On October 29th and November 6th, 1864, I colonized a number of these agamous gall-flies, that I had bred myself from oak-apples, upon three different isolated black oaks, that I knew to have not been previously infested by these galls for many years back. Two of these trees were very large--say about 2.5 feet in diameter at the butt--and I placed the gall-flies upon one particular overhanging bough of each of them, and on no other part of the tree. The third tree was small--say 1 foot in diameter at the butt--and I placed the gall-flies on the trunk of this tree, at the point where the main branches took their origin.
On May 21, 1865, I examined all these three trees. The first large tree had no galls at all on it. The second large tree had produced four Q spongifica galls, partly on the very bough on which I had placed the gall-flies in the preceding autumn, and partly on some boughs that immediately adjoined it. I estimated that the portion of this last tree thus occupied by galls did not form more than one-twentieth of the whole tree; so that, even if we suppose that one or more wandering Cynips q aciculata had flown on to this tree from the neighboring woods in the preceding autumn--and this insect, coming out as it does so late in the year, flies as reluctantly and as dully as a plant-louse--the chances are about 19 to 1 that they would not have occupied the particular portion of it found to bear galls in the following spring. On the other had, as there certainly is not more than one-hundredth part of the black oaks growing near Rock Island that produce any oak-apples at all in any particular year, the chances are at least 99 to 1 against any particular black oak there, taken indiscriminately, bearing oak-apples in any particular year. Consequently, the compound chance of any particular black oak, taken at random from those growing near Rock Island, not only bearing them in a particular year, but bearing them exclusively upon a particular portion of its boughs, previously designated and forming only one-twentieth part of the entire mass of its boughs, is, according to the theory of chances, 1/2000; or in ordinary parlance, the chances are 1999 to 1 against such an improbable event happening. But these chances are founded on the supposition of the tree that was experimented on having been taken at random without any selection; whereas, instead of taking it at random, I exercised the greatest possible care in every instance, 1st, to select a black oak that grew a long way off from any other black oaks, and 2nd, to select one that I was familiar with and had watched for years, and knew not to have borne any oak-apples for several years preceding.
The third black oak, visited on May 21, 1865--which was the small one--was absolutely loaded down with galls, and I estimated their number at 50 or 60 at least. At this early period all these galls were still small and immature, and it was necessary to leave them for a week or two upon the tree to ripen and mature.
On June 5th, 1865, I climbed the small gall-bearing black oak, and stripped it of every gall that I could see. From it I harvested only 18 normal spongifica galls, exclusive of 2 or 3 that had been destroyed by lepidopterous larvae, and about 40 specimens of a particular form of gall (Q pseudotinctoriae, Walsh) which occurs commonly but sparingly among the normal Q spongifica galls, and also, in a slightly modified type, upon red oaks infested by the Q inanis gall. For a long series of years this Q psuedotinctoriae gall has been a great puzzle to me; for, whether obtained from black oak or from red oak, although I have bred from hundreds of them, and have kept them on hand for years, I have invariably bred nothing from them but great numbers of a very large and very handsome Chalcis fly, belonging to the Pteromalides with concealed ovipositors, which I have never reared from any other gall, and a few stray specimens of such Chalcis flies (Callimome and Eurytoma) as I have bred also from the normal Q spongifica galls. My friend Baron Osten Sacken, to whom I had before this period communicated this peculiar form of gall, suggested that it was a true Q spongifica gall, modified by the action of the parasite that inhabited it; and the negative fact that I could never breed anything but parasites from it, after experimenting with hundreds of specimens in three or four different years, compels me to acquiesce in this most anomalous and, so far as I am aware, unprecedented conclusion. The gall in question never exceeds 0.85 inch in diameter, while the normal Q spongifica gall often attains a diameter of 1.75 inch, and is shaped like a normal gall, except that it is often studded outside with sharp prickle-like tubercles similar to those of the exotic gallae-tinctoriae gall--whence the name that I have given it. The central cell is round and about 0.2 inch in diameter, with an external crust which is only about 0.02 inch thick, instead of forming a dense woody mass as in the normal form. The external crust of the gall itself is similar to that of the normal gall; but, instead of its being connected with the central cell by homogeneous spongy matter, with a few subobsolete slender radiating filaments among it, as in that gall, it is connected with the central cell solely and exclusively by dense, opaque, coarse, whitish cottony fibers, radiating from the central cell, as in the Q inanis gall, but differing widely from those of that gall by being very much coarser, by being cottony instead of smooth, and by being placed so close together as to occupy the whole space between the cell and the external crust of the gall, instead of being separated from each other by very wide interspaces. On my cutting into 24 galls that remained unbred from, on this 17th day of March, 1869, out of the above-mentioned lot of about 40 Q pseudotinctoriae galls harvested June 5th, 1865, either of them were found to contain dead Pteromalidous imago already spoken of, seven what was probably its mature dead larva, one what was probably its mature dead pupa, one the pupal shell of a female Callimome, one a Eurytoma studiosa female, Say, and in six the tenant of the cell must have perished in early life, for in these six the central cell was empty.
