structure it is to be noted that the spongy parenchyma has in this case remained normal. The folds are produced in the same manner as in the preceding species, except that in this form it is the upper epidermis that undergoes the folding process. The nature of this folding can be seen in Fig. 1.

Eriophyes Sp. 
Chadwick's No. 93. 

Host Prunus nigra Ait. 

A very much elongated pouch gall, greenish or whitish in colour, found on the upper side of the leaf with the opening on the under side. 

All the characteristics of the normal mesophyll have been completely altered in the affected part of the leaf. Its cells, which, with the epidermis, constitute the wall of the gall, are larger than the normal and are elongated parallel to the long axis of the outgrowth. The upper epidermis which forms the epidermis of the gall has not been affected, but the cells of the lower epidermis which line the gall cavity have become much enlarged and in addition have produced a large number of closely set trichomes which project into the gall cavity. The nature of these is shown in the upper part of Fig. 5. These structures are often from 2 to 3 cells in length. Around the opening of the gall a circle of closely set acicular hairs occurs. The hairs on the outside of the gall and on the normal leaf are also of the acicular type. The vascular strands are much larger than those of the normal leaf but appear to be simply the stimulated normal veins.

Eriophyes abnormis (Garman).

Chadwick's No. 144

Host Tilia americana L.

 A very much lobed pouch-gall, usually on the upper side of the leaf, found rarely on the under side. The opening which is on the opposite side of the leaf to the pouch is surrounded by a dense growth of acicular trichomes. 

The anatomical structure of this gall shows it to combine the characteristics of a pouch-gall with a type in which the epidermis lining the larval cavity is thrown into folds, as described in the case of the galls on Populus. The foldings of the lining of the gall cavity often coalesce, and practically divide the cavity into a number of compartments. Morphologically this is one of the most highly differentiated of our Phytoptocecidia. The lining epidermis has produced a large number of acicular, unicellular trichomes; these in some cases almost fill the cavities. The hairs surrounding the gall aperture are of the same type, as are also the normal hairs of the leaf. Glandular cells are much more abundant in the gall tissues than in those of the normal leaf.

Eriophyes serotinca Beut.

Chadwick's No. 100

Host Prunus serotina Ehrh.

This is a club-shaped gall, produced on the upper side of the leaf of the host. The aperture of exit, which is on the lower side, is surrounded by fine white hairs. It varies in length from 5 to 8 mm. and at a distance from the apex of about two-thirds the total length, it is narrowed into a stalk with an average diameter of 1 mm. In color it varies from green to a distinct red. 

The stimulation has not produced very marked changes in the structure of the leaf near the origin of the gall. Apart from the fact that the spongy parenchyma has divided more actively, the mesophyll is normal. The upper epidermis of the leaf that forms the outer covering of the gall, has its cell walls abnormally thickened. The lower epidermis that lines the gall cavity has larger cells than the unstimulated epidermis, and from these cells originate elongated, unicellular trichomes with bulbous bases. The hairs on the normal leaf are acicular and unicellular. While the cells in the neck of the gall are arranged in rows parallel to its length, the larger cells that form the main body of the gall are not regularly placed. The vascular strands pass up from the leaf at a distance of about three cell layers from the gall cavity.

Aphid Gall (Unclassified), 

Host Populus balsamifera L. 

A pouch-like gall on the under surface of the leaf, produced by a fold in the blade near the base of the midrib. One edge of the fold is attached along this midrib. The slit-like opening, which is on the upper surface of the leaf, extends the full length of the gall. This species resembles very closely the gall produced by Cecidomyia majalis Bass. The general structure is shown in Fig. 7.

Dimensions: — Length along line of attachment to midrib, 10-12 mm. ; Width, 4-5 mm. 

In the part of the leaf blade that forms the gall all resemblance to the normal mesophyll has disappeared. A compact mass of tissue has taken its place, the cells of which are much larger than normal mesophyll cells. Towards the interior of the gall the cells become smaller and richer in protoplasmic contents. The upper epidermis forming the interior of the gall remains practically normal, except that it produces longer and more abundant trichomes than when unstimulated. These trichomes are usually three cells in length. A cross section of this gall shows two small groups of cells with porous laminated sclerenchymatous walls, one of which is situated near the midrib and the other exactly op- posite on the other side of the gall opening. Thus each side of the gall aperture is bordered by a band of sclerified cells, as shown in the trans- verse section of Fig. 8. 

