R. T. Holway.
Preliminary Note on the Structure of the Pretarsus and Its Possible Phylogenetic Significance.
Psyche 42:1-24, 1935.

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PSYCHE
--
VOL. XLII
MARCH 1935 No. 1
PRELIMINARY NOTE ON THE STRUCTURE OF THE PRETARSUS AND ITS POSSIBLE PHYLOGENETIC
SIGNIFICANCE
Biological Laboratories, Harvard University The uncertain reliability of morphological structures for determining ancestral relations is well known and the ex- treme care and attention to the vagaries of parallelism, spe- cialization, etc. so necessary for the development of valid theories have been rightly emphasized of recent years. How- ever, in spite of its late disrepute, there is yet much to be learned from comparative morphology if sufficiently long series are studied and checked with the trends of parallel structures and if a most judicious interpretation of the evi- denoe is demanded.
The primary purpose of this paper is to indicate the type of claw-segment found in the important insectan orders together with the common basic plan occurring throughout and also to outline possible homologies which further study and examination of selected and thoroughly representative series from each order may support. Although it is impos- sible at the present stage to offer any evidence substantial enough to warrant forming exact conclusions as to homolo- gies from order to order, to. say .nothing of attempting to solve phylogenetic problems, there are without doubt most significant agreements in many cases which provide definite possibilities to be tested by further study. Especially im- portant are those correlations which fall in with phylogen- etic concepts already accepted (or debated) from the evi- dence of the comparative morphology of other structures. Pacht 42:)-24 11935). http//psyche eniclub.mg/42/42-001 him)



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2 Psyche [March
For example, the concurrence of Trichoptera and Lepidop- tera is not questioned and some investigators have suggested that these two groups might'even be conceived of as one valid order. The nature of the claw-segment of Astenophy- lax (Fig. 21) and Automeris (Fig. 22) shows at a glance the proximity of these structures even in such advanced re- presentatives of the respective orders.
The relative value of a structure like the pretarsus as a phylogenetic guide may be uncertain, but the above example indicates that such consistency of the claw-segment from order to order should make it of considerable utility in checking theoretical relationships. Although the basic plan of the pretarsus presents a certain conformity throughout the orders, there are sclerites which are variable enough to warrant their investigation from the standpoint of taxon- omy. For example, within the Hymenoptera the orbicula may be important in this respect. Hayes and Kearns (1934) have studied the empodium of Coleoptera and find that the Adephaga is the only group showing any consis- tancy in structure; they conclude, in general, that there is in some a tendency towards generic or family uniformity but that the super-families are decidedly heterogeneous. Arolia and pseudarolia have long been used as the basis of classification for the Miridae and for other Hemiptera the empodium may prove to be of taxonomic value. Acknowledgements
The writer is indebted to Professor C. T. Brues and Pro- fessor G. C. Crampton for their advice during the prepara- tion of this paper, and to them and to the Museum of Com- parative Zoology for much of the material herein figured. Terminology.
The general structure of the pretarsus has been figured many times for different orders: Snodgrass gives detailed descriptions of the condition in the honey-bee (1925) and figures several other types (1927) ; Crampton (1923) com- pares the claw-segment of Periplaneta with those of Leptid and Asilid Diptera; Hayes and Kearns (1934) have studied the pretarsus of Coleoptera and summarize past work on



