Smithsonian Miscellaneous Collections

SMITHSONIAN MISCELLANEOUS COLLECTIONS VOLUME 80, NUMBER 11 THE LEGS AND LEG-BEARING SEGMENTS OF SOME PRIMITIVE ARTHROPOD GROUPS, WITH NOTES ON LEG-SEGMENTATION IN THE ARACHNIDA (With 12 Plates) BY H. E. EWING Federal Bureau of Entomology (Publication 296J1 CITY OF WASHINGTON PUBLISHED BY THE SMITHSONIAN INSTITUTION APRIL 23, 1928 Zf)c Bovb QSaUtmore (preee BALTIMORE, 'MD., U. S. A. THE LEGS AND LEG-BEARING SEGMENTS OF SOME PRIMITIVE ARTHROPOD GROUPS, WITH NOTES ON LEG-SEGMENTATIO'N IN THE ARACHNIDA By H. E. EWING Federal Bureau of Entomology (With 12 Plates) CONTENTS PAGE Introduction 2 primitive The arthropod appendage 3 The primitive arachnid leg 4 Legs of Solpugida 4 The leg segments of primitive phalangids 6 The leg segments in some of the Acarina 7 Leg-segmentation in various arachnid groups with special reference to the Pseudoscorpionida 8 The Pycnogonida and Ricinulei 10 Conclusions in regard to arachnid limbs 11 Leg-segmentation and musculature in the Protura 12 Leg-segmentation and musculature in Pauropoda 14 The so-called coxal appendages in Pauropiis 15 The Pauropus type of leg 16 Leg-segmentation and muscle attachments in Symphyla 16 The coxal appendage in Symphyla 18 The legs in Thysanura 18 Remarks on the thysanuran type of leg 20 The nature of the coxal appendage in the Machilidae 20 The leg segments of Collembola 20 The collembolan type of leg 22 The primitive type of tarsal claws in Insecta 22 Postcephalic body-segrrtentation in Arthropoda 24 The first body segment of Symphyla 25 The first body segment in Pauropoda 25 Evidence of a cervical segment among insects 26 The segments included in the insect thorax 26 Abdominal segmentation 27 The doubling of body segments 27 Embryonic reduction of abdominal segments 28 The tergal plates of the leg-bearing segments 28 Conclusions regarding the terga of leg-bearing segments of primitive arthropods 3° SmithsonianIMiscellaneous Collections, Vol. 80, No. 11 I 2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o PAGE The sternal plates of the leg-bearing segments 30 Sterna in Arachnida 30 Sterna in Pauropoda 31 Sterna in Symphyla 31 Thoracic sterna in Protura 32 Thoracic sterna in Thysannra and Collembola 32 Pleurites in leg-bearing segments 34 The origin of insect pleurites 35 General conclusions 36 References to literature Z7 Abbreviations used on plates 39 Explanation of plates 40 INTRODUCTION In attempting to work out the homologies of leg segments in a number of the different groups of arthropods, an effort has been made to find the generalized type for such classes as the Crustacea, the Arachnida, and the Insecta, and by comparing intergrades with these established types to interpret the homologies as far as possible for each group. In such a comparison the position of the segment in the series is determined, but the procedure of prime importance is the definite establishment of a known segment in the series which may be used as a point of reference. Different methods have been employed for the establishment of a " land mark " segment or, what amounts to the same, a "land mark" articulation. Borner (1921) lays great stress on the bend of the leg at the knee, usually regarding such a bend as taking place at the distal articulation of the femur. Snodgrass (1927) uses chiefly the trochantero- femoral articulation in insects as a starting point. In the Arachnida, where legs appear to show the maximum number of true segments, eight in addition to the claws, it is assumed that all segments are present. Such being the case one may begin at the base of the leg in legs of this sort by calling the first segment the subcoxa. Where there is a reduction of the number of leg segments, it may yet be possible to establish the homology of the remaining segments if the method of reduction can be established. In insect legs it appears, according to the results of recent investigation, that the tarsus never gives rise to muscle fibers, the extensor claw muscle having been lost, and the flexor taking its origin from the tibia or" from the tibia and other segments proximal to it. Using this as an established principle the writer has in many instances regarded the last segment giving rise to muscle fibers as the tibia in the case of insects and also in NO. II LEGS OF PRIMITIVE ARTHROPODS—EWING 3 primitive groups related to the insects. Nevertheless the trochantero- femoral articulation should be regarded as a " landmark " of great importance in interpreting the segmentation of insect legs. The question of the segmentation of the body and the segmentation and musculature of the legs in primitive arthropods has been approached from the standpoint of a taxonomist, using that term in its broadest sense as applying to one who has been interested in classi- fication primarily as bearing on natural relationships and evolution. It is lamentable that there is