The Gall Wasp Genus Cynips: A Study in the Origin of Species/Basis of Present Study
BASIS OF PRESENT STUDY
The present study is based upon more than 17,000 insects and an estimated 54,000 galls—a total of over 71,000 specimens representing the 93 species known in the genus Cynips. Every one of the insects has been examined under the higher powers of a binocular microscope. Thousands of the specimens have been repeatedly re-examined in direct comparison with every specimen whose affinities might in any way throw light on the interpretation of the characters under observation. Exactly 16,899 of the insects and nearly all of the 54,000 galls of the genus are in our own collections where they have been available for all the comparisons and endless re-comparisons that have been necessary during the four years of intensive study given to this group of species. There are 96 Schmitt boxes of the mounted material of Cynips which constitute the permanent validation of our conclusions on this group, and this collection will be available at all times to future students. Part of our material is being distributed among museums and other workers on Cynipidae. Outside our own collections, I have studied the Cynips material in the American, U.S. National, Philadelphia Academy, Harvard (Museum of Comparative Zoölogy), Field, Illinois Natural History, and Stanford Museums.
The specific distribution of the material used in the present study is as follows:
| Species | Insects Examined |
Galls Examined | ||
| Agama (1 var.) | 57 | 177 | ||
| Arida (1 var.) | 24 | 406 | ||
| Bella (3 var.) | 56 | 1,182 | ||
| Cava (1 var.) | 54 | 157 | ||
| Centricola (4 var.) | 118 | 245 | ||
| Conica (1 var.) | 3 | 38 | ||
| Cornifex (1 var.) | 4 | 12 | ||
| Divisa (2 var.) | 1,090 | 1,092 | ||
| Disticha (1 var.) | 13 | 5 | ||
| Dugèsi (4 var.) | 110 | 360 | ||
| Echinus (6 var.) | 589 | 6,460 | ||
| Folii (4 var.) | 646 | 935 | ||
| Fulvicollis (7 var.) | 5,170 | 14,581 | ||
| Gemmula (4 var.) | 746 | 547 | ||
| Guadaloupensis (3 var.) | 18 | 67 | ||
| Hirta (7 var.) | 1,457 | 5,240 | ||
| Longiventris (2 var.) | 129 | 355 | ||
| Mirabilis (2 var.) | 26 | 393 | ||
| Maculosa (2 var.) | 29 | 70 | ||
| Mellea (11 var.) | 431 | 1,010 | ||
| Multipunctata (1 var.) | 175 | 1,189 | ||
| Nubila (3 var.) | 64 | 185 | ||
| Pezomachoides (8 var.) | 5,890 | 18,135 | ||
| Plumbea (1 var.) | 30 | 38 | ||
| Teres (3 var.) | 55 | 180 | ||
| Villosa (7 var.) | 367 | 1,400 | ||
| Totals—93 varieties | 17,351 | insects | 54,460 | galls |
We should have had thousands of specimens of every one of these insects. Failing that, our conclusions on certain matters have been based to a large degree upon the more available species, and the intensive study of those species has offered interpretations of our smaller bodies of data which would not otherwise have been possible.
To obtain data on geographic distribution and the relation of geographic isolation to the origin of species, I have attempted to secure representative series of each Cynips from as many and as widely distributed localities as possible. It was in the fall of 1917 that I made my first collections of the genus, and in the twelve years that have intervened I have travelled over thirty-two thousand miles in the pursuance of the distributional data originating from my own collections. In 1919 and 1920 I went as a Sheldon Travelling Fellow of Harvard University to the South and to the Far West. For a number of years I have been relieved from part of my teaching to pursue research under a grant of the Waterman Institute of Indiana University, and in this connection I have engaged in extensive field work thruout the northern Middle West and in the southeastern quarter of the United States. In the course of these contacts with gall wasps in the field, as well as thru the longer hours spent over the microscope, I have gradually developed my present concept of species.