On the supposition of the peculiar character of the Q pseudotinctoriae gall being caused by the action of the large, Pteromalidous parasite that generally, but not always, inhabits it, and never inhabits the normal type of gall, it may be asked how it happens that this very same Q pseudotinctoriae gall sometimes produces the same species of green Callimome which is commonly bred from the normal gall, and occasionally a Eurytoma, which is also bred occasionally from the normal gall? I can only suggest that, in these two latter cases, the Callimome and the Eurytoma are parasitic upon the large Pteromalidous parasite, and that the peculiar character of the gall was determined in the first instance by the Pteromalidae. The Chalcididae are, to a much greater extent than is commonly supposed, secondary and not primary parasites.
From the 18 normal Q spongifica galls, obtained on June 5th, 1865, as specified above, from the small black oak, I bred on June 11th 1865, one C q spongifica female, and another female of the same type on June 14th, 1865. On cutting into the remaining 16 galls, on October 15th 1865, I obtained five living C q aciculata. Thus it results that q aciculata generates sometimes q aciculata and sometimes q spongifica.
Lest it should be supposed that there was any reasonable probability of this small black oak having been attacked by any other gall-fly producing upon black oaks the Q spongifica gall besides those that I placed on it myself in the preceding autumn, it is proper to add here that it grew on the Bluffs, where black oaks are very scarce, and that I am quite confident that there is not another black oak within a quarter of a mile of it. So rare, moreover, are the galls Q spongifica upon the Bluffs, that in the course of six years' careful observation I have only noticed there three or four black oaks bearing these galls, and even then there were only from 3 to 6 on a tree; whereas on the sandy bottom land, which swarms with black oaks, these galls are comparatively quite common.
If, however, we choose to believe that the very same insect that produces the Q inanis gall also produces the Q spongifica gall, then the above mode of reasoning will not apply with such force; for the Q inanis gall, and the red oak on which it grows, are nearly as common on the Bluff as the Q spongifica gall, and the black oak on which it grows, are on the bottom land. Still, even upon this hypothesis, it is exceedingly improbable that Cynips q inanis should have attacked this particular small black oak in the spring of 1864; for 1st, I know that this tree bore no oak-apples for many years previous to 1865; 2nd, there were no red oaks growing anywhere within two or three hundrd yards of it, the few oaks that grew near it being either white oak or bur oak, which never produce either the Q spongifica or the Q inanis gall; 3rd, I noticed that, in the spring of 1866, this very same small black oak swarmed again with the Q spongifica gall, almost as abundantly as in the spring of 1865. Doubtless these galls had been generated by gall-flies that escaped from some of the galls before I harvested them on June 5th, 1865, or from green galls that had previously fallen off the tree on to the ground, as they will very often do in very great numbers when there is a high wind blowing, and be carried along the surface of the ground for hundreds of yards by the action of the wind; 4th, if we assume that the two galls that had produced C q spongifica June 11th and 14th, 1865, had been generated by C q inanis, and that only the five galls that produced C q aciculata October 15th, 1865, had been generated by the C q aciculata that I placed on the small black oak in the autumn of 1864, how does it come about that the Q spongifica gall is so very rare on those very Bluffs where the Q inanis is so common? Surely if C q inanis, bred from red oak, is capable of generating the Q spongifica gall on black oak, the Q spongifica gall ought to be as numerous on the Bluffs, in proportion to the number of black oaks growing there, as it is on the sandy bottom land, whereas it is no such thing. 