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193.51 Structure of the Pretarsus 3
this structure; de Meijere's investigations are more extlen- sive and cover all the important orders but his German terminology is not used at all in this country. A complete history of the terminology of the numerous parts of the claw-segment must bfe left for a later and more comprehen- sive study of this structure, therefore that system accepted by the more recent investigators is followed throughout. In some cases the same terms are used from order to order, even though true homology is not yet proved, instead of of- fering new names which in all. probability would have to be discarded aftler further study supplied evidence to substan- tiate suspected homologies.
However, all doubtful cases
are indicated as such.
The following is a list of the terms herein used with their definitions and equivalents.
Apodeme (Unguitractor tendon, Sehne) .-This is the tendon which runs from the unguitractor plate to the tibia1 musculature, the flexor of the claws? Supposedly the claws spring back into their normal position by natural resiliency when tension through the apodeme is released as there is no l'evator of the pretarsus in insects. (Crampton 1923) (Snodgrass 1929).
Arolium. In general the term arolium is restricted to a median paired or unpaired, pad-like structure which is ar- ticulated to the unguifer or arises from a position midway between the claws. Whether the complicated median struc- ture of Hymenoptera, Lepidoptera and Trichoptera, the paired ribbon-like parts known as arolia in certain Hemip- tera (Miridae), and the single median pad of Diptera and orthopteroid insects are all strictly homologous is prob- lematical. However, the term arolium is here used to des- ignatle these structures in every order in which they occur, according to the definition above.
Basipulvilli. (auxillise). This term was first applied by Crampton (1923) to the small lateral scleritlea at the bases of the pulvilli of Diptera and also to similar sclerites lat- erad of the unguitractor in Periplaneta although true pul- villi are not found in Orthopteroids. The auxillise (Hymen- optera), of MacGillivray, are evidently homologous with



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4 Psyche [ March
these structures but wherever such parts are found they are called basipulvilli in this paper.
Camera. "(Bugel, or bow of Arnhart). Camera was originally applied to the curved narrow sclerite supporting the paired lobes of the arolium in Hymenoptera (MacGil- livray). It is herein also used to designate the evident ho- mologue in Trichoptera, Lepidoptera and Mecoptera. Distitarszis. (onychium, digitus, ungula, etc.) . The dis- tal tarsal segment.
Empodium. (onychium). This term has been used for a variety of structures. Crarnpton (1923) finds that it should be restrictled to a process of the unguitractor as found in Asilid Diptera and other orders. Hayes and Kearns (1934) accept this view and apply the term in their work on Coleoptera.
Empodium is often used erroneously
to refer to a pulvillus.
Flexor membranes. These are the membranous areas be- neath the claws which apparently serve to transfer the ten- sion of the apodeme on the unguitractor to the claws thus forcing them downwards. They are the areas from which, it has been suggested, the puvilli may have developed. Gleitrinne or Gleitflache (de Meijer'e)'. A prolongation of the ventral wall of the distitarsus. It is especially devel- oped in Odonata and in some Coleoptera where it extlends forward between the claws.
Onychium.
This term has been used in such a variety of ways that it appears impossible to define it satisfactorily and therefore is not used in this paper. Orbicula. In Hymenoptera, a small dorsal sclerite at the base of the arolium and distad of the unguifer. The
orbicula is quite variable as to size and shape in this order and may be of taxonomic value.
Parempodia. (paronychia) . Bristle-like appendages of the empodium.
(Coleoptera and Hemiptera) .
Planta. As used by Snodgrass, Crampton and MacGil- livray planta applies only to the sole of the foot (typically in Hymenoptera and Orthoptera) . However, the examples