so frequently shown a lack of cooperation and helpfulness between the taxonomist and the morphologist. The separation between these two groups of workers has in some instances become so great that not only is there a lack of the proper cooperation in conducting the investigations, but the taxonomist does not even read the literature dealing with the morphology of his own group, and the morphologist seldom consults taxonomic papers, even outstanding monographs. The result is that taxonomy and general entomology have suffered greatly from a multiplicity of terms, from the use of several terms to indicate the same structure, and in con- sequence many have become befogged in regard to obvious facts in evolution and relationships. Before passing to the results of this investigation the writer wishes to express his indebtedness to R. E. Snodgrass, whose recent treatise on the " Morphology and Mechanism of the Insect Thorax " (Snod- grass, 1927) has been of great help to him in undertaking this investigation. THE PRIMITIVE ARTHROPOD APPENDAGE Two questions of importance arise concerning our concept of the primitive type of arthropod limb : Was it a simple, filiform, unbranched structure or was it biramous ; and how many true segments did it contain? There has been a general tendency to look to the Crustacea in searching for the answer to these questions, and doubt- less with much reason, yet the writer is of the opinion that a study of the arachnid limbs is eqvially important if not more so. Probably the Crustacea have been chiefly considered because of their supposed relationship to that very large and very important class, the Insecta, and because of their evident affinity with the much studied, ancient and extinct group, the Trilobita. Hansen (1893 & 19^5), in two highly important papers, has led the way in his investigations in regard to the search for the generalized type of appendage for the Crustacea. He pointed out that an addi- 4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o tional segment should be recognized in the unbranched base of the typical crustacean limb, and he recently (1925) holds to the possibility that in a primitive state the crustacean limb was uniramous. He " states : But it seems to me impossible to deny the possibility that the exopod may be anologous with the epipod, and if so the primitive appendage is uniramous." Hansen's contention that the basal or un- divided part of a crustacean limb should be considered as three- segmented instead of two-segmented is recently criticized by Bor- radaile (1926) who states in his paper: "From the foregoing con- siderations it is evident that the recognition of a three-segmented protopodite in the Crustacea is purely empirical, for the actual third segment is not homologous in all cases, being sometimes the third segment of the primary series, sometimes compounded of the third and fourth, sometimes the fourth alone, and probably in a few instances compounded of the fourth and fifth, the actual second segment in the latter two cases being compounded of the primary second and third." According to Hansen (1925) the crustacean limb is composed of eight true segments which he names as follows from the body : Prae- coxa, coxa, basis, ischium, merus, carpus, propodus and dactylus. If the old terminology were employed these would be: Pleuropodite, coxopodite, basipodite, ischiopodite, meropodite, carpopodite, propo- dite and dactylopodite. THE PRIMITIVE ARACHNID LEG Because the ancient group Arachnida has apparently evolved from extinct types of the same sort as those that gave rise to the Crustacea, it is apparent that a study of the appendages of the more generalized arachnid groups should throw light on the controversy in regard to the leg type for the primitive Crustacea. In working on the homology of arachnid legs the writer follows Hansen's interpretation of the Crustacean leg by calling the first segment in the generalized leg the subcoxa. LEGS OF SOLPUGIDA The Solpugida, or sun spiders, have long been considered as being one of the most generalized of the arachnid groups. This is due particularly to the fact that in this group some of the cephalothoracic segments remain free and movable. Much research has been done on the body segmentation in the solpugids and on the morphology of certain specialized organs. The legs, however, have not attracted the attention that they deserve, although it has long been known that NO. II LEGS OF PRIMITIVE ARTHROPODS EWING 5 the number of segments in a solpugid leg is high, being seven or eight in addition to the claws. In eight-segmented legs (pi. i, fig. i) there is a somewhat flat- tened, short basal segment, then three short doubly-hinged cylindrical segments, followed by three long segments and lastly by the terminal claw-bearing segment, which has two or more false rings.

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