The morphology of the jumping apparatus was studied comparatively between specimens of Orchesella cincta with extended and flexed furca (Figs. 2A-C). Abdominal segments 2–6 were reconstructed, including cuticular structures such as basal plates and sclerites, as well as internal musculature. The architecture of the abdomen in Orchesella cincta is defined by visible and clear segmentation. The 4th abdominal segment is longer than the others, with a well-developed furca which inserts ventrally. A complex muscular system with muscles oriented predominantly parallel to the longitudinal axis is characteristic in O. cincta. Recognizable at the base of the furca are the basal plates - mobile and elastic cuticular structures via which the furca articulates with the abdomen on a longitudinal axis. Three basal plates BP1, BP2 and BP3 are found ventrolaterally in the abdomen, connecting to each other at the edges. Their architecture, mobility and elasticity are intrinsically related to the jumping behavior. The movement of the basal plates and the abdominal segments occurs mainly by muscular action. The furca is extended via an anteroposteriorly oriented rotary movement capable of a 180º execution angle. When extended, the furca protrudes posteriorly and is highly exposed. When flexed, the furca folds ventrally together with the basal plates, hiding in the inner part of the abdomen while attached to the retinaculum (Figs. 3A-B, 4, 5A-B).
In the following, the form and functioning of the main parts of the jumping apparatus are described. An interpretation of the morphofunctional mechanisms is provided on the basis of a comparison between individuals with flexed and extended furca. MicroCT reconstructions of all muscles and the cuticular walls of the 2-6th abdominal segments can be found in Figs. 3A-B and 4. A descriptive list of reconstructed muscles and information about attachment points is provided below in Table 1.
Table 1
Table describing the muscles of 2nd -6th abdominal segments and their respective attachment points in Orchesella cincta.
Name of the muscle | origin | insertion point |
M.IIa-dlm1 | tergite of IIa anterior dorsal medial | tergite of IIIa anterior dorsal medial |
M.IIa-dlm2 | tergite of IIa anterior dorsal medial | tergite of IIIa anterior dorsal medial |
M.IIa-dlm3 | tergite of IIa anterior dorsal medial | tergite of IIIa anterior dorsal medial |
M.IIa-llm1 | tergite of IIa anterior lateral medial | tergite of IIIa anterior lateral medial |
M.IIa-llm2 | tergite of IIa anterior lateral medial | tergite of IIIa anterior lateral medial |
M.IIa-llm3 | tergite of IIa anterior lateral medial | tergite of IIIa anterior lateral medial |
M.IIa-llm4 | tergite of IIa anterior lateral medial | tergite of IIIa anterior lateral medial |
M.IIa-llm5 | tergite of IIa anterior lateral medial | tergite of IIIa anterior lateral medial |
M.IIa-isdlm1 | muscle centre dorsomedial in transition area between Ia and IIa | muscle center dorsolateral in transition area between IIIa and IVa |
M.IIa-isdlm2 | muscle centre dorsomedial in transition area between Ia and IIa | muscle center dorsolateral in transition area between IIIa and IVa |
M.IIa-isllm1 | muscle centre ventral lateral in transition area between IIa and IIIa | tergite of IIIa ventral lateral in transition area between IIIa and Iva |
M.IIa-isllm2 | muscle centre ventral lateral in transition area between IIa and IIIa | tergite of IIIa ventral lateral in transition area between IIIa and Iva |
M.IIa-isllm3 | muscle centre ventral lateral in transition area between IIa and IIIa | tergite of IIIa ventral lateral in transition area between IIIa and Iva |
M.IIa-isllm4 | muscle centre ventral lateral in transition area between IIa and IIIa | tergite of IIIa ventral lateral in transition area between IIIa and Iva |