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19351 Structure of the Pretarsus 5
of most of the various orders observed show a plate which is probably homologous with the hymenopterous planta and therefore this term is used wherever practicable. Coleop- tera, Diptera, Hemiptera, Dermaptera, Odonata and Eph- emeroptera do not exhibit the planta in its typical condi- tion although an empodium is present in these orders (ex- cept Ephemeroptera) which suggests the possibility that the empodium is merely a modified planta, closely associ- ated with the unguitractor. Further study is needed to de- termine the correct relationships of these structures. Snodgrass (1929) suggests that the planta may possibly be a subdivision of the unguitractor. However, the nature of the planta under high magnification is unlike that of the unguitractor which always gives a granulose appearance, is heavily sclerotized, and is sharply demarked in every case observed 'even to the primitive Odonata and Ephemer- optera. On the other hand the planta is variable in extent and degree of sclerotization and never shows the heavy, rugose aspect of the unguitractor. It seems more likely that the planta represents merely an area of the mem- branes distad of the unguitractor which has become sclero- tized in greater or less degree according to the stress to which it is subjected by the tension of the apodeme and un- guitractor. Of course the unguitractor itself is a sclerotized area of the membrane but it is always distinct in outline and not an indefinite structure as the planta often appears to be. (Note.-Planta has also been used to refer to the basal joint of the post tarsus in pollen-gathering Hymen- optera, to the soles of the post tarsal joints and to the anal clasping legs of catferpillars) .
PseudaroZia.-Paired structure which occurs beneath the claws of some Herniptera (Miridae) and which possibly will prove to bear some relation to pulvilli. Pulvillus.-Originally referred to the pad-like structure beneath each claw in many Diptera.
Other orders in which
undoubted homologous parts occur are Lepidoptera and Trichoptera. Membranous areas are found beneath the claws in most orders but it is impossible to consider them as homologous with the pulvilli. In this paper they are



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6 Psyche [ March
termed flexor membranes. Crampton (1923) suggests two possible derivations for the pulvilli: either they arose as portions of a divided arolium (see condition in Hymenop- tera, Fig. 26, Plecoptera, Fig. 7, and Mantispa, Fig. 19.) or they are detachled membranous areas from the under side of the claws (see condition in some Ephemeroptera, Fig. 2, Homoptera and Hemiptera, Figs. 14 and 15, and the relation of basipulvilli to such membranous areas in Blat- tids.)
Ungues.-The claws.
Unguif er.- (tubercula, and Gelenkhocker of de Mei jere) refers to a small dorsal sclerite by which the claws are ar- ticulated with the distitarsus.
Unguitractor- (tarsulus, calcanea) . This sclerite is the most constant structure of the pretarsus and is easily identified in all insects thus far observed because its gen- eral appearance is always the same though varying in size and shape. The surface of the unguitractor is sculptured to a greater or less degree with nodules which are some- times almost spinelike. These nodules appear to corres- pond with the polygonal cells of the hypodermis and these cells may be seen through the chitin with high magnifica- tion. Snodgrass (1927) shows that in Tibicen this plate may be divided into two sclerites and also he figures in the membrane distal to the unguitractor, two small plates which he says may represent the arolium. It seems more likely that thley are a divided planta but there is little evidence as yet for 'either supposition.
Discussion.
ORDER ODONATA
Anax junius Drury. (Fig. 1)
The claw-segment of Anax is simple and consists of the unguitractor plate (ut) with the expanded distal portion which is probably an empodium (em?) and of the lateral fleshy pads, flexor membranes (fm.) ,-which transmit tension to the ventral surfaces of the claws. The Gleit-
flache of de Meijere, or thickened ventral portion of the



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19351 Structure of the Pretarsus 7
distitarsus over which the unguitractor slides, is well de- veloped in Odonata and a process of this structure extends beyond the bases of the ungues (un) . The only other orders in which this condition has been observed are Coleoptera and Hemiptera.
ORDER EPHEMEROPTERA
Siphlonurus sp. (Fig. 2)
Thle unguitractor is a sma1,l pear-shaped plate and the claws (un) are simple. The important feature in this case is the membranous pad-like structure (pv?) closely applied to the ventral surface of each of the ungues. It is this con- dition which has led Crampton to suggest that possibly the pulvilli (Diptera) were derived from some such situation, that is, a splitting off of a ventral membrane from the claw itself. Proximally these pad-like structures are closely as- sociated with the membranous areas transmitting tension from the unguitractor to the claws and in fact give evidence of being continuous with them.
Although these examples of Odonata and Ephemeroptera both show their ellementary position, (absence of arolium, planta, basipulvilli or other significant structures), they give no evidence of any close relationship for the presence of an empodium in Anax indicates a certain amount of spe- cialization.
Thus, if these cases prove to be typical, the con- clusions to be derived from the comparative morphology of the pretarsus will evidently be in agreement with the general conception of the relative positions of these orders. CURSORIAL ORTHOPTERA
Blattidae-Pmmiplanta mericana L. ( Fig. 3) The arolium (ar) is large in this family, although its structure is simple compared with the complicated types found in the higher orders. Vientrally it is membraneous and concave, so that it forms a most efficient adhering organ. The unguitractor is typically large and heavily sclerotized. Two other structures of importance appear in this group. The planta
(pi) is a median quadrate plate just distal to the unguitractor and seems to be lightly sclerotized; it bears a single large seta. On either side of the planta is a



