Fig. 3.

(A) Schematic view of full ANN-based chemotaxis agent. A chemical field $c\left(x\right)$, swimmer arm lengths ($L_1$, $L_2$, $L_{\mathrm{T}}=L_1+L_2$), and respective arm velocities ($V_1$, $V_2$, $V_{\mathrm{T}}$) are used as input. By measuring the CG through the CG block, the swimmer controls the forces $F_1$ and $F_3$ in order to perform directed locomotion toward an ascending gradient of $c\left(x\right)$. Directed locomotion is split into two PCLs, which permute input and output of the SAL (Fig. 2 A, Insets) according to a predicted sign $D$ of the CG. The prediction of $D$ by the CG block (cyan) can be performed either by directly measuring $D=\text{sign}\left(\nabla c\left(x_{\mathrm{c}}\right)\right)$, by (B) spatial resolution of the chemical field, or by (C) temporal sensing at the COM position $x_{\mathrm{c}}$. The respective solutions for the ANNs (gray and dark gray) found by NEAT are shown in D and E; details are in SI Appendix.