Hormaphis hamamelidis Fitch.

Host Hamamelis virginiana L.

The gall formed by this species is found on the upper side of the leaf of the host, but the larvae escape from an opening on the under side. The mature gall is conoidal in shape with the apex usually slightly bent over. General structure is illustrated in Fig. 12. A circular ring of tissue covered with pubescence surrounds the gall opening which is shown in Fig. 12. 

Dimensions: — Average length of gall 10.5 mm.; diameter at base 4 mm. 

The gall is composed of small cells placed close together, forming a compact and very uniform tissue. The cells are arranged with their longer diameters pointing in the direction of the gall apex. In a longi- tudinal section the vascular strands are seen to pass up each side at a depth of about three cells from the gall cavity. The beaks of the larvae, often found imbedded in the wall of the gall, were inserted far enough to almost reach these vascular strands. The hairs that surround the gall aperture are acicular and unicellular. 

Hamamelistes spinosus Shimer. 

Host Hamamelis virginiana L. 

The galls in this species are modified flower buds. These are some- what elliptical in outline with gradually tapering stalks. They are covered with spines, which are usually curved. The opening is situated at the union of the stalk and the gall proper. This opening is funnel- shaped and is surrounded by a circular ring of tissue as in the preceding species on the same host. The pubescence is absent in this case.

Dimensions: — Average length including stalk 21 mm.; average width 10 mm. 

This gall is covered by an unusually small-celled epidermis. The spines that are so noticeable a feature are found to consist of projections of the epidermis, filled with cells in continuity with the mesophyll. The cells of the gall are almost perfectly circular in outline and packed together very closely. This tissue is very uniform except in the four or five layers adjoining the gall cavity. In that zone the cells are smaller and richer in protoplasmic contents, constituting a fairly well marked nutritive layer.

In cross section of the gall about thirty main fibro-vascular bundles are cut; these are comparatively large and situated near the larval cavity. Two of these have been cut in the section shown in Fig. 10. Other smaller strands are cut further out. The gall receives all the fibrovascular strands that, under normal conditions, would have passed up into the flower. The interior of both this and the preceding species is almost perfectly glabrous.

Aphid Corrugations on Birch.

Betula lenta L.
Betula alba var. papyrifera (Marsh) Spach.

The primary folds in the leaf that form this gall run parallel to the main veins, with the latter as boundaries between them. Their crests are on the upper side of the leaf, while the hollows which form the larval chambers are on the under side. The primary folds are divided into secondary folds, and these again into depressions resembling minute Acarina dimple-galls. This complex arrangement is conditioned entirely by the veining of the leaf, since each fold, primary or secondary, is sup- ported along its edges by veins. The folding can be seen in Fig. 9. The anatomical characteristics of these galls show that the folding of the leaf has not entirely changed the structure of its normal mesophyll. Around the gall cavities the spongy parenchyma is nearly normal through- out and the palisade layer is recognizable in different places. The cells, however, are considerably larger than the cells of the normal mesophyll. The cells of the lower epidermis, that form the lining of the gall cavities, are well filled with food materials for the larvae. The supporting veins on each side of the fold send out branches that supply the gall with an adequate vascular system. Hormaphis hamamelidis Fitch and Hamamelistes spinosus Shimer, as worked out by Pergande,^° show that they inhabit alternately Betula nigra L. and HamameUs virginiana L. The galls on the birch leaves are produced by the fourth genera- tion of Hamamelistes spinosus Shimer. Pergande described them as "pseudo-galls or corrugations". The witch-hazel galls produced by the stem-mother of this species are plentiful in this locality, but Betula nigra L. is not found here. The Aphids have consequently been compelled to extend their list of food plants to include B. lenta L. and B. alba var. papyrifera Spach.

Pemphigus vagabundus Walsh.

Host Populus deltoides, Marsh.