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8 Psyche [March
small sclderite, the basipulvillus (bp) of Crampton, from which the retractile membrane leads to the under side of the claw. The relation of these membranes to the basipulvilli (where they are present) is the same throughout the or- ders and with further study should present interesting evi- dence with respect to the hypothesis that the pulvilli are evolved from such membranes.
Mantidae-Tenodera sinensis Saussure. (Fig. 4) An a'rolium has not been found in Mantids thus far ob- served and such a structure is probably absent in this fam- ily. The general plan follows closely that of the roach al- though a seta is not present on the planta. The flexor mem- . branes are large and especially noteworthy are their lat- eral expansions which if further produced might 'easily take on the appearance of pulvilli !
Termopsis angusticollis Walker (Fig. 5)
Again, in this case the basic plan follows closely that of the Blattid; however, the planta is much smaller than in either Mantid or Blattid and the seta does not arise from the planta itself but from thfe membrane. This condition might be taken as evidence for the view that the planta is a variable sclerotized area of the membrane between the claws. Although there is no arolium we find between the bases of the claws in winged termites a small plate (ar?) which undoubtedly represents an abortive arolium for in winged specimens of Mastotermes, a definite arolium, even though small and rudimentary, is present. Unguitractor and basipulvilli, as in the Mantid, show no significant var- iations from thae Blattid type.
The proximity of these three groups has been noted many times by students of insect morphology so that such a cor- relation in type of pretarsus is not surprising. This com- bination appears even more natural when contrasted with the equally coherent union (from the standpoint of Pre- tarsus) of the Saltatorial Orthoptera.




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19351 Structure of the Pretarsus 9
ORDER EMBIOPTERA
Oligotoma sp. (Fig 6)
In this order we again find the Blattid type; the planta is more extended laterally and is in contact with the basi- pulvilli on either side. There is no arolium and the flexor membranes are large and slightly expanded. There seems to be nothing particularly outstanding about the claw-seg- ment of this insect but in general it approaches that of the Plecopteran to such a degree as would not prohibit the con- ception that these two orders are related. ORDER PLECOPTERA
Pteronarcys dorsata Say (Fig. 7)
Pteronarcys shows the lateral basipulvilli and median planta as found in cursorial Orthoptera and termites. The arolium is a large and complicated structure which is made up of three lobes : a median portion or true arolium, and two lateral smaller pads beneath the claws which may be sec- ondary divisions of the arolium or pulvilli which have be- come fused with the arolium. A more complete study of the Plecoptera may settle this question if intermediabe forms can be found. On the other hand, it is possible that this structure represents a new development with no traceable homologies to the parts referred to above. (This also ap- plies to similar lobies beneath the claws of many Homoptera and Hemiptera, to be discussed later). Dorsally, the mem- branous pads are supported by chitinous plates (indicated by broken lines) and the median lobe blears two clusters of small set= on its ventral surface. The presence of planta and basipulvilli make it unlikely that the Plecoptei-a should be considered with the Archipterygota,
SALTATORIAL ORTHOPTERA
Locustidae-Melanoplus bivittatus Say (Fig. 8) A large arolium has been found in all Locustids observed as contrasted to Tlettigoniids and Gryllids in which this structure probably does not occur.
It should be noted that
the presence or absence of an arolium is not a condition which is constant for an order, but further study should



