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. 2016 Apr;13(117):20150984.
doi: 10.1098/rsif.2015.0984.

Bridging the gap: wound healing in insects restores mechanical strength by targeted cuticle deposition

Eoin Parle  1 , Jan-Henning Dirks  2 , David Taylor  3
Affiliations

Bridging the gap: wound healing in insects restores mechanical strength by targeted cuticle deposition

Eoin Parle et al. J R Soc Interface. 2016 Apr.

Abstract

If an insect is injured, can it repair its skeleton in a manner which is mechanically strong and viable? Previous work has described the biological processes that occur during repair of insect cuticle, but until now, there has been no biomechanical assessment of the repaired area. We analysed the biomechanics of the injury repair process in the desert locust (Schistocerca gregaria). We show that after an incision, a healing process occurred which almost doubled the mechanical strength of locust tibial cuticle, restoring it to 66% of the original, intact strength. This repair process occurred by targeted cuticle deposition, stimulated by the presence of the injury. The cut surfaces remained unrepaired, but a patch of endocuticle was deposited, reinforcing the area and thus increasing the effective fracture toughness. The deposition rate of endocuticle inside the tibia increased fourfold compared with uninjured controls, but only on the dorsal side, where the incision was placed. The limb is highly loaded during jumping, so this partial restoration of strength will have a profound effect on the fitness of the insect. A finite-element model provided insights into the mechanics of the repair, predicting that the patch material reaches its ultimate strength before the fracture toughness of the existing cuticle is exceeded.

Keywords: cuticle; insects; repair; strength; toughness; wound.

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Figures

Figure 1.
Figure 1.
(a) Cross section of the locust tibia: we made an incision of length L on the upper (dorsal) side. Cuticle thickness was measured on all sides: dorsal (T1); ventral (T2) and lateral/medial (T3). (b) Tibiae were loaded in cantilever bending applying a force F at a distance l from the fixed end; the incision was placed on the top surface a distance x from the loading point such that (l–x) was typically 2 mm. Failure occurred at the incision (see right-hand photo). (c) Typical force/deflection results. (Online version in colour.)
Figure 2.
Figure 2.
(a) The finite-element model. (b) A close-up view showing the slot and also the layer of new endocuticle (in this case of thickness 20 µm). (c) Typical results from the model: maximum principal stress plotted on a cross section through the cut, showing high stresses in the layer of new cuticle. (Online version in colour.)
Figure 3.
Figure 3.
(a) Cuticle thickness as a function of time post-injury for the 15 subjects in which repair occurred; (b) cuticle deposition rate post-injury at the injury location (T1) and elsewhere (T2, T3), compared with uninjured controls in the immediate post-moult modelling period (less than 20 days) and the later dormant period (more than 20 days). (Online version in colour.)
Figure 4.
Figure 4.
SEM photos of repaired and non-repaired cuticle. Panels (a) and (b) show the fracture surface of a tibia that has received an incision (dashed line) and subsequently repaired the area by depositing cuticle (enhanced on the photo by red shading). At high magnification (b), there is a clear difference between the scalpel-cut surface and the fracture surface of the new cuticle. Panels (c) and (d) show the fracture surface of a tibia in which repair did not occur. Panels (e) and (f) are longitudinal sections of tibiae 30 days after receiving incisions. In (e), the incision (dashed line, arrowed) caused a relative displacement of the two sides of the cuticle, which are indicated by white lines. No repair occurred. In (f), there was no such displacement and new cuticle formed (red area) to repair the injury. (Online version in colour.)
Figure 5.
Figure 5.
Results for strength and fracture toughness of three groups: injured tibiae for which no repair occurred (tested 20–50 days after incision), those for which repair did occur (also tested after 20–50 days) and uninjured controls (tested 20–63 days post-moult). Error bars indicate standard deviation.
See this image and copyright information in PMC

References

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