All the leaf rudiments of the terminal bud appear to be concerned in the production of this gall. Yet it is in reality a large pouch-gall with its wall thrown into smaller secondary folds. The apex of the stem, from which the gall originates, is usually swollen to nearly twice its normal diameter. These galls often remain on the host plant until the next season's galls begin to appear. The cells composing this gall form a compact and fairly uniform tissue and a nutritive layer is not clearly differentiated. About one- third of the thickness of the gall wall, on the side next the larval chamber, however, is composed of cells that are somewhat larger than the remaining cells. The epidermal cells, lining the gall cavity, are elongated into short trichome-like structures. The phyllome origin of this gall is revealed by the presence of well defined stomata on its epidermis. These structures are numerous and appear to be quite normal. Glandular cells are plentifully distributed. Vascular strands pass irregularly throughout the wall of the gall.

Pemphigus rhois, Fitch.

Host Rhus typhina L.

A balloon-shaped gall with the regularity of its outline destroyed by the elongated lobes that cover its surface. A gall is shown in Fig. 14. The epidermis is slightly pubescent and coloured red, shading into yellow and green. It originates from the under side of the leaf, and the point of attachment on the upper side is indicated by a small papilla covered with a dense pubescence. These galls vary in size from very short types less than i cm. to those that are 4 to 5 cm. in length. composed of the ordinary vascular elements of the normal leaf that have been stimulated to increased activity. There is not a special tracheary system originated for the gall. In the galls the strands occupy a definite position, since in the normal leaf they occupy a definite place in relation to the spongy and the palisade parenchyma. Large glands are present in the gall tissue, as shown in Fig. 14. A gland is found invariably asso- ciated with a fibro-vascular strand and seems to have its counterpart in the very small gland that runs through each vein of the normal leaf. In some cases the abnormal glands have acicular trichomes projecting into their cavities.

Chermes abietis Choi.

Picea abies (L) Karst.
Picea mariana (Mill) B.S.P.

A polythalamous gall produced by the swelling of the base of the young shoots. Since the twigs are not usually killed the galls are sur- mounted by a variable length of normal stem. The galls in general vary from conoidal to nearly spherical in shape, but in some cases, owing to the stimulation not having aftected the entire circumference of the stem, the gall does not extend completely around it and is consequently less regular in outline. The surface of the gall is covered with the en- larged bases of the aborted needles. These give a faceted appearance to the gall and produce a likeness to a miniature pineapple. 

Dimensions: — Longer diameter 2-3 cm.; shorter diameter 1-2 cm. 

The gall in this case is a joint production of the cortex of the stem and the bases of the leaves of the host. The cells of the epidermis, lining the gall cavities, in some cases have been prolonged to form very short trichome-like structures. The hairs at the aperture of exit, as seen in Fig. II, are composed of one or two cells. The resin ducts that occur in the normal cortex are found consider- ably enlarged in the gall. In addition to these, there are out near the gall periphery numerous smaller resin ducts, as shown near the margin of Fig. II, that do not have corresponding structures in the unstimulated tissues. These additional ducts pass in from the swollen bases of the aborted leaves. A cross section near the base of these leaves cuts from four to six resin ducts, while a normal leaf does not contain more than two of these structures. 

Pachypsylla celtidis-mamma Riley. Host CelHs occidentalis L. A complicated form of pouch-gall produced in the mesophyll of the leaf of the host. On the upper surface of the leaf the gall is indicated by a decided depression in the centre of which is a slight elevation that marks the opening of the gall. The part projecting from the lower surface, is oblate-spheroidal in shape, attached to the leaf by a slightly tapering cylindrical stalk. The average number of galls found on a leaf is usually about ten, but in some cases much higher. The surface of the gall is smooth except for a few fine scattered hairs, glaucous and greenish- yellow in colour. Dimensions: — Height from point of attachment 6-7 mm.; width 7-8 mm. The anatomical structure of this gall shows it to be a more complex type than any other of the Hemiptera discussed in this paper. Besides the folding of the leaf the blade has been further changed in thickness and in the character of the cells. The production of the greater part of the abnormal tissue is due to a wide, well differentiated cambium layer, that extends right across the gall and at its margin passes into the tissues of the normal leaf between the palisade and the spongy parenchyma (Fig. 15). The larval chamber is lined by this cambium sheath which thus functions as a nutritive layer. Bordering this zone is a well developed protective tissue composed of cells with uniformly thickened walls. The sclerenchyma is laminated and pene- trated by branched canals, presenting the same character as that found in the galls of the Cynipidae. Outside of this protective zone lies the chief mass of the gall, com- posed of thin-walled irregularly shaped cells, as illustrated in Fig. 15. Typical cells of the protective sheath are also found scattered throughout this tissue. As the gall becomes older these cells increase in number.