M.IIa-isllm5 | muscle centre ventral lateral in transition area between IIa and IIIa | tergite of IIIa ventromedial in transition area between basal plate 1 and tergite IIIa |
M.IIa-vtrm1 | tergite of IIa ventre lateral | sternite of IIa medial |
M.IIIa-llm1 | muscle centrecentrallateral in transition area of IIa and IIIa | muscle centrecentrallateral IIIa |
M.IIIa-llm2 | muscle centrecentrallateral in transition area of IIa and IIIa | muscle centrecentrallateral IIIa |
M.IIIa-llm3 | muscle centrecentrallateral in transition area of IIa and IIIa | muscle centrecentrallateral IIIa |
M.IIIa-istm1 | inner side of the sternite III | muscle center dorsolateral in transition area between IIIa and IVa |
M.IIIa-dvm1 | muscle centrecentrallateral IIIa | tergite of IIIa mediolateral |
M.IIIa-dvm2 | muscle centrecentrallateral IIIa | tergite of IIIa mediolateral |
M.IIIa-dvm3 | muscle centrecentrallateral IIIa | tergite of IIIa mediolateral |
M.IIIa-dvm4 | muscle centrecentrallateral IIIa | tergite of IIIa mediolateral |
M.IIIa-trm1 | muscle centrecentrallateral IIIa | linked to M.IIIa-ret |
M.IIIa-trm2 | muscle centrecentrallateral IIIa posterior | dorsallateral in transition area between IIIa and IVa |
M.IIIa-trm3 | muscle centrecentrallateral IIIa posterior | basal plate 1 mediolateral anterior |
M.IIIa-isdlm1 | tergite of IIIa, anterior dorsal medial | tergite of Va, anterior dorsal medial in transition area between IVa und Va |
M.IIIa-isllm1 | muscle centre lateral in transition area between IIa and IIIa | sternite of IVa posterolateral in BP3 |
M.IIIa-ret | retinaculum at lateral side of ramus | linked to M.IIIa-trm1 |
M.IIIa-llm1 | tergite lateral in transition area between IIa und IIIa | tergite lateral in transition area between IIIa und IVa |
M.IIIa-llm2 | tergite lateral in transition area between IIa und IIIa | tergite lateral in transition area between IIIa und IVa |
M.IIIa-llm3 | tergite lateral in transition area between IIa und IIIa | tergite lateral in transition area between IIIa und IVa |
M.IIIa-llm4 | tergite lateral in transition area between IIa und IIIa | tergite lateral in transition area between IIIa und IVa |
M.IIIa-dlm1 | muscle centre dorsomedial in transition area between IIa and IIIa | muscle centre dorsomedial in transition area between IIIa and IVa |
M.IIIa-dlm2 | muscle centre dorsomedial in transition area between IIa and IIIa | muscle centre dorsomedial in transition area between IIIa and IVa |
M.IIIa-ldvm1 | BP1 median lateral point | tergite IIIa lateral (very long longitudinal point) |
M.IIIa-ldvm2 | BP1 lateral transition area between BP1 and tergite IIIa | tergite IIIa lateral (very long longitudinal point) |
M.IIIa-te.ret | muscle centrecentrallateral IIIa | retinaculum at lateral side of ramus |
M.IVa-dlm1 | tergite of IVa anterior dorsal medially | tergite of Va anterior dorsal medial |
M.IVa-dlm2 | tergite of IVa anterior dorsal medially | tergite of Va anterior dorsal medial |
M.IVa-dlm3 | tergite of IVa anterior dorsal medially | tergite of Va anterior dorsal medial |
M.IVa-dlm4 | tergite of IVa anterior dorsal medially | tergite of Va anterior dorsal medial |
M.IVa-dlm5 | tergite of IVa anterior dorsal medially | tergite of Va anterior dorsal medial |
M.IVa-llm1 | tergite of IVa anterior dorsal medially | ventrally in between the sternites BP1 and BP2 |
M.IVa-llm2 | tergite of IVa anterior dorsal laterally | ventrally in between the sternites BP1 and BP2 |
M.IVa-dvm1 | in the BP1, laterally in the basal rod | in the middle of IVa, laterally |
M.IVa-isdlm1 | tergite of IVa anterior dorsal medially | tergite of Va posteriorly in the BP3 |
M.IVa-isdlm2 | tergite of IVa anterior dorsal medially | tergite of Va posteriorly in the BP3 |
M.IVa-isdlm3 | tergite of IVa anterior dorsal medially | tergite of Va posteriorly in the BP3 |
M.IVa-isllm1 | in the middle of IVa, lateral medially | tergite of Va posteriorly in the BP3 |