My own cross-country field work has been supplemented with collections made thru every season and in some cases for several years by collectors working in their native areas. Over a hundred such collectors have contributed data, sending material from localities in nearly every one of the United States and from England, France, Germany, Denmark, Italy, Hungary, Austria, Czecho-Slovakia, Finland, and Russia among European countries. This coöperation has been indispensable, and specific acknowledgment of my indebtedness to each collector is made with each item in the systematic portion of this paper. The accompanying map (Fig. 6) will show the extensive—albeit still inadequate—sources of our American material of Cynipidae.
The accumulation of these geographic data has received some impetus from repeated analyses we have made of our field technic. By using the automobile we reach twice as much territory in a given time as we were formerly able to reach by railroad transportation. We find that the densest populations of Cynipidae are, for some reason not yet apparent to us, located on isolated oak trees or at edges of woodlands, and it is there that we now concentrate most of our efforts while in the field. It was some years before we learned that the distribution of most species of gall wasps is very local, and that it is a waste of time to attempt to make collections of populations that are sparsely scattered over the countryside. If large populations are not readily available at the first stopping place, we take a sample and move a hundred yards to a half-mile across country, continuing the procedure until we have discovered some tree or thicket heavily laden with galls. Many a rural community will bear witness to the peculiarities of our conduct in their countryside, but our cabinets of insects and galls bear more eloquent testimony to the effectiveness of this method.
Because of the peculiar host relations of the higher Cynipidae, it is necessary to collect from every species of oak in each locality in order to obtain a satisfactory set of specimens. To avoid being misled, in our interpretations of species, by such local populations as might segregate into Mendelian races, we make it a point to secure material from separated trees of each host wherever possible. Our returns are more than doubled when two of us work together in the field. We have given from a few hours to several days to the exploration of each locality, but upon careful accounting we find that, with two or more of us working together, we can take a fair sample of a region in four or five hours.
In order to secure approximate data on the furthest extension of the range of each species, we do not make our collecting stations too near each other. Usually they are not less than fifty miles apart. In some instances it has been possible to revisit important regions and make further collections which would more precisely establish the limits of ranges, or give more extended data on the variation of species in transition areas. As already noted, this cross-country field work has been supplemented by year-round collections from many localities.
Since the coördination of taxonomic studies depends upon an accurate understanding of the Latin binomials that have been used for previously described species, I have made a thoro study of the holotypes—the single specimens which are absolute criteria of the correct application of each published name—for 76 out of the 78 American species of the genus. Practically all of these holotypes were studied after this study had advanced toward completion and when it was possible to make critical comparisons with my own material. Of the 15 European forms in this genus, I have seen types of four. The types of seven are not in existence, and the remaining five are established by types which I have not seen. Wherever I have not seen the types, the original descriptions are quoted in full in the systematic portion of this study.
Finally, among the material resources available for the present study mention should be made of the technical assistance and museum equipment that has been available for several years chiefly thru the interest of the Zoölogy Faculty and the Administration of Indiana University and grants elsewhere acknowledged from the Waterman Institute of Indiana University, from the Elizabeth Thompson Science Fund, and from the Bache Fund of the National Academy of Sciences.
These are the bases of the present study of the genus Cynips. This is our warrant for undertaking an interpretation of species in this group of insects. We offer no further extenuation of the fact that our conclusions are not in accord with systematic work that has been based on more meager material. We believe it no coincidence that our conclusions more nearly accord with those of Dunn who studied 12,600 specimens of the 86 forms of the salamanders of the family Plethodontidae, or of Mickel who studied approximately 10,000 specimens of the genus Dasymutilla. In handling even such large numbers of individuals of a limited group of species, one becomes convinced that accurate information on the variability or constancy of species over wide areas, the extent of host and geographic isolation between related species, the correlation of biologic and structural facts, and the attainment of sound conclusions as to the affinities of species within any group and consequent interpretations of species and biologic phenomena cannot emanate from anything short of the most extensive data that modern facilities can provide.
FIG 6. AMERICAN COLLECTIONS OF CYNIPS