Fig. 1. Embryogenesis in the two-spotted spider mite. (A-D) Early cleavages. (A) Uncleaved egg. The nucleus is just starting to become visible in the centre of the egg. (B) First division, the egg nucleus (dark area) has divided. A clear membrane between the two blastomeres is visible (arrow). (C) Second division. (D) Blastoderm stage embryo. Nine divisions have taken place forming a cellular blastoderm. (E-F) Formation of the `germ disc', and the germ band. (E) At 20-22 hours AEL a thickened portion of the germ band is visible (arrow), probably in ventral regions, which we interpret to be the germ disc. The germ disc starts as an ovoid swelling (E,F), and then flattens (G). Flattening of the germ disc is quickly followed by the appearance of leg primordia on both sides of the ventral midline (viewed from the anterior in H). (I-L) Formation of the germ band, limb primordia and limb outgrowth (lateral view). All embryos are viewed with anterior to the left and dorsal up. (I) The initial limb primordia form by 23 hours AEL. P, pedipalps; 1-4, walking legs. Primordia for the chelicerabearing segment and the opisthosoma germ band become visible soon after. (J) C, chelicerae. (K,L) Limb buds grow and become jointed. (M-P) Formation of the germ band, limb primordia and limb growth (ventral view). All embryos are viewed from the ventral side, with anterior to the left. Embryos are of the same stage as those in I-L. Scale bars: 50 µm.

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