Functional morphology of tarsal adhesive pads and attachment ability in ticks Ixodes ricinus (Arachnida, Acari, Ixodidae) | Journal of Experimental Biology

Legs appeared to be highly flexible because of their elastically articulated segments, all roughly similar in length and width (Fig. 2; Table S2). They were mostly held arcuately curved, and could be instantly folded tightly to the ventral body after a disturbance (thanatosis) and during quiescence.

During waving and questing with the forelegs, the three remaining pairs of legs were used for locomotion and provided sufficient hold while walking horizontally, upside-down on the ceiling and vertically up and down (Fig. 2A). The forelegs were observed to contact the passing host first for transition from plants.

On human skin, ticks moved without hindrance, spanning several skin microfolds with their legs (Fig. 2B,C). In the ceiling situation (upside-down position), female I. ricinus walked at a velocity of up to 2.3 mm s−1 on glass using four pairs of legs for locomotion (Movie 1).

Three pairs of legs generated sufficient adhesion while resting (Fig. 2D).

 

Light microscopic images of female I. ricinus. (A–D) Ticks are shown with six legs in contact with the substrate while walking on a sloped gramineous leaf surface (A), horizontally on a resin replica of human female skin from the ventral forearm (B,C) and upside-down attached to glass (D). (E) Ventral detail of claws held apart: between them, the expanded, translucent pad and dorsal plates are visible. (F,G) Shiny blue areas in fluorescence light microscopy images demonstrate the presence of resilin in the tarsal–pretarsal articulation, in the lateral view of the pad and claw cuticle (F) and dorso-lateral view of the pretarsus with a partly unfolded pad (G). Arrowheads in B indicate the elastic tarsal–pretarsal articulation in contact with the substrate, while the pretarsus is held dorsally backwards. c, claw; e, expanded pad; f, folded pad; p, pad; t, tarsus; tp, tarsal–pretarsal articulation; u, unfolded pad.

” data-icon-position=”” data-hide-link-title=”0″>Fig. 2.

Fig. 2.

Light microscopic images of female I. ricinus. (A–D) Ticks are shown with six legs in contact with the substrate while walking on a sloped gramineous leaf surface (A), horizontally on a resin replica of human female skin from the ventral forearm (B,C) and upside-down attached to glass (D).

(E) Ventral detail of claws held apart: between them, the expanded, translucent pad and dorsal plates are visible. (F,G) Shiny blue areas in fluorescence light microscopy images demonstrate the presence of resilin in the tarsal–pretarsal articulation, in the lateral view of the pad and claw cuticle (F) and dorso-lateral view of the pretarsus with a partly unfolded pad (G).

Arrowheads in B indicate the elastic tarsal–pretarsal articulation in contact with the substrate, while the pretarsus is held dorsally backwards. c, claw; e, expanded pad; f, folded pad; p, pad; t, tarsus; tp, tarsal–pretarsal articulation; u, unfolded pad.

On rod-shaped substrates, such as plant stems and bundles of hairs, arcuate legs entirely surrounded the substrate, supporting clinging and climbing.

Tarsal morphology

Corresponding to previous reports (Babos, 1964), each leg terminated with an ambulacrum (pretarsus), which was composed of an elongated apotele bearing long, paired, rod-shaped, curved claws (Figs 2E–G and 3A,E).

They appeared lunate and transparent, having an average thickness of 15.1±1.31 µm (n=5) and a length of 135.2±17.58 µm (n=5). A thin outer cuticular layer surrounded the claw interior, which was completely filled with an amorphous material (see below).

They tapered into a mean claw tip diameter of 1.2±0.36 µm (n=10). The claw base was slightly broadened, reaching about 20.1±1.62 µm in width (n=5).

The mean diameter of a circle fitting the claw curvature measured 54.7±8.08 µm (n=10). Between the claws, a foldable pad arose (Figs 2E–G and 3A–D). The fluorescence microscopy images (Fig. 2F,G) revealed the presence of resilin, an elastic protein, in the materials of pads, claws and membranous areas surrounding the tarsal–pretarsal articulation.

The pad was composed of three lobes held together by dorsal plates. The surface of the pad was covered with 376.9±58.47 nm (n=14)-wide and 296.4±7.42 nm (n=5)-deep folds (folded condition). On the ventral pad surface, centrally located folds were perpendicularly arranged; laterally located ones ran at an angle of 45 deg.