M.IVa-isllm2 | in the middle of IVa, lateral medially | tergite of Va posteriorly in the BP3 |
M.Va-trm1 | tergite of Va anterior dorsal medially | dorsal anteriorly at the BP3 |
M.Va-dlm1 | tergite of Va anterior dorsal medially | tergite of VIa anterior dorsal medial |
M.Man-dvm1 | ventral medially at the anterior portion of the manubrium | dorsal medially at the anterior portion of the manubrium |
M.Man-dvm2 | ventral medially at the middle portion of the manubrium | dorsal medially at the middle portion of the manubrium |
M.Man-lm1 | basal plate 3 posterior dorsal medially | anterior laterally at the articulation point of manubrium |
(please, insert the Table 1 here)
Basal plate 1 (BP1) and the basal rod (BR)
BP1 is the most prominent basal plate, originating from the inner posterior border of the 3rd abdominal segment, medially close to the retinaculum. Posteriorly it forms a border with BP2. BP1 has the most complex architecture. It is shaped like a swim float and has 2 walls, one on the inner side and another on the lateral border, allowing the board to assume a folded shape when seen in transversal or frontal perspective (Figs. 5A-F, 6A-B). The basal rod (BR) is the basal sclerite of BP1. It begins anteriorly, almost at the edge of the 3rd abdominal segment, on the side of the inner wall. The BR is initially narrow, assuming a thicker and flatter surface from the middle of BP1 up until the posterior end, where it connects to BP2 (Figs. 6A-B, 7A-K, 8A-B). At the posterior end of the basal rod are two finger-shaped depressions into which the basal condyle (bc) fits in the flexed furca state (Figs. 8C-E).
The muscles working anteriorly on BP1 and BR are M.IIIa-ldvm1 and M.IIIa-ldvm2, which connect the internal and external lateral wall of BP1, respectively with the lateral region of the 3rd abdominal segment. M.IIIa-istrm1, originating medially at the inner wall of BP1, connects anteriorly to the muscular center of the retinaculum, laterally to abdominal segments III and IV, as well as posteriorly to the lateral wall of BP1. In the middle of BP1, connected to the internal lateral wall, M.IVa-dvm1, the main dorsoventral muscle, can be found. This muscle is responsible for deforming BP1 and BR (Figs. 3A-B, 4, 8B, 9A-B).
Basal plate 2 (BP2) and basal sclerite 2 (BS2)
BP2 is triangular in shape and is positioned ventrolaterally, forming a corner between the abdomen and the furca. In the flexed furca state, this plate is telescoped internally. BS2 originates ventrally in BP2 (between BP1 and BP3) and extends vertically and laterally along the posterior edge of BP2, connecting to the ventral lateral edge of tergite V at the border between the abdomen and the furca (Figs. 5A-F, 7A-K, 8A-C, 10A-C)
Five muscles connect to BP2, two of them - M.IVa-llm1 and M.IVa-llm2 - to the anterior ventrolateral portion (on the border between BP1 and BP2). These two muscles run laterolongitudinally together, assume a spiral shape, cross and support the muscle M.IVa-dvm1 and connect dorsally at the border between the 3rd and 4th abdominal segments. Laterally, on the dorsal surface at the corner of BP3, the muscles M.IVa-isdlm1, M.IVa-isdlm2 and M.IVa-isdlm3 connect (i.e. to the basal plate) (Figs. 3A-B, 4, 10A-E).
The Basal Plate 3 (BP3) and Basal Sclerite 3 (BS3)
At the base of the furca, is the basal plate 3 (BP3), the point of articulation between the basal sclerites BS2 and BS3. Here I recognize as BS3 the anterior part of the furca that articulates with the abdomen, and in flexion it pushes the basal sclerites to the center of the body. This sclerite connects to BP3 laterally and then extends ventrally in a V-shape (in transverse view) and merges in the midline. At the base of the BS3 there is a finger-shaped basal condyle that forms the point of contact between the furca and BR when the furca is flexed (Figs. 5A-F, 7A-K, 8A-C, 10A-E, 11A).