Rhabdophaga batatas Walsh.

"On Salix humilis Marsh. A polythalamous gall of very variable shape and size, pale green when young, the colour of the bark when mature, growing on twigs .06-. 19 inch in diameter and always some distance from the tip of the twig. Sometimes it resembles a small kidney-potato pierced lengthways by a twig, and has then most generally a smooth polished surface studded with a few buds, one or two of which occasion- ally give birth to a shoot, and it then reaches 1.35 inch in length and .6 inch in diameter. Sometimes it resembles a young apple pierced lengthways by a twig and attains a diameter of .3 inch." — Walsh.** In this gall the larval cells are situated in the pith of the host plant, just inside the line of the fibro-vascular bundles. The epidermis has a much thicker cuticle than that borne by a normal stem of corresponding age. The cortex is approximately four times as thick as the cortex of the normal stem. This is due principally to the increased size of the cells. The cells of the nutritive layer are very similar to those of the surrounding tissues but a well marked protective zone defines its outer limits. This is clearly shown in Fig. 25. The cells of the protective zone present a characteristic very rare in Dipterous galls, although frequently found in the Cynipid galls, namely, the walls of the cells are not uniformly thickened but are much heavier on the side next the nutritive zone. This unequal sclerification is illustrated in Fig. 26. Crystals of calcium oxalate, that were so characteristic a feature of this zone in C. triticoides Walsh, seem to be entirely absent in this gall.

Cecidomyia triticoides Walsh.

 "On Salix cordata Muhl. A polythalamous woody gall .70-1.23 inch long and .30-. 37 inch in diameter, bearing a remote resemblance to a head of wheat with the kernels elongated, naked, pointed and very protuberant, its general outline oval or elongate-oval, and formed by the swelling of a twig to 2 or 3 times its former diameter, the swelled portion being very much contracted longitudinally, so as to bring each kernel- like bud nearly or quite into contact with the base of the one that pre- cedes it in the same row, the whole number being arranged in four irregular rows."— Walsh.

The larval chambers in this gall are placed usually along the line of the fibro-vascular bundles of the stem, and wherever a chamber is situated the vascular tissues are not developed. 

Rhabdophaga strobiloides Walsh.

Host Salix cordata Muhl. [eriocephala]

"The galls are very uniform in size and form, usually top-shaped, some inclining to spherical, a little oblate below and prolate above, and as the female oviposits but one egg in the terminal bud of the willow shoot, the galls are terminal and monothalamous. "The gall is a rather tightly and regularly arranged mass of from 70 to 80 aborted leaves, representing perhaps about i m. of the leafage of a normal branch. "The average measurement of 200 galls was 12 mm. X15 mm., and the length of the deformed part of the branch included in the gall around which the aborted leaves were packed was 6 mm." — Brodie. 

The leaves that constitute the principal mass of this gall do not take any part in supplying the larva with food. The tissue that has this function is composed of a mass of small, thin-walled cells. It terminates the stem axis and the larva is in immediate contact with it (Fig. 24). This tissue which really furnishes a nutritive layer seems to originate from a cambium-like tissue at the base of the mass of cells. An important factor in the production of this gall is the practical cessation of growth of the bud axis. The aborted leaves that compose the gall exhibit very slight anatomical aberrations. 

Pontania pisum Walsh

on S. discolor Muhl.

"On Salix discolor. A subspherical, pea-like, hollow, pale yellowish- green gall, always growing on the under side of the leaf and almost always from one of the side veins (in one case from the midrib), and attached to the leaf by only a minute portion of its surface; 0.18 to 0.28 inch in diameter and a few miniature, only 0.08 inch in diameter. Almost invariably there is but one gall to the leaf, but on four leaves there were two, and occasionally two are confluent. Surface in some smooth and even without pubescence; in others a little shriveled, generally studded in the medium-sized ones with four to twelve small, robustly conical nipples, which in the larger ones have burst into a scabrous brown scar. Only in three out of sixty-two was there any rosy cheek as in P. pomum. The point of attachment is marked on the upper side of the leaf by a brown sub-hemispherical depression." — Walsh .