Dorsally, folds were oriented transversely to the pad base.

 

Cryo-SEM micrographs of the pretarsus of I. ricinus. (A) The lateral view shows (1) paired curved, lunate, tapered claws, (2) sclerites and (3) extendable, folded pad. (B) Detail of cuticular folds on the ventral surface of the folded pad. (C) Ventral view of the pretarsus with an unfolded pad and spaced claws. (D,E) Ventral aspect of the folded pad (D) and dorsal view of the folded pad and tightly held claws (E). c, claw; d, dorsal plate; tp, tarsal–pretarsal articulation; p, pad; t, tarsus; v, ventro-lateral plate.

” data-icon-position=”” data-hide-link-title=”0″>Fig. 3.

Fig. 3.

Cryo-SEM micrographs of the pretarsus of I. ricinus. (A) The lateral view shows (1) paired curved, lunate, tapered claws, (2) sclerites and (3) extendable, folded pad. (B) Detail of cuticular folds on the ventral surface of the folded pad.

(C) Ventral view of the pretarsus with an unfolded pad and spaced claws. (D,E) Ventral aspect of the folded pad (D) and dorsal view of the folded pad and tightly held claws (E). c, claw; d, dorsal plate; tp, tarsal–pretarsal articulation; p, pad; t, tarsus; v, ventro-lateral plate.

 The internal material of the pad was composed of a hierarchical network of fibres embedded in an amorphous matrix or lumen (Figs 4 and 5).
Five distinct layers of fibre arrangement could be distinguished. (1) Densely packed nanofibres connected the pad to the 3 µm-thick sclerital backing, the so-called dorsal plate, composed of cuticle layers which were crossed by large pore channels (Fig. 5A,E,F).
(2) Single nanofibres were linked to fibre bundles, creating an 8 µm-thick and 39 µm-wide meshwork, which merged into larger, solid rope-shaped fibre bundles (Figs 4A and 5B). (3) The meshwork of 1.6±0.20 µm (n=10) thick bundles filled up a 50 µm-thick portion of the pad. Interspaces between bundles were 1.9±0.55 µm (n=20).
(4) Ventrally, large fibre bundles diverged into 0.3±0.06 µm (n=10)-thick fibre bundles, which split into single, 65.5±17.17 nm (n=20)-thick nanofibres (Fig. 4B,D).
(5) The 5 µm-thick fibre bundle-fibre layer was terminally covered with a 23.2±1.39 nm (n=20)-thick, folded membrane (epicuticle) (Figs 4C and 5A–D).
The highly electron-dense, clear layer of epicuticle was interrupted by numerous, diffuse, permeable nanofolds, containing accumulated secretion (Fig. 5C,D).
The thickness of the pad gradually increased in the distal direction, from about 22 to 96 µm. In the proximal half of the pad, rope-shaped and thinner fibre bundles were oriented perpendicularly to the sclerital backing.
In the distal half, they were aligned distally at an angle of 45 deg. Nanofibres were arranged perpendicularly to the membrane and backing. Neither transversely running fibres nor pore channels were detected inside the pad.

 

Freeze fractures of the ixodid pretarsal pad (cryo-SEM). (A) Sagittal aspect of the internal hierarchical arrangement of backing rope-shaped fibre bundles, emerging smaller fibre bundles and nanofibres that split from these, covered with a membrane. (B) Transition zone between fibre bundles and nanofibres. (C) Detail of nanofibres covered with the membrane. (D) Detail of fibre bundles splitting into nanofibres. Arrows point ventrally. me, membrane; nf, nanofibre; fb, fibre bundle; rf, rope-shaped fibre bundles.

” data-icon-position=”” data-hide-link-title=”0″>Fig. 4.

Fig. 4.

Freeze fractures of the ixodid pretarsal pad (cryo-SEM). (A) Sagittal aspect of the internal hierarchical arrangement of backing rope-shaped fibre bundles, emerging smaller fibre bundles and nanofibres that split from these, covered with a membrane.

(B) Transition zone between fibre bundles and nanofibres. (C) Detail of nanofibres covered with the membrane.

(D) Detail of fibre bundles splitting into nanofibres. Arrows point ventrally. me, membrane; nf, nanofibre; fb, fibre bundle; rf, rope-shaped fibre bundles.