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10 Psyche [March
produce information concerning its consistency in groups below ordinal rank. The absence of basipulvilli is charac- teristic of all the leaping Orthoptera and it is this factor which gives such a clear-cut division between the two or- thopteroid groups. The planta of Melanoplus is more heav- ily sclerotized than the representatives of Tettigoniidae and Gryllidae which were examined.
Tettigonfidae-Ceuthophilus maculatm Say (Fig. 9) Except for the absence of arolium and basipulvilli, the condition of the planta seems to be the only important fea- ture of Ceuthophilus. This structure is not as heavily scler- otized as it is in Melanoplus and is quite different in shape. Often the planta is so lightly demarked, especially in small insects, that its boundaries are almost indistinguishable from the surrounding membrane.
Gryllidae-Gryllus pennsylvanicus Burm. (Fig. 10) Gryllus gives us the first radical variation in the ungui- tractor for here the form is quite different from anything else found among the Orthoptera. This sclerite is charac- terized by a longitudinal V-shaped ridge or protuberance, the significance of which must remain in doubt until serifes are studied in groups below the order. It is possible that the less notable variations (chiefly in outline) of thle ungui- tractor observed in these orthopteroid families will prove to be consistent to some daegree.
The planta may be easily
compared with that of Ceuthophilus and differs mainly in the length of the lateral arms and the nature of the pos- terior margin.
Gryllotalpidae-Gryllotalpa borealis Burm. (Fig. 11) Considerable modification of the planta is exhibited by Gryllotalpa and its condition in this insect is one of the factors suggesting a close relationship between planta and empodium. Note the numerous large setae and the manner in which the anterior portion of the unguitractor merges with the flexor membranes and planta. Comparison of the latter with the corresponding structure in Ceuthophilus and Gryllus indicates that it is a planta.
Empodium by definition refers to a process of the ungui- tractor, but this term must also apply to certain structures



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19351 Structure of the Pretarszis 11
closely associated with the unguitractor which are not act- ual processes of it. Eg,, the setiform empodium of Asilid Diptera is articulated with the unguitractor rather than continuous with it; also the coleopterous empodium in many cases gives some evidence that it may be retracted within (or beneath?) the unguitractor as pointed out by Hayes and Kearns. At any rate the empodium is not necessarily a distinct part of the unguitractor, so that the condition in Gryllotalpa may be intermediate between the typical iso- lated planta (as of Melanoplus) and the empodium of many Coleoptera where there is no discernable break between this structure and the unguitractor. Ceuthophilus and Gryllus may represent a step beyond the Melanoplus type towards closer association of planta with unguitractor. About all that can be definitely said for the Saltatorial Orthoptera (sensu strictu) at the present time is that they are linked by a universal absence of basipulvilli and by the nature of other more variable features such as planta and flexor membranes.
In any case there appears to be a wealth of material to be investigated within this group. Phasmida+AnISomorpha buprestoides (Stoll) (Fig. 12) The absence of basipulvilli would place the Phasmids with Saltatorid rather than Cursorial Orthoptera and this is in agreement with conclusions presented by students of other structures. The unguitractor is narrower than in other Orthopteroids, while the planta is more heavily sclerotized and is triangular in outline, but is distinct from the ungui- tractor, a condition nearer to that of Locustids than to the other Salfcatorial Orthoptera. The arolium is large and is reinforced dorsally by thickened ridges. On the whole the condition of the claw segment of this insect may be con- sidered as supporting the general conception of the position of this family, i. e., related to the Saltatorial Orthoptera but developed along its own line of specialization. ORDER DERMAPTERA
Pscdis sp. (Fig. 13)
Apparently basipulvilli and planta are missing in Psalis.