Walsh is incorrect in supposing that this gall originates from a midrib or vein. A section shows that it is clearly a product of the mesophyll and is attached to that part of the leaf. The side vein, near which it is always placed, is cut by the ovipositor, however, and vascular strands pass out from it into the gall body. 

The mature gall consists of a peripheral layer of thin-walled cells, irregular in outline surrounding a central cavity (Fig. 81). This tissue is clearly derived from the mesophyll and epidermis of the leaf, but a stage was not secured young enough to show the relative amounts produced from each. The epidermis bears numerous lenticels, organs which Kuster mentions as occurring on the gall produced by Pontania salicis. 

At the point of attachment of the gall the blade of the leaf is strengthened by several rows of cells derived from the upper epidermis and the palisade parenchyma, as shown in Fig. 81. These cells seem to have remained unmodified in any way, since their arrangement in rows is still clear in fairly old stages of the gall. Consequently they differ very markedly from the irregularly arranged cells of the main part of the gall body.

Euura S. gemma Walsh.

"On Salix humilis. The lateral bud of a twig enlarged so as to be twice or thrice as long, wide and thick as the natural bud before it begins to expand in the spring ; its external surface otherwise entirely unchanged both in texture and colour. Internally, instead of the normal downy embryo leaves, it contains early in the autumn a homogeneous grass-green fleshy matter, which is afterwards gradually consumed by the larva, leaving nothing at last but a mere shell partly filled with excrement. The gall is monothalamous, sometimes one only on a twig, some- times two or three or more at irregular intervals, very rarely as many as three or four formed out of three or four consecutive buds. 

Length .17 to .36 inch 
Breadth .10 to .17 inch. "— Walsh.

The anatomy of this gall presents little differentiation of tissue. A cross section shows that the entire mass of the gall consists of small thin-walled cells, shown in Fig. 68. The bud scales surrounding this group of cells resemble those of the normal bud except that the cuticle of the epidermis is abnormally thickened. 

Euura S. ovum Walsh.

"On Salix cordata [eriocephala]. An oval or roundish, sessile, monothalamous swelling, .30 to .50 inch long, placed lengthways on the side of small twigs, green wherever it is smooth, but mostly covered with shallow longitudinal cracks and irregular rough scales which are pale opaque brown. Its internal substance fleshy in the summer like that of an apple, but with transverse internal fibres. When ripe in the autumn filled with reddish-brown spongy matter, with close-set transverse internal fissures at right angles to the axis of the twig. On cutting down to the twig at any time a longitudinal slit about .20 inch long becomes plainly visible." — Walsh . 

As already noted the host of this gall in this locality is Salix humilis, it remains to be determined whether there are two distinct species of producers or one species with two hosts [Zinovjev and Smith imply this is more likely [Euura salicisovulum](https://www.gallformers.org/gall/3652)]. Walsh's description of the gall on Salix cordata corresponds to the form occurring here on S. humilis.

The ovipositor of the producer has in this case made a longitudinal cut in the stem. A transverse section at the place where the gall is located shows that this wound extends in from the epidermis to the boundary of the pith. The activity of the young tissues, abnormally stimulated, soon fill this fissure with a mass of small, angular parenchyma. The rapid division of these cells forces the exposed edges of the cortex and central cylinder apart so as to form a wedge-shaped opening which is filled up with the gall mass (Fig. 71). It should be stated that the newly formed cells originate mainly from the division of a cambium bordering the pith at the bottom of the fissure. But other tissues also respond to the stimulation initiated by the ovipositor of the insect. Thus a section of the stem at a short distance from the gall shows that the outlying cambium has become abnormally active and has produced a layer of bast nearly one-third thicker than that found in the normal stem. Likewise the activity of a cork-cambium layer has thrown off a strongly cuticularized epidermis present in the earlier developmental stages .

Euura S. ovum Walsh.