 

TEM images of a sagittal section of the ixodid pretarsal pad and claw. (A) Pad and backing plate (sclerite). (B) A set of hierarchically arranged rope-shaped fibre bundles, smaller fibre bundles splitting into branching nanofibres, and an outer membrane. (C) Detail of nanofibres and membrane. (D) Detail of the membrane composed of a clear and diffuse epicuticle and covered with secretion. (E) The layer of fibres and branching nanofibres connecting the pad with the backing sclerites. (F) Detail of dorsal plate sclerite. (G) Aspect of the claw. (H) Detail of the distal claw. In A, the black arrow points distally; the white arrow indicates the site shown in detail in F; and the white arrowhead indicates the claw base and orientation of the claw tips. The black arrowheads in D show the superficial secretion. am, amorphous material; ce, clear epicuticle; de, diffuse epicuticle; dp, dorsal plate; epi, epicuticle; exo, exocuticle; endo, endocuticle; fb, fibre bundle; lu, lumen; me, membrane; nf, nanofibre; op, opening; po, pore; rf, rope-shaped fibre bundles; sc, sclerite; se, secretion.

” data-icon-position=”” data-hide-link-title=”0″>Fig. 5.

Fig. 5.

TEM images of a sagittal section of the ixodid pretarsal pad and claw. (A) Pad and backing plate (sclerite). (B) A set of hierarchically arranged rope-shaped fibre bundles, smaller fibre bundles splitting into branching nanofibres, and an outer membrane. (C) Detail of nanofibres and membrane.

(D) Detail of the membrane composed of a clear and diffuse epicuticle and covered with secretion. (E) The layer of fibres and branching nanofibres connecting the pad with the backing sclerites. (F) Detail of dorsal plate sclerite.

(G) Aspect of the claw. (H) Detail of the distal claw. In A, the black arrow points distally; the white arrow indicates the site shown in detail in F; and the white arrowhead indicates the claw base and orientation of the claw tips.

The black arrowheads in D show the superficial secretion. am, amorphous material; ce, clear epicuticle; de, diffuse epicuticle; dp, dorsal plate; epi, epicuticle; exo, exocuticle; endo, endocuticle; fb, fibre bundle; lu, lumen; me, membrane; nf, nanofibre; op, opening; po, pore; rf, rope-shaped fibre bundles; sc, sclerite; se, secretion.

 The pad folded and unfolded similar to a fan at different degrees of opening, depending on the body posture, abiotic conditions and acting forces.
Three conditions of the pad were observed: folded, unfolded and expanded (Figs 2 and 3; Movie 1). While folded, the pad was widely covered by dorsal and ventral plates, and the claws were held close together, parallel to each other (Figs 3E and 6A).
We observed that in this condition, the entire pretarsus was frequently kept uplifted in the perfectly fitting cavity of its counterpart, the distal, dorsal tarsus (Fig. 2B).
Walking horizontally without disturbance, pretarsi mostly remained in that position, and ticks used the soft tarsal–pretarsal articulation to contact the substrate.
Adhesive pads were applied to the surface, unfolding them trapezoidally by V-shaped spreading of both dorsal plates and claws (Figs 2C,D,G and 6B). This posture was observed in, for example, ticks attached to human skin, where pads and claws frequently grasp the edge of skin microfolds (Fig. 3C).
Human skin generally provided a rather non-structured substrate for ticks’ feet, because claws and pads matched only a small area of the plateau between microfolds, which appeared less irregular or slightly rough (Fig. 6A). In particular, on the ceiling or after a disturbance like vibration, ticks totally expanded their pads.
They spread their claws apart until the claw tips almost faced each other, and the shape of the pad in contact became ovoid (first legs) or circular (Figs 3C and 6B).
At the upside-down position on the glass ceiling, the contact area of the pad differed statistically between legs, sexes and the unfolded and expanded condition (Table S3).
It was larger (1) in first legs than in the other legs, (2) in females than in males, and (3) under expansion. The fully expanded pad of (questing) forelegs created instant contact with the surface of a passing host.

Lähde: Functional morphology of tarsal adhesive pads and attachment ability in ticks Ixodes ricinus (Arachnida, Acari, Ixodidae) | Journal of Experimental Biology

Mainokset
Kategoria(t): Riistanhoito. Lisää kestolinkki kirjanmerkkeihisi.