There was an error published in J. Exp. Biol.217, 2130-2138. Eqns A12 and A13 in the Appendix were cropped. The online pdf and full-text versions (but not the print version) of the article have been corrected. The correct versions of the equations also appear below.

(A12)
(A13)

We apologise to the authors and readers for this error.