"On Salix cordata [eriocephala]. An oval or roundish, sessile, monothalamous swelling, .30 to .50 inch long, placed lengthways on the side of small twigs, green wherever it is smooth, but mostly covered with shallow longitudinal cracks and irregular rough scales which are pale opaque brown. Its internal substance fleshy in the summer like that of an apple, but with transverse internal fibres. When ripe in the autumn filled with reddish-brown spongy matter, with close-set transverse internal fissures at right angles to the axis of the twig. On cutting down to the twig at any time a longitudinal slit about .20 inch long becomes plainly visible." — Walsh . 

As already noted the host of this gall in this locality is Salix humilis, it remains to be determined whether there are two distinct species of producers or one species with two hosts [Zinovjev and Smith imply this is more likely [Euura salicisovulum](https://www.gallformers.org/gall/3652)]. Walsh's description of the gall on Salix cordata corresponds to the form occurring here on S. humilis.

The ovipositor of the producer has in this case made a longitudinal cut in the stem. A transverse section at the place where the gall is located shows that this wound extends in from the epidermis to the boundary of the pith. The activity of the young tissues, abnormally stimulated, soon fill this fissure with a mass of small, angular parenchyma. The rapid division of these cells forces the exposed edges of the cortex and central cylinder apart so as to form a wedge-shaped opening which is filled up with the gall mass (Fig. 71). It should be stated that the newly formed cells originate mainly from the division of a cambium bordering the pith at the bottom of the fissure. But other tissues also respond to the stimulation initiated by the ovipositor of the insect. Thus a section of the stem at a short distance from the gall shows that the outlying cambium has become abnormally active and has produced a layer of bast nearly one-third thicker than that found in the normal stem. Likewise the activity of a cork-cambium layer has thrown off a strongly cuticularized epidermis present in the earlier developmental stages.

Undescribed Sawfly Gall (Euura N.S.) on Salix serissima Fernald.

This gall is produced by the abnormal swelling of the petiole of the leaf. The leaves infected are those borne on the branchlets from which spring the pistillate catkins. In the majority of cases, the leaf that bears the gall is the one from the axil of which the peduncle of the catkin arises. The swelling is so close to the branchlet that after the leaves have fallen, the gall appears to have originated from it. This misleading appearance is due to the petiole of the leaf breaking just above the gall. The galls are cone-shaped with the apex towards the blade of the leaf. They are marked deeply by three or four grooves that meet at the tip. 

Dimensions: — Height of gall 7-8 mm.; diameter at base 6-7 mm. 

The anatomy of this gall presents a very compact tissue, owing to the cells being placed close together without intervening air spaces. The very much thickened cuticle of the epidermis is the greatest departure from the normal tissue. Of the three bundles of the normal petiole two are lacerated by the ovipositor of the insect, as shown in Figs. 69, 70. In the mature gall the halves of these two are found in four widely separated regions (Fig. 69), owing to the abundant production of tissue between them. Indeed practically all the abnormal tissue is produced from the undifferentiated cells stimulated by the cutting of the bundles.

Pontania pomum Walsh.

"On Salix cordata (and very rarely on S. discolor). A smooth, fleshy, sessile, globular or slightly oval, monothalamous gall, resembling a miniature apple, .30 to .55 inch in diameter, growing on one side of the midrib of a leaf, and extending to its edge or sometimes a little beyond it. The principal part of the gall generally projects from the under side of the leaf, and only about one-sixth of its volume from the upper side, although very rarely it is at most equally bisected by the plane of the leaf. Scarcely ever more than one gall on a leaf and very rarely two of them, more or less confluent so as to seem like one kidney-shaped gall. Ex- ternal colour greenish-yellow, generally with a rosy cheek like an apple especially on the upper surface and often with many dark little dots on its surface." — Walsh .

The ovipositor of the producer of this gall has been thrust laterally through the midrib of the leaf into the mesophyll. The wound has completely severed the bundle of the midrib, as seen in Fig. 76. 

The full-grown gall presents an epidermis with a very thick cuticle. The remainder of the gall consists of a complex of thin-walled cells arranged so as to constitute a typical aeriferous tissue (Fig. 77). A similar arrangement of cells is not found in the normal leaf, the mesophyll of which consists of a fairly compact tissue. The vascular strands growing out from the wounded bundle form a complete ring around the gall, situated about half way between the epidermis and the centre. 