Two abdominal muscles connect to the dorsal lateral part of BP3, they are: M.IVa-isllm1, M.IVa-isllm2, both originate laterally, in the tergite of the 4th abdominal segment. M.IIIa-isllm1, starts dorsolaterally in the 3rd abdominal segment and extends posteriorly to connect at the base of the furca, in the BS3 (Figs. 3A-B, 4, 10A-E). Medially at the BP3 originate the M.Man-lm1, which connect posteriorly at the base of the posterior manubrium membrane (pmm) (Figs. 3A-B, 11A-E).
The furca: manubrium and dens
The furca connects ventrally to the 4th abdominal segment through BP3 and BS3, and when flexed it extends anteriorly along the midline to the first abdominal segment. This medial appendix is made up of 2 main parts, anteriorly the manubrium and posteriorly, the dens (Figs. 2A-C, 5A-B, 10A-E, 11A-J). With a tubular shape the manubrium is fused medially, and in the posterior portion there are two structures, dorsally a T-shaped furcular sclerite of the manubrium (fms), and ventrally a flexible membrane (pmm), both elastic structures border the dens. Anteriorly the dens is formed by two pads that together form the dens lock (dl), and then bifurcate posteriorly and with a series of crenulations assume the shape of a circular spring. These structures have internal channels through which the hemolymph flows between furca and abdomen, in both directions. Internally the dens pad, there is a transverse membrane of dens (dm). At the base of the dens, internally between each of its parts, there is a series of locks, where the retinaculum connects when the furca is flexed (Figs. 11A-E).
There are three pairs of muscles in the furca, present exclusively in the manubrium. M.Man-lm1 is a longitudinal muscle that originates medially in the anterior portion of BP3 and connects posteriorly on the lateral side of the manubrium, right at the base of pmm, the articulation point with the dens. The other two pairs are dorsoventral. M.Man-dvm1 is present anteriorly, and M.Man-dvm2 solely in the middle portion of the manubrium (Figs. 3A-B, 11A-E).
The retinaculum
Ventromedially in the posterior portion of the 3rd abdominal segment, the retinaculum is situated. This very small and unremarkable structure has an elastic cuticular composition that permits deformation, and an architecture capable of holding the furca at rest. It is a triangular-shaped structure with 2 arms at its end (the rami), each one with 4 teeth. Internally at the base of each of the rami, a pivot point is located (Figs. 2A-C, 12A-E).
Internally a complex muscular system supports the inherent movements of this structure. Connecting internally to the retinaculum in the region of the point of articulation are the short muscles M.IIIa-ret. At the base of this, a connection point with M.IIIa-trm1 and the M.IIIa-te.ret tendon can be found. Both these muscles connect internally to a complex muscle center composed of M.IIIa-trm2, M.IIIa-istrm1 and M.IIIa-istm1 (Figs. 3A-B, 4, 12A-E, 13A-D).
Morphofunctional aspects related to jumping
Spring mechanism, triggers and energy storage
The cLSM and MicroCT images revealed the areas of the jump apparatus that are most elastic and have energy storage potential. The spring mechanism is generated in BP1, BP2, BP3, mainly from the deformation of their respective basal sclerites. Such deformation and energy storage is made possible by the elasticity of these structures, which are rich in resilin. The tension in the sclerites is generated by muscular action, hydrostatic pressure and in large part by tension between the furca and the basal sclerites when furca is flexed. Four main kinematic zones can be identified in the furca: BS3 right at the base of the manubrium, which extending posteriorly connects the furcular area of the manubrium zone (FMZ) with strong posterior sclerite (fms), and laterally the posterolateral manubrial zone (PLMZ) where a flexible membrane (pmm) is present. Both posterior manubrial zones, articulate with the anterior zone of dens (ADZ), the main point of contact between the furca and the surface (Figs. 7A-K, 8A-C, 11A-H).
