Fig. 5. Back vertex distance (BVD) as a function of entry position of an He—Ne laser beam into the (A) right (1.65 mmx1.88 mm diameter) and (B) left (1.63 mmx1.85 mm diameter) lens of a Pterygoplichthys etentaculus individual. The triangles represent individual data points, and the solid lines represent fitted non-linear regression lines [(A) x=2.50-0.92y²+0.80y⁴; (B) x=2.44-0.68y²+0.55y⁴]. The beam-entry positions have been normalised so that the edges of the lens represent 1 and -1.