I was successful in obtaining this gall at such an early stage that the egg membrane was still unbroken (Fig. 76). This phase shows that the epidermis, the palisade and the spongy parenchyma mutually take part in the gall production. Counting along a line passing through the centre, four of the cell layers are seen to have arisen from the lower epidermis and six from the upper, eight from the palisade and fifteen from the spongy parenchyma of the leaf. Hence it is noteworthy that the new tissues are not the product of a cambium but have been contributed to by every morphological region of the leaf. The cells that are produced at this stage are in rows generally in exact alignment with the cells from which they have arisen. They thus do not have the arrange- ment of the aeriferous tissue of later stages to which they give rise. The cuticle, so marked a feature of the older stages of the epidermis, is exceedingly thin. The epidermis bears trichomes springing from the bottoms of deep pits (Fig. 76). This condition has arisen through the circumstance that the primary epidermal cells from which hairs have grown out have not experienced the periclinal divisions participated in by their fellows and so have been left far below the general surface as shown in the text fig. below.

Pontania desmodioides Walsh.

"On Salix humilis. A smooth, flattish, fleshy, sessile, yellowish- green monothalamous gall of a semicircular outline, the chord of the semicircle adjoining the midrib of a leaf; its general shape like the seed of a Desmodium or like the so-called "quarter" of an orange, the thin inside edge of the "quarter" closely hugging the midrib of the leaf, and the robust outer surface not biangulated but rounded off. No rosy cheek. The volume of the gall is generally about equally divided between the upper and lower sides of the leaf but sometimes the lower portion is rather the larger. Usually there is but a single gall on a single leaf, but occasionally there are two of them, either on the same side or on opposite sides of the midrib." — Length .23 to .50 inch Walsh.

When mature this gall shows in cross section a cavity surrounded by a peripheral layer of little differentiated tissue. The epidermis has given rise to a very thick cuticle that is not present in the normal leaf. The bundle of the midrib has been injured only slightly by the ovipositor of the producer. The vascular strands given off from it almost encircle the gall along a line half way in from the epidermis. 

A stage of the gall so young that the larva was unhatched shows the gall tissue to have been produced by cell division in the upper epidermis, the spongy parenchyma and the palisade parenchyma of the normal leaf (Fig. 80). At the thickest part of the gall, when it is in this stage, the upper epidermis has produced four layers of cells, the spongy and palisade parenchyma seven layers each. The lower epidermis has not divided as yet, and probably takes no part in the production of the gall. The abnormal cells from the palisade parenchyma show clearly their origin by their arrangement in rows at right angles to the surface of the leaf. The cells produced by the spongy parenchyma, on the other hand, are not regularly placed but include air spaces. The result is that the abnormal tissue in this case also resembles the normal tissue from which it is derived. 

This stage of the gall shows that the cavity present in the mature gall has arisen between the tissue produced by the spongy and that derived from the palisade parenchyma of the normal leaf. 

Pontania hyalina Norton.

"Fleshy galls occurring in two parallel rows, one on either side of the midrib, sometimes touching but not originating from the latter, and rarely extending to the edge of the leaf; sometimes as many as twenty on a single leaf; in other cases confined to a row on one side of the leaf or occasionally occurring singly; shape irregular elongate-ovate, projecting equally on both surfaces of the leaf; length 7 to 10 mm.; the abortive ones smaller. Colour on upper side more or less brownish red ; beneath white, with slight purplish tinge." — Marlatt.

The anatomy of this gall presents scarcely any differentiation of tissue. When mature it consists of a mass of thin-walled chlorophyll- bearing cells, the innermost of which are arranged in rows almost at right angles to the blade of the leaf, as seen in Fig. 75. Cells are so much alike that they afford no clue as to their origin. 

Again I was able to obtain material so young that the larva was still confined within the egg membrane (Figs. 73, 74). It shows that the spongy parenchyma, the palisade parenchyma and the epidermis of the normal leaf were jointly concerned in the production of the abnormal tissue. The spongy parenchyma has contributed nearly half of the entire mass, the epidermes three layers each, and the row of cells immediately overlying the lower epidermis three layers. The remainder has been derived from the palisade parenchyma. 

Undescribed Sawfly Gall on S. lucida Muhl.