5th, on the hypothesis of C q inanis generating Q spongifica galls, there again recurs the inevitable question "Why does C q inanis, if it is specificaly identical with C q spongifica, produce swarms of an autumnal dimorphous female--C q aciculata--on the black oak, and none at all on the red oak?" Or shall we take refuge in the anomalous hypothesis that one and the same bisexual species, variously known as C q spongifica sexgen and C q inanis sexgen, produces two such entirely different galls as Q spongifica and Q inanis upon black oak and red oak respectively, the same type of gall being always found upon the same species of oak; and that a distinct agamous species--C q aciculata--generates upon the black oak, galls which are utterly undistinguishable from those of C q spongifica upon the same oak, and which occur upon the same oak promiscuously intermixed with these last galls in scores of different localities, and yet that this agamous species never under any circumstances generates any galls at all--whether of the Q spongifica type or of the Q inanis type--upon the red oak? To such a supposition I can only oppose what, from long experience with galls of all kinds, I consider as an established axiom; namely, that the characters of the gall depend entirely upon the insect that makes it, and in no wise upon the plant, or the particular part of the plant, from which it grows. Consequently, I could as readily believe that a cow could produce sometimes a calf and sometimes a lamb, as that Cynips q inanis could produce sometimes a Q inanis gall upon red oak, and sometimes a Q spongifica gall on black oak. If it produced any gall at all upon Black Oak, instead of upon red oak which is its normal habitat, it would inevitably, in my opinion, produce a gall having all the characters of the Q inanis gall that is commonly found upon red oak.
I am well aware that much of the above reasoning will lack its due weight with the reader, because he has not, as I have, watched particular trees in a grove of Black Oaks swarming with oak-apples for year after year, while the neighboring trees bear none at all, or only a few scattering specimens, and because he has never seen, as I have twice seen with astonishment, that even a particular bough on a particular tree will bear numerous oak-apples for year after year, while the rest of the tree will bear none at all. Hence I have derived a profound conviction that the gall-flies that make these oak-apples, although they have full-sized wings, yet scarcely ever use them; whereas persons who are unacquainted with these insects would naturally suppose that they fly about the woods as freely as a bee or a butterfly. Out of the thousands that I have bred in my office, I never knew a single individual, whether of the vernal or of the autumnal type, to take wing at all; and only on one or two occasions, when I have been placing them upon oaks to experiment on the laws of their reproduction, have I seen one or two of them take wing, and then it would only fly a yard or two.
On June 11th, 1865, I gathered the four galls off the large gall-bearing Black Oak previously referred to. From these I bred no q spongifica at all; but on cutting into them on October 15th, 1865, I obtained therefrom three living C q aciculata.
On October 16th, 1865, having now in my possession two lots of living and lively q aciculata, one consisting of 5 females and the other of 3 females, that I knew to be generated by q aciculata of the preceding season, I determined to see whether they would all or any of them continue to generate q aciculata in the succeeding season, or whether, as had been the case with two of their predecessors, they would revert one or more of them to q spongifica. I therefore placed them, each lot by itself, on two fresh isolated black oaks, that I knew to have not been previously infested by these galls, for several years back at all events.
On May 31st, 1866, I gathered off one of these two black oaks, upon which I had colonized the 5 q aciculata in the preceding autumn, 5 q spongifica galls, four of them badly eaten by lepidopterous larvae, and only one in a perfect state. They were at this date too young and immature to gather with safety, but I feared to leave them longer on the tree on account of the caterpillars, which will very frequently eat away all the sponge and starve out the larva in the central cell. From this lot of 5 galls, generated by 5 C q aciculata in the preceding autumn, which 5 C q aciculata had themselves been generated in the autumn next but one preceding by the 8 C q aciculata that I had colonized upon another isolated black oak, I bred, on the 14th and 17th of the ensuing June, 2 Cynips q spongifica females. The remaining 3 galls produced nothing.
The other isolated Black Oak, upon which I had