The spring mechanism is intensified by the presence of the trigger, the retinaculum, which increase the tension between the basal plates and prevents the basal sclerites from returning to a relaxed state. At the base of the furca a basal condyle (bc) at BS3 intensifies the tension on the basal sclerites and extends posteriorly along with the manubrium as a keel, coming into direct contact with the ventral part of the abdomen when the furca is flexed (Figs. 8D-E, 10A-E, 12A-G, 13C-D).
Hydrostatic pressure
Hydrostatic pressure plays a passive role in jumping by increasing the tension of the basal plates, and an active role by increasing the efficiency of the jump in coordination with the muscles. Thereby the hemolymph is concentrated inside the body cavity, tensing its walls in a dorso-ventral and longitudinal movement when the furca is flexed. With the release of the furca, this hydrostatic volume is directed ventrally and posteriorly toward the furcular cavity (Fig. 11A-E).
In the abdomen, the muscles related to the increase in hemolymphatic pressure are mainly the dorso-longitudinal muscles (M.IIa-isdlm1, M.IIa-isdlm1, and the dlm), dorsoventral (M.IVa-dvm1), ventral and lateral longitudinal muscles. The contraction of these muscles causes a reduction in the space between the segments and, consequently, an increase in hydrostatic pressure. At the moment of extension, M.IIIa-istm1 may have an important role in directing pressure to the basal plates by deforming the 3rd abdominal segment dorsoventrally and decreasing the opening for hydrostatic flow to the posterior abdominal region. The pleural muscles M.IIIa-istrm1, M.IIIa-ldvm1 and M.IIIa-ldvm2 and the ventral longitudinal lateral intersegmental muscles MIIa-isllm1, MIIa-isllm2, MIIa-isllm3, MIIa-isllm4 and M.IIa-isllm5 could act by regulating the hydrostatic pressure between the lateral tergites (IIIa-IVa), sternite III and BP1 in the anteroposterior flow towards the opening cavity of the furca (Figs. 3A-B, 4, 8A-E, 9A-B, 10A-E, 14A-B).
In the furca, the dorsoventral (M.Man-dvm1, M.Man-dvm2) and longitudinal muscles (M.Man-lm1) are involved in controlling hemolymphatic pressure by compressing the cuticle walls, effectively injecting hemolymph into the dens cavity. As already mentioned, the muscles (M.Man-lm1) potentially also act by releasing the furca from the retinaculum (Figs. 11A-E, 14A-B).
Transition between "flexed furca" and "extended furca” phases
The flexed furca phase starts with the engagement of the retinaculum in the locks of the dens, with the help of muscles M.IIIa-trm1, M.IIIa-te.ret and M.IIIa-ret, which through their contraction set up the hook, when the furca returns to the ventral side of the abdomen (Figs. 12D-E). The return of the furca to the abdomen potentially occurs by the combined contraction of muscles M.IVa-dvm1 (which connects to BP1), M.IVa-ll1 and M.IVa-ll2 (which connect at the border between BP1 and BP2) and the long M.IIIa-isllm1 (which connects to BS3 at the base of the furca) (Figs. 10A-E, 14A-B). The furca extension phase begins with the release of the furca by the retinaculum when extension is needed. It has been proposed that the furca is released from the retinaculum by hydrostatic pressure alone (this will be addressed in the discussion section), but here I propose an alternative hypothesis. The contraction of the longitudinal manubrial muscle M.Man-lm1 through the articulation with the dens (which is almost like a knee), creates the pull on the posterior side of manubrium, the posterior manubrial membrane (pmm) and their sclerites. This causes an opening of the dens pads, resulting in the opening of the dens lock, and the release of the furca (Figs. 12F-G). Subsequently, the contraction of muscles M.IVa-isdlm1, M.IVa-isdlm2, M.IVa-isdlm3, M.IVa-isllm1 and M.IVa-isllm2 could extend the furca (Figs. 10A-E, 14A-B, 15A-B) and, together with the spring and the hemolymphatic mechanisms (or even without, but with loss of efficiency), result in jumping.