This gall consists of an enlargement of either the petiole or midrib of S. lucida. Neither of these organs bears, as a rule, more than one gall at a time, but occasionally the petiole of a leaf carries two or even three and the midrib in rare instances two. 

The midrib galls are fairly regularly elliptical in outline with the shorter diameter across the leaf. The swellings in most cases are nearly equally divided between the upper and the lower leaf surfaces. The petiole galls vary from spherical to ovoid in shape. In the latter case the smaller end of the gall is towards the apex of the leaf. 

Dimensions of the gall: — Longer diameter 6-12 mm.; shorter diameter 3-7 mm. 

The very marked proliferation of tissue in this gall is not accompanied by a differentiation that presents many points of interest. The cells are larger than those of the normal leaf and the nuclei are correspondingly larger. The bundle is cut nearly through by the ovipositor (Fig. 82). The free ends of the bundle thus stimulated grow out until in some cases they almost surround the gall. This elongation is produced in part by the increased diameter of the vessels but also by the production of new cellular elements. 

The pith, exposed by the cutting of the bundle, produces almost all the abnormal tissue (Figs. 83, 84), but the cortex contributes some. The cells are arranged in curved lines that pass across from one elon- gated end of the bundle to the other. Between these rows are many air spaces which are elongated in the direction of the lines of cells (Fig. 84). 

Undescribed Sawfly Gall on Salix humilis

This is a monothalamous gall produced by the abnormal swelling of the leaf petiole of S. humilis. It is conoidal in shape with a long tapering apex which is towards the blade of the leaf. As it is situated at the base of the petiole the uniform enlargement of that organ is often pre- vented by the axillary bud. This causes the gall to project to a greater extent on the outside of the petiole and produces an irregularity in the outline of the gall. The surface is quite glabrous in spite of the fact that the epidermis of the leaf is decidedly pubescent. 

Dimensions : — Length of gall 6-9 mm. ; width 3-4 mm. 

Nearly the entire mass of this gall originates from the vascular bundle of the petiole which has been stimulated to activity by the in- sect's ovipositor. The small thin-walled cells of the gall substance spring from a cambium layer produced in the pith of the bundle near the ovipositor wound. The cells arising from this tissue force the severed ends of the bundle apart until the vascular elements form only a narrow line of cells between the gall proper and the cortex of the petiole; it is shown at this stage in Fig. 85. This cortex is not materially thickened but shows signs of stimulation in that there appears a small amount of aeriferous tissue located near the place of entrance of the ovipositor. This tissue is shown on each side of the wound in Fig. 85. This tissue was not found in the normal cortex of the petiole. The cuticle is very much thickened. 

Undescribed Sawfly Gall {Pontania N.S.) on Salix humilis Marsh. 

This is a monothalamous gall found on the leaves of Salix humilis. It is spherical in shape and in that feature resembles P. pomum, but in other respects it differs very markedly. It extends from the side of the midrib almost out to the margin of the leaf and is divided into two hemispheres by the leaf blade. In consequence the gall protrudes nearly equally from each leaf surface. Usually there are two or three galls on a leaf. When two are present they come in contact with the mid- rib at the same place but on opposite sides, as illustrated in Fig. 79. In a few cases four and even five galls were found on one leaf. The galls are pubescent but not as densely as the under surface of the leaf. 

Dimensions: — Average diameter 1 cm. 

The mesophyll of the leaf and the upper epidermis are mutually concerned in the production of this gall. In one, sufficiently immature to show the relative amount of tissue arising from each source, it was found that the upper epidermis had produced two cell layers, while the lower had not responded to stimulation ; and that the palisade and spongy parenchyma had each produced one-half of the remaining mass. The hollow in the gall, present from the earliest stages, has been formed between the tissue arising from the palisade and the spongy parenchyma respectively. 

When only one gall originates from the midrib at any point, the vascular bundle is cut approximately half through (Fig. 78). But more frequently two galls are located opposite one another, one on each side of the midrib, in which case the two incisions meet and completely sever the bundle, as seen in Fig. 79. 

Vascular strands pass almost completely around the gall, along a line half way between the epidermis and the gall cavity. These strands originate from the midrib in the neighborhood of the injury and pass in opposite directions. 

A contribution to the morphology and